precise , flexible , and specialized regulation of membrane trafficking is key to tissue patterning and differentiation during the development of multicellular organisms .
it underlies the development of cell and tissue polarity ( apodaca et  al . , 2012 ; farr et  al . , 2009 ; winter et  al . ,
2012 ) and allows cells to specialize in the secretion or absorption of specific cargo in response to different signals or functional demands ( cao et  al . , 2012 ;
, it is essential to be able to visualize and manipulate specific membrane trafficking pathways .
the rab protein family provides a unique entry point to study membrane trafficking pathways and their function in development and differentiation .
rabs comprise a large family of lipid - modified gtpases that localize to specific subcellular membrane compartments .
they cycle through gtp- and gdp - bound states , acting as molecular switches to recruit effector proteins that control compartment biogenesis ( seabra and wasmeier , 2004 ; zeigerer et  al . , 2012 ) , functional properties ( liu and storrie , 2012 ) and composition ( behnia and munro , 2005 ; zerial and mcbride , 2001 ) , and direct vesicle motility ( horgan and mccaffrey , 2011 ) , tethering ( sinka et  al . , 2008 ) , and fusion ( schimmller et  al . , 1998 ) .
a set of five rabs ( 1 , 5 , 6 , 7 , 11 ) , which we denote the core rabs , has been maintained in almost all eukaryotes  from unicellular organisms to metazoans , fungi , and plants ( pereira - leal , 2008 ; pereira - leal and seabra , 2001 ) .
functional studies in yeast and in tissue culture cells have revealed their key functions in regulating the core secretory and endocytic pathways common to all cells .
interestingly , recent genomic phylogeny studies have suggested that the putative last eukaryotic common ancestor ( leca ) had a much larger repertoire  between 1523 rab proteins .
this group includes the core rabs , but also many others that are lost in different eukaryotic lineages .
in contrast , the rab family underwent a tremendous expansion correlated with the emergence of metazoans ( bock et  al .
, 2011 ; elias et  al . , 2012 ; klpper et  al . , 2012 ; pereira - leal and seabra , 2001 ) . this expansion has been proposed to reflect the greater complexity of membrane trafficking pathways required for cell communication , tissue patterning , and differentiated cellular functions .
systematic analysis of the tissue specificity and subcellular localization of rabs is an important first step in understanding how membrane trafficking pathways are organized in different cell types and how they are deployed during development and differentiation .
they maintain distinct protein and lipid compositions on their apical and basolateral surfaces through targeted delivery , endocytosis , and recycling of specific cargo ( apodaca et  al . , 2012 ) .
neurons also polarize trafficking of membrane and secreted proteins to organize the somatodendritic and axonal domains ; these have been suggested to rely on sorting mechanisms similar to the basolateral and apical domains of epithelial cells , respectively ( dotti and simons , 1990 ; siegrist and doe , 2007 ) .
if so , are the rabs that first appeared in metazoans more likely to be deployed in this way ?
much has been learned about the subcellular localization of rab compartments by expressing tagged rab proteins .
however , unphysiological expression levels can alter membrane trafficking and distort the appearance of the relevant membrane compartments ( mottola et  al . , 2010 ) .
we therefore generated a toolkit for the systematic analysis of rab protein expression and function in drosophila melanogaster .
the drosophila genome predicts 33 rab proteins based on sequence similarity , and there is evidence that 27 are expressed ( chan et  al . , 2011 ; zhang et  al . , 2007 ) .
fourteen of these rabs were already present in the leca ( between 23 billion years ago ) , and the other 13 first arose by duplication and divergence at the root of the metazoan lineage ( 500 million years ago ) .
while the genes encoding many rab proteins have undergone further expansions in vertebrates , this is not the case in drosophila ( diekmann et  al .
, 2011 ; elias et  al . , 2012 ; klpper et  al . ,
each drosophila rab exists in only a single copy , facilitating analysis ( http://flybase.org ) .
, 2008 ; rong and golic , 2000 ) to fuse yfp to the n terminus of each drosophila rab .
furthermore , the yfp tag provides a target for knockdown approaches , allowing specific and controllable reduction of rab protein function ( brankatschk et  al . , 2014 ;
we systematically analyze rab protein tissue distribution and subcellular localization in six different tissues comprising over 23 cell types .
we construct an online image database flytrab in which each rab can be viewed in whole tissues at subcellular resolution and a searchable annotation database for the systematic analysis and comparison of rab expression and localization ( tomancak et  al . , 2007 ) .
analysis of these data provides insights into the evolution of membrane trafficking pathways and how they are deployed during cell polarization and tissue differentiation .
to generate a novel set of endogenous n - terminally tagged rab proteins , we used homologous recombination to target the coding sequence for each drosophila rab gene and replace the predicted start codon with dna encoding yfp ( figure  1a ) .
western blotting indicates that the yfp tag does not alter rab4 protein levels ( figure  s1a ) .
furthermore , tagging endogenous rab1 , rab6 , and rab7 with yfp does not perturb golgi morphology in salivary gland or fat body cells ( figures s1b  s1i ) .
finally , except for yrabx5 , all new yrab alleles produce viable homozygous flies , further confirming that their functions are not disturbed ( figure  1b ) . to confirm the presence of yrab fusion proteins , and to examine their endogenous levels and proportions in different tissues , we performed semiquantitative western blotting against the yfp tag in three larval tissues : the fat body ( fb ) , wing disc ( wd ) , and salivary gland ( sg ) ( figures 2a and 2b ) .
we blotted equal amounts of tissue lysate from each homozygous yrab line and compared the signal strength of each yrab to a recombinant yfp standard ( figures 2a and 2b ) .
several yrabs ( 3 , 14 , 32 , 26 , x4 , x5 ) were not detected in any of the three tissues ( figures 2a and 2b ) . to confirm that these fusion proteins were produced at all , we blotted extracts of adult heads .
this identified yrab3 , yrab32 , yrab26 , yrabx4 , and yrabx5 , but failed to detect yrab14 ( figures s2a and s2a ) .
qualitatively , detectability of the different yrab proteins in wd , sg , and fb is consistent with mrna expression data from modencode ( figure  s2b ) .
yrab14 , yrab32 , and yrabx4 are exceptions  we can not detect these proteins in wd , fb , or sg although modencode ( chen et  al . , 2014 )
predicts moderate expression . despite general qualitative agreement , yrab protein levels are not always proportional to reported mrna levels in modencode ( figure  2b )
it has been proposed that core rabs that have been conserved since the leca regulate pathways that are required in all cells , whereas rabs that first arose in metazoans regulate tissue - specific pathways .
consistent with this , core rabs ( yrab1 , yrab5 , yrab6 , yrab7 , yrab11 ) are present in all three tissues examined by western blotting ( figures 2a and 2b ) .
leca - rabs that are more often lost and those that arose in metazoans are more tissue - specific ( diekmann et  al . , 2011)many of these are undetectable in some or all of the three tissues ( figures 2a and 2b ) .
we estimate that the most abundant yrabs ( figure  2b , red ) are present at concentrations of at least 10  m ( for comparison , citrate synthase is present at 10  m ) ( srere , 1967 ) , while the least abundant detectable yrabs ( figure  2b , dark blue ) are present at 20-fold lower concentrations . while rab gefs and gaps can modulate rab protein activity and flux through the corresponding compartment ( cabrera and ungermann , 2013 ) , rab protein concentration may also help determine the capacity of the compartment .
if so , then a large fraction of secreted and transmembrane proteins move through pathways controlled by core rab proteins . finally , comparing yrab protein levels
reveals unexpected heterogeneity in the proportions of even the highly conserved leca - rabs in different tissues . conserved rabs of the protein secretion machinery ( rab1 , rab2 , rab6 , rab8 ) are present in different proportions in wd , sg , and fb cells .
similarly , rabs with conserved functions in endocytic trafficking and recycling ( rab4 , rab5 , rab7 , rab11 ) are also present in different proportions in different cell types
. it will be interesting to see whether these differences are reflected by changes in trafficking through the corresponding compartments . to examine expression and subcellular localization in a broader range of cell types
, we imaged larval wd , sg , fb , and brains , as well as adult ovaries and testes ( janning , 1997 ) .
we collected an array of 3d tiles from each tissue and stitched them ( preibisch et  al . , 2009 ) to form an image of the entire tissue at subcellular resolution . to analyze these data
, we developed a hierarchical annotation scheme using a controlled vocabulary describing yrab expression and subcellular localization at the tissue and cell type level .
the terms  expressed/not expressed  describe the presence of a yrab in a tissue or cell type . to describe differential yrab expression in particular cell types within a tissue , we use the annotation term  elevated levels . 
we use three independent terms to describe subcellular localization in general : cortical , intracellular punctate , and intracellular diffuse .
a diffuse appearance could reflect either non - membrane - associated yrabs or small vesicles below the resolution of the light microscope .
a particular yrab might exist in different pools within the cell and therefore be described by one or more of these terms .
for polarized cell types , we add polarity annotation terms that separately describe the polarized distribution of each of the three possible pools ( cortical , punctate , diffuse ) .
for example , a particular yrab might have a diffuse intracellular pool that is not polarized , but a cortical pool that is polarized .
thus , each yrab in each tissue or cell type is characterized by multiple features .
we annotated at total of 23 cell types in six tissues : three cell types in the cns , ten in ovaries of different stages , four in testes , three in sg , one in the fb , and two cell types in both early and late third instar wd .
to perform hierarchical clustering of yrabs according to specific chosen features , we constructed a binary matrix that displays all features of every yrab / tissue combination .
all the yrab imaging data and annotations , including the binary matrix can be accessed through the functional yfp - tagged rab ( flytrab ) database .
( 2007 ) and provides a  link to collaborative annotation toolkit for massive amounts of image data ( catmaid ) ( saalfeld et  al . , 2009 ) to browse the original 3d data set for each yrab and tissue type .
the open source software catmaid ( http://www.catmaid.org ) , originally used for em data , allows online browsing of terabyte - scale image data .
we extended the capabilities of the catmaid software to allow visualization of multichannel immunofluorescence data .
we also added tools that facilitate co - localization analysis , as well as cropping and downloading image data .
only the five core yrabs are expressed in all three tissues examined by western blotting . to ask whether this principle held over a broader range of cell types , we surveyed the flytrab database . to help visualize these findings , we clustered yrabs according to their expression in different cell types ( figures 2c and s2c ) .
the y axis positions of the branch lines in the dendrogram indicate the similarity of cell - type expression between the two groups connected by the line .
yrabs connected by a branch line with a similarity level of 1 are expressed in identical cell types .
the set of yrabs that are expressed in all annotated cell types is highlighted in red .
yrabs that separate at lower levels of similarity ( orange and yellow branch lines ) are increasingly dissimilar to the ubiquitous group ( i.e. , they are expressed in fewer cell types ) .
yrabx5 and yrab32 form an out - group since they are not expressed in any annotated cell type ( figure  2c , gray ) .
yrab 5 is not detectable in migrating border cells , but is present in all other cell types .
in contrast , leca yrabs that are often lost during evolution ( figure  2b , group iv ) tend to be expressed in fewer cell types , as do yrabs that arose in metazoans .
the metazoan yrab26 , yrab27 , yrab3 , and yrabx4 form an extremely tissue - specific group ( figure  2c , yellow ) .
together , these yrabs are expressed in only four annotated cell types and share the common feature of being expressed in neurons .
yrab26 and yrab27 are expressed identically only in neurons , whereas yrab3 and yrabx4 are expressed in one and two additional cell types , respectively .
these data generally support the idea that core rabs organize the basic membrane trafficking machinery common to all cells , whereas other rabs are more likely to have tissue - specific functions .
to investigate the relationship between rab protein localization and cell polarity , we examined the subcellular distribution of each drosophila yrab in the neurons of the early second instar cns and in three epithelial cell types with different functions ( early third instar sg cells , ovarian follicle cells of different stages , along with early and late third instar wd cells ) .
different cell types within the cns can be easily identified by a combination of brain morphology and immunostaining .
the cns is surrounded by a layer of specialized glial cells that constitute a blood brain barrier ( bbb ) .
neuronal cell bodies are located peripherally and extend axons and dendrites into more central regions , forming a neuropil .
cell bodies of primary neurons , which have already formed synaptic connections , are located closest to the neuropil .
secondary neurons are beginning to be born , and their cell bodies lie in more outer regions .
nuclei of both primary and secondary neurons stain with the post - mitotic marker elav .
pluripotent neuroblasts and ganglion mother cells are mostly located in very peripheral regions , are large , and do not stain for elav .
thus , we can easily compare the relative proportions of different yrabs in neuroblasts , cell bodies of primary and secondary neurons , and neuronal projections ( figure  s3a ) .
nine of the 22 yrabs detectable in neurons of the larval cns are roughly equally distributed between cell bodies and the neuropil ( figures 3i3q and s3j , black ) . six yrabs ( 1 , 7 , x1 , 4 , 6 , 39 ) are enriched in neuronal cell bodies ( figures 3c3h and s3j , blue ) .
seven yrabs ( 3 , 19 , 23 , 26 , 27 , 30 , x4 ) are clearly enriched in the neuropil ( figures 3r3x and s3j , magenta ) .
compared with yrabs that are uniformly distributed or enriched in cell bodies , yrabs that are enriched in the neuropil comprise predominantly metazoan yrabs .
this group includes the highly cns - specific yrab3 , yrab26 , yrab27 , and yrabx4 .
yrab3 and yrab27 have well characterized roles in synaptic vesicle release ( mahoney et  al . , 2006 ; sdhof , 2013 ) , and
it would be interesting to examine whether other neuropil - enriched rabs may have related functions .
mrna localization to growth cones and to synaptic termini is thought to contribute to axon guidance and synaptic plasticity .
these mrnas appear to be locally translated , indicating that dendrites and axons have a functional er / golgi protein synthesis machinery ( jung et  al .
do these outposts rely on the same rab machinery as er / golgi assemblies in the cell body ? examining yrab protein distribution shows that canonical golgi and er rabs ( rab1 , rab2 , rab6 ) are present in both cell bodies and the neuropil ( figures 3 and s3j ) .
thus , canonical rab machinery likely regulates transport of secreted and membrane proteins in neuronal projections .
does membrane trafficking change as cells differentiate ? to examine this , we looked for yrabs with different expression levels in neuroblasts , neurons , and bbb glia .
the expression of most yrabs ( e.g. , yrab6 ) does not differ between these cell types ( figures s3c and s3c ) .
however , neuroblasts express higher levels of some yrabs ( 4 , 18 , 21 , 40 ) compared to neurons and bbb glia ( figures s3d  s3f and flytrab ) .
these include not only tissue - specific yrabs ( x4 , 3 , 26 , 27 ) but also yrab5 ( figures s3g  s3i and flytrab ) .
in contrast , yrab9 is undetectable in neurons and neuroblasts , but is expressed in bbb glia ( figures 3b and 3b ) .
thus , the differentiation of glia and neurons may entail changes in membrane trafficking resulting from altered rab protein expression . to analyze rab protein localization in an epithelium specialized for apical secretion
the post - mitotic sg cells produce and release digestive enzymes and , later , glue proteins into the apical lumen .
each sg cell contains multiple large cup - shaped er / golgi compartments , which stain for the golgi marker lava lamp .
these occupy the medial and basal regions of each cell , but are excluded from the most apical regions ( szul et  al . , 2011 ) .
to help visualize the different yrab localization patterns in the sg we clustered them according to annotation features describing the polarized distribution of cortical , punctate , and diffuse pools ( figure  4 g ) .
as expected , yrabs with known functions in er - golgi trafficking ( yrab1 , yrab2 , yrab6 ) localize to large compartments in the basal / medial cytoplasm ( figures 4d4e , s4n , s4o , and s4 t ) .
several yrabs ( 19 , 30 , 11 ) are enriched in a diffuse apical pool ( figures 4b4c and 4 g , magenta , and s4i  s4k ) .
yrab11 does not cluster identically with yrab19 and yrab30 because it is also present in larger medial and basal structures like yrab39 and yrab7 ( figures 4 g , green , and s4k  s4m ) .
the only yrab that is basally polarized in the sg is yrab4 ( figures 4f , 4f , 4 g , blue , s4s , and s4s ) .
the apical group of rabs ( yrab11 , yrab19 , yrab30 ) are interesting candidates for mediating apical secretion in sg cells .
interestingly , yrab19 and yrab30 are also enriched in neuronal projections , which may indicate a common function in polarized secretion in these cells .
rab30 localizes to the golgi in s2 cells  in contrast to its apical localization in sg cells .
its interaction partners in s2 cells include not only golgins , but also components of the exocyst  a complex that targets golgi - derived vesicles to the plasma membrane ( gillingham et  al . , 2014 ) .
if rab30 mediates delivery of vesicle the golgi to the plasma membrane , then its different localization in the sg and s2 cells may simply reflect steady - state differences in its distribution . to what extent
is rab compartment architecture a conserved feature of polarized epithelia ? to address this , we examined other epithelia , beginning with the follicle epithelium ( fe ) of the ovary . when egg chambers emerge from the germarium , the fe is cuboidal and its apical surface contacts the germline cells .
fe cells proliferate between stages 2 and 6 , and their growth and patterning is controlled by apical signals from the enlarging germ cells ( gonzlez - reyes and st johnston , 1998 ; lpez - schier and st johnston , 2001 ) . at stage 6 , fe cells stop dividing and begin to grow by becoming polyploid . at stage 8 , fe cells contacting the oocyte start to elongate , becoming columnar over increasingly larger regions as the oocyte enlarges . at the same time , fe cells overlying the nurse cells flatten and become squamous . to visualize intracellular yrab protein distribution features in early ( stages 28 ) and late ( stage 9 ) fe , we clustered them according to annotations describing the polarized distribution of cortical , punctate , and diffuse pools ( figures 5i and 5j ) .
yrab proteins in the fe display two different architectures depending on the developmental stage : during early stages , the compartments organized by yrab4 , yrab5 , yrab7 , yrab11 , yrab19 , yrab30 , yrab39 and yrabx1 polarize apically toward the overlying germline ( figures 5b5c and 5i , magenta )
. however , as the fe becomes columnar by stage 9 , a new rab architecture develops .
apical yrab polarization is lost ( figures 5f and 5f ) , and yrab1 , yrab2 , yrab6 , yrab7 , yrab8 , yrab10 , and yrab18 now polarize toward the basal side of the cell ( figures 5g5h and 5j , blue ) .
basal yrabs fall into two distinct groups in the dendrogram ( figure  5j , blue ) because their compartment morphologies differ . comparing dendrograms
depicting subcellular localization of yrabs in the sg , early fe , and late fe reveals no consistent pattern ( figure  s5 ) .
although the early fe and sg share the feature that yrab11 , yrab19 , and yrab30 localize apically , this feature disappears in the late fe .
furthermore , yrab4 is basal in the sg and apical in the early fe . to analyze rab compartment organization in a fourth epithelium
, we turned to the larval wd , which later gives rise to the adult wing and thorax .
the wd is a folded epithelial sac with an apical lumen and a basal side that is bathed in hemolymph .
as the disc grows , it becomes pseudostratified on one side and squamous on the other  a feature common to many other developing epithelia such as the neural tube and retina .
we  examined and annotated yrab localization in third instar wd before and after  pseudostratification ( figure  6 and flytrab ) . before pseudostratification
yrab5 and yrabx1 are apically enriched , and yrab8 is basally enriched ( flytrab and figure  6f ) .
yrab1 , yrab2 , yrab4 , yrab5 , yrab7 , yrab8 , and yrabx1 become apically localized ( figures 6c , 6c , and 6 g , magenta ) , while yrab11 ( figures 6d , 6d , and 6 g , magenta / blue ) localizes to the extreme apical and basal ends of the cells .
more than half of the yrabs in the wd are reproducibly represented in a characteristic structure found in the apical cytoplasm at the level of the junctions ( figures 6d6f and 6 g , dashed line ) .
it contains not only apically enriched rabs like yrab1 ( figures 6c , 6c , and 6 g , magenta dashed line ) , but also rabs with an otherwise uniform distribution such as yrab6 ( figures 6e , 6e , and 6 g , black dashed line ) .
strikingly , many rabs in both the secretory ( rab1 , rab2 , rab6 , rab8 ) and endocytic pathways ( rab4 , rab5 , rab7 , rab11 ) are represented in this apical hub even when they also localize to other positions in the cell .
their localization is consistent with independent markers of er and golgi compartments ( figures 6h6i ) .
the dramatic rab redeployment during pseudostratification is clearly seen by comparing the dendrograms shown in figures 6f and 6 g . taken together ,
systematic analysis of rab localization in these epithelia reveals little similarity and provides no support for the idea that apical - basal epithelial polarization relies on a common rab compartment architecture . to illustrate this dissimilarity , we clustered yrabs according to features describing polarity and compartment morphology , generating a dendrogram for each tissue ( figure  s5 ) .
these clustering patterns highlight the differences between epithelial tissues and even between different developmental stages of the same tissue .
thus , epithelial cells deploy different subsets of rab compartments in a polarized fashion depending on their specialized tasks .
to examine whether rab tissue distribution and subcellular localization was related to rab protein phylogeny , we performed clustering analysis based on annotation terms that describe compartment morphology and polarity in different cell types ( figure  s6 ) .
this revealed several groups of rabs with strong similarities in localization and cell type expression .
comparing this analysis with sequence - based phylogenetic trees shows that two of these groups ( rab19 and rab30 and rab3 , rab26 , rab27 , rabx4 ) consist of rabs that are evolutionarily related , but that diverged in the earliest metazoans .
the fact that compartment morphology / polarity and tissue distribution of rabs within these groups have evolved together for the last 500 million years suggests that their functions are interdependent .
other rab pairs with similar subcellular and cell type distribution are not related phylogenetically ( e.g. , rab23 and rab35 ) .
their shared features , however , suggest that it would be interesting to investigate whether they have similar functions . yrab23 and yrab35 represent a phylogenetically unrelated rab pair with similar subcellular localization patterns .
these rabs are both found at the cell cortex in all cell types where they are present ( figure  s6 , yellow box ) .
we therefore wondered whether they might fulfill redundant functions in tissues where they are co - expressed .
while yrab35 is generally expressed , yrab23 is mainly expressed in epithelial tissues such as the developing wing ( flytrab ) .
rab23 mutants disturb tissue polarity in the adult wing ( pataki et  al . , 2010 ) . normally , each wing cell produces a single , distally oriented hair .
when rab23 is lost , cells produce multiple hairs and hair orientation shifts away from the proximal - distal axis toward the wing margin ( figure  7b ) .
however , dominant - negative rab35 expression and targeted rab35 knockdown disturb actin organization in thoracic bristles ( zhang et  al . , 2009 ) .
anti - gfp rnai and anti - gfp nanobody expression have been used to knock down endogenously gfp- and yfp - tagged proteins ( brankatschk et  al . , 2014 ; caussinus et  al . , 2012 ; neumller et  al . ,
2012 ) . to ask whether these methods were generally effective at reducing levels of yfp - rabs
indeed , both methods strongly reduce levels of yrab23 ( figures s7a  s7c ) and produce phenotypes similar to rab23 mutants ( figures 7e , s7j , and s7k ) .
although gfp rnai efficiently reduces yrab35 levels ( figure  s7e ) , its depletion causes only occasional subtle disturbances in hair morphology ( figure  7d and data not shown ) . to look for interactions between rab23 and rab35
, we knocked down both rabs in the developing wing blade of yrab35;yrab23 homozygous flies using nubbingal4 .
g ) produces defects in hair polarity and morphology that would be expected from the sum of the individual knockdowns .
the additional removal of yrab35 does not enhance hair polarity defects caused by loss of yrab23 . however , knockdown of both rabs produces a phenotype not seen in either knockdown alone : the formation of ectopic cross veins ( figures 7f and s7h ) .
thus , rab23 and rab35 function redundantly to organize the pattern of veins in the wing .
membrane trafficking is key to the regulation of many signaling systems that determine the wing vein pattern ( restrepo et  al . , 2014 ; wang and struhl , 2004 )
. it will be interesting to further investigate how rab35 and rab23 influence signal transduction .
here , we describe a genetic resource for visualization of endogenous rab proteins . we provide a catmaid - based annotated 3d image database ( flytrab ) that allows the user to browse rab protein distribution with subcellular resolution in 6 different tissues comprising over 23 cell types .
dedicated search algorithms connect image data with the defined vocabulary in annotation trees and allow clustering of rabs according to features of interest .
all original recorded data are accessible and downloadable online  a feature that will allow users to investigate specific aspects of interest more deeply .
furthermore , hypotheses generated by searching this database can be easily tested with knockdown strategies that target the yfp tags .
these will provide powerful tools to explore the different functions and specializations of membrane trafficking pathways in  vivo .
we exploited the comprehensive nature of this resource to address questions in cell differentiation , cell polarity , and rab protein evolution .
our analysis shows that cell differentiation is accompanied by quantitative and qualitative changes in rab protein expression and localization , suggesting that rewiring of membrane trafficking pathways is a key feature of differentiation .
for example , quantitative western blotting of fbs , sgs , and wds revealed not only tissue - specific rab protein expression , but also changes in the relative levels of rabs that control basic cellular processes such as secretion , endocytosis , and recycling .
furthermore , neuronal differentiation is accompanied not only by expression of neuron - specific rabs involved in synaptic vesicle release ( rab3 , rab26 , rab27 ) , but also quantitative changes in levels of other rab proteins .
2014 ) and it would be interesting to know how changing their relative proportions influences the endocytic pathway in neurons .
rab4 , rab18 , and rab40 are more highly expressed in neuroblasts than in neurons .
interestingly , rab18 mutations have been associated with warburg micro syndrome , which is characterized by postnatal microcephaly ( bem et  al . , 2011 ; cheng et  al . , 2014
. it would be interesting to know whether neural stem cells function normally in patients with this syndrome .
bbb glial cells also derive from neuroblasts ( desalvo et  al . , 2011 ) .
rab9 expression increases with differentiation of bbb glia , suggesting that it may have special functions there . finally , not only do rab levels change as tissues develop  their subcellular localization can change as well .
rab proteins undergo a striking redeployment during development of both the ovarian follicular epithelium and the wd epithelium .
interestingly , although rab proteins throughout the wing pouch re - localize as cells become pseudostratified , there appear to be few obvious differences in their expression levels or distribution within the wing pouch  despite the fact that this region is already being patterned by the activity of several different morphogens ( crozatier et  al .
the single exception is rabx6 , which is expressed at elevated levels in the anterior compartment and in sensory organ precursor cells ( flytrab ) .
thus , morphogen spreading and signaling probably operates in a field of cells with relatively homogeneous membrane trafficking capabilities .
although we initially wondered whether epithelial polarity might depend on a standard polarized membrane trafficking architecture ( farr et  al .
, 2009 ; weisz and rodriguez - boulan , 2009 ) , our observations do not support this idea . while many different rabs can assume a polarized localization in specific epithelia at specific developmental stages
thus , different polarized distributions of rab compartments may simply reflect differences in epithelial function .
for example , one set of rabs ( 4 , 5 , 7 , 11 , 19 , 30 , 39 , x1 ) is apically localized in early follicle cells in the ovary , but not at later stages .
apical localization of these rabs correlates with a time of intense communication between the germline and the overlying follicle cells that is mediated by notch / delta signaling  a pathway whose regulation critically depends on endocytosis and recycling ( deng et  al . , 2001 ;
this may account for the enrichment of endocytic / recycling rabs ( 4 , 5 , 7 , 11 ) in the apical regions of these cells .
the apical polarization of rabx1 , rab39 , rab19 , and rab30 suggests that they could also be involved in communication with germline cells . in later egg chambers follicle cells overlying the oocyte undergo columnarization , during which the area of the basolateral membrane increases . by this time , the same set of rabs is no longer apically localized . instead , a large number of rabs in the secretory pathway ( 1 , 2 , 6 , 8 , 10 ) as well as three others ( 7 ,  11 , 35 ) become basally localized
. it would be interesting to investigate whether increased basal delivery of secretory vesicles expands the basolateral membrane in these cells .
completely different changes in rab protein architecture and polarity occur during pseudostratification of the wd . during pseudostratification , many rabs ( 1 , 2 , 4 , 5 , 7 , x1 ) become apically localized  including rab8 , which reverses its polarity .
pseudostratification also correlates with the development of a novel cluster of rab compartments located at the level of apical junctions , which we call the apical hub .
this structure coincides with an apical web of microtubules ( eaton et  al . , 1996 ) that arises during pseudostratification in response to dpp signaling ( gibson and perrimon , 2005 ; shen and dahmann , 2005 ) .
although this area of the cell is only a few microns in diameter , over half of the rabs expressed in the wd are reproducibly represented here .
thus , the apical cytoplasm of wing epithelial cells is a major membrane trafficking hub .
it will be interesting to discover whether the apical hub is a general feature of pseudostratified epithelia and whether it might facilitate rapid intercellular communication during patterning of these developing tissues . finally , these studies show that the ability to polarize is not limited to rabs that arose first in metazoans ; almost all leca and metazoan rabs are distributed in a polarized way at least in some cell types .
the only widely expressed rabs for which we never detected a polarized distribution were rab21 ( present in the leca but often lost ) and rab9 and rab40 ( that evolved in metazoans ) .
this suggests that the metazoan expansion of the rab family provided a wider variety of membrane trafficking routes that could be targeted in different ways , rather than specific pathways required for cell polarity .
they generally rapidly diverge in their expression patterns , acquire mutations , and may eventually adopt novel beneficial functions ( neo - functionalization ) .
duplicates may also divide functions once present in a single ancestral protein ( taylor and raes , 2004 ; zhang , 2003 ) .
our analysis has revealed two instances in which a group of phylogenetically related rab proteins show extremely similar patterns of cell - type - specific expression and even subcellular localization .
one group comprises rab19 and rab30 and the other rab3 , rab26 , rab27 , and rabx4 .
the members of both groups diverged from each other over 500 million years ago ( elias et  al . , 2012 ) .
this suggests that there has been selection to preserve their coordinate expression and subcellular localization , and their functions somehow depend on each other .
when a rab is duplicated , both rabs are recruited to the same compartment by the same signals .
of course the rab specifying the derivative branch might not function without the rab that specified the original branch , explaining the requirement for coordinate expression . in the future ,
the tools for rab visualization and knockdown described here will facilitate these and other analyses .
yfp - tagged rab alleles were generated by ends - in  homologous recombination and the initial genomic duplication was resolved using the i - cre system as described by rong and golic ( 2000 ) and maggert et  al .
embryo collections and staging protocols were performed at 25c ; flies were raised on conventional cornmeal agar under a 12  hr light/12  hr dark cycle at 25c . nubbin - gal4 ( # 42699 ) and uas - gfprnai ( # 41559 and # 9331 ) are available from bloomington stock center , rab from dgrc ( # 125902 ) .
uas - gfpnanobody flies were provided by the affolter lab and en(105)-gal4 flies are from c. dahmann .
larval brains ( 4448  hr after egg collecting [ aec ] ) , salivary glands ( 7280  hr aec ) , early ( 6872  hr aec ) , and late ( 110120  hr aec ) wing discs were dissected in ice - cold pbs ( cns , salivary glands ) or graces medium ( wing disc ) , fixed with 4% pfa at room temperature and probed with one or more of the following : anti - gfp ( invitrogen ) , dapi ( roche ) , anti - hrp - cy5 ( dianova ) , anti - elav ( developmental studies hybridoma bank [ dshb ] ) , anti - dlg ( dshb ) or anti - crbs2.8 ( gift from e. knust ) . the larval fat bodies ( 7280  hr aec )
were fixed with 4% pfa for 30  min at room temperature and stained with anti - gfp ( invitrogen ) , anti - dlg ( dshb ) , and dapi ( roche ) .
anti - lva ( gift from e. knust ) and anti - kdel ( enzo life science ) were used to mark organelles .
aged ( 57  days ) flies were fed for 1  day at room temperature ( rt ) with yeast paste ; ovaries and testis were dissected and fixed with 4% pfa in pbs and stained for 3  days at room temperature with anti - gfp ( invitrogen ) , phalloidin-555 ( roche ) , anti - dlg ( dshb ) , and dapi ( roche ) .
all samples were mounted in vectashield mounting medium ( vector labs ) and photographs were acquired with an olympus1000 confocal microscope and evaluated using fiji imaging software .
catmaid image data sets were analyzed manually and annotated using a defined terminology to describe subcellular localization of individual yrabs ( see supplemental experimental procedures ) .
multiple annotation features that are represented in a binary matrix characterize the tissue distribution and subcellular localization of each yrab .
we used this binary matrix to perform hierarchical clustering of yrabs according to specific annotation features using past 3.01 ( http://folk.uio.no/ohammer/past/ ) .
. rooted trees ( dendrograms ) were generated according to similarity coefficients based on unweighted pair group method with arithmetic mean ( upgma ) linkage ( see supplemental experimental procedures ) .
wing discs , fat bodies , and salivary glands from feeding 3 instar larvae ( 110120  hr aec ) were dissected in ice - cold graces medium , homogenized with a plastic pestle in 1% tx-100 pbs lyses buffer , and pelleted at 20,000   g for 5  min at 4c .
protein content from recovered supernatants was measured using bca ( manufacturer protocol , invitrogen ) and 8  g protein lysatelane were loaded on 12.5% sds - pages .
additional blots ( figure  s1 ) were probed for rab4 ( gift from m. zerial ) and -tubulin ( gift from e. knust ) .
comparing yrab signals to recombinant yfp standards shows that there is more than 10  ng of the most abundant yrabs per lane , corresponding to 0.2 pmol .
each lane contains four wing discs , and a wing disc has a volume of 0.005  l .
 summarymembrane trafficking is key to the cell biological mechanisms underlying development . 
 rab gtpases control specific membrane compartments , from core  secretory and endocytic machinery to less - well - understood compartments . 
 we tagged all 27 drosophila rabs with yfpmyc at their endogenous chromosomal loci , determined their expression and subcellular localization in six tissues comprising 23 cell types , and provide this data in an annotated , searchable image database . 
 we demonstrate the utility of these lines for controlled knockdown and show that similar subcellular localization can predict redundant functions . 
 we exploit this comprehensive resource to ask whether a common rab compartment architecture underlies epithelial polarity . 
 strikingly , no single arrangement of rabs characterizes the five epithelia we examine . 
 rather , epithelia flexibly polarize rab distribution , producing membrane trafficking architectures that are tissue- and stage - specific . 
 thus , the core machinery responsible for epithelial polarization is unlikely to rely on polarized positioning of specific rab compartments . 
Introduction
Results
Discussion
Experimental Procedures
Author Contributions