after their biosynthesis at the endoplasmic reticulum , secreted proteins are transported to the golgi complex , where they are post - translationally modified and sorted for secretion , plasma membrane delivery , or delivery to prelysosomes .
the golgi contains multiple subcompartments , termed cis ( early ) , medial ( middle ) , and trans ( late ) cisternae ; each of these subcompartments houses different sets of glycosyltransferases and other enzymes .
proteins enter the golgi at the cis compartment and exit at the trans compartment , but how they move from one cisterna to the next is still being determined .
two possible models are widely discussed [ 1 - 3 ] . according to the cisternal maturation ( or progression ) model ,
cargo remains in a given compartment and different enzymes arrive there to convert a cis cisterna into a medial one or a medial cisterna into a trans cisterna .
alternatively , cargo moves from one golgi compartment to the next , encountering different enzymes in each subsequent compartment until it reaches the trans cisterna , where it is then sorted into carriers bound for post - golgi destinations .
this second class of model could use vesicles to transport cargo from one compartment to the next or compartment - connecting tubules through which cargo could pass .
conversion of one golgi compartment into another by high - resolution , live cell video microscopy .
a limitation of those studies is that one of the compartment markers that was monitored is a peripheral membrane protein that is likely to reversibly bind to and release from the golgi surface .
the situation may be more complex in mammalian cells , where golgi cisternae are stacked tightly together , unlike yeast ; it is hard to imagine a single cisterna moving from one side of the well - stacked structure to the other .
nevertheless , large procollagen cargo traverses the golgi without ever leaving a cisterna , in support of a maturation model . to complicate matters ,
membrane tubules have been detected between golgi cisternae under conditions of active secretion ; this scenario would permit cargo movement from one side of the stack to the next without maturation or vesicle transfer .
important new clues to how golgi compartments might  mature come from a study of golgi - localized , ras - related , rab family gtpases in yeast .
rab gtpases are localized to different membrane compartments and catalyze the formation of functionally distinct , membrane microdomains that are important for transport vesicle formation , vesicle motility , and vesicle ( or compartment ) docking and fusion .
rab gtpases help early endosomes mature into later endosomes by a process called rab conversion .
the early endosomal rab5 protein recruits a specific guanine nucleotide exchange factor ( gef ) that activates rab7 .
rab7 then recruits rab7-specific effectors to that compartment , thereby converting an early endosome into a late endosome .
this type of rab cascade ( figure 1b ) was first described for a yeast golgi rab , ypt32p , recruiting the gef for the subsequent acting sec4p rab .
( a ) endoplasmic reticulum , golgi , and endosome membranes are capable of homotypic fusion and fission .
( b ) rab cascades occur when sequentially acting rabs recruit guanine nucleotide exchange factors ( gefs ) and gtpase - activating proteins ( gaps ) to membranes .
raba recruits a gef that will convert the subsequent acting rab to its active form .
rabb can then recruit a gap that will inactive the previous acting rab , thereby removing it from the newly formed , second compartment .
rivera - molina and novick   have now used live cell video microscopy to detect rab conversion at the yeast golgi : they see compartments containing the early golgi rab , ypt1p , convert into a compartment containing the late golgi rab , ypt32p .
( although the light microscopy method employed could not resolve structures smaller than about 200 nm , the images were nevertheless highly compelling . )
the process involves the recruitment of ypt32p by the gtpase - activating protein ( gap ) that inactivates ypt1p : gyp1p . upon inactivation
, ypt1p becomes a substrate for removal from membranes by another protein , gdi ( gdp - dissociation inhibitor ) .
the removal of rabs from the membranes makes this work subject to one of the same limitations of the previous studies ; nevertheless , these markers permitted the authors to detect an important molecular transformation .
the data provide a direct molecular mechanism for compartment inter - conversion at the golgi , reminiscent of maturation in the endocytic pathway .
golgi compartments were seen to be dynamic , undergoing a certain amount of fission and fusion . in some cases ( 30% )
, a ypt32p compartment appeared to fuse to a ypt1p compartment to yield a mixed compartment or a mixed compartment appeared to undergo segregation and fission to yield separate ypt1p and ypt32p compartments  .
what this means is that , yes , yeast golgi compartments undergo apparent maturation by rab conversion , and at the same time , cargo may get the 
fast track from one compartment to the next by intermittent cisternal fusion and fission events ( figure 2a , b ) . importantly , although a compartment will seem to mature , it is actually forming from a stable predecessor .
early golgi has the capacity to fuse with other early golgi cisternae and so on .
fission is favored in the absence of microtubules or when golgin proteins are depleted from cells .
glycosyltransferases are localized to cis ( red ) , medial ( yellow ) , and trans ( orange ) golgi compartments but rarely have a perfectly sharp distribution .
thus , for example , cis - golgi homotypic fusion events can occur with another cis compartment ( step 1 ) or perhaps a medial ( yellow ) compartment ( or both ) .
similarly , a medial cisterna may be able to fuse with a trans cisterna ( orange ) . in this manner
, a large cargo may be able to encounter all golgi - processing enzymes without entering a transport vesicle , indicated by fusion steps ( arrows ) 1 , 2 , and then 3 .
the ability of golgi cisternae to undergo fission and fusion has been known since the 1970s : simple , nocodazole - triggered depolymerization of microtubules causes the mammalian golgi to fragment into mini - stacks that disperse throughout the cytoplasm , and drug washout leads to rapid stack reassembly ( figure 2a ) .
this indicates that the golgi is capable of fission as soon as microtubules are lost and of fusion with itself as soon as microtubules repolymerize .
compartment collisions likely enhance fusion , and cytoskeletal motor proteins that decorate the golgi and connect it to both microtubules and actin cables are sure to contribute to both fusion and fission events , as is true in the endocytic pathway ( figure 1a , c ) . are intercisternal fusion / fission connections required for membrane traffic ?
transport is only partially blocked in nocodazole - treated cells and this condition favors stack fission .
but homotypic fission and fusion are likely much more prevalent than previously anticipated because cellular depletion of any one of many different golgi proteins (  golgins ) generates mini - stacks that are clustered near the microtubule - organizing center .
such transient fusion and fission could yield the tubules that have been detected in electron micrographs of mammalian cell golgi complexes .
fission and fusion would make it possible to accommodate extra - large cargoes , such as collagen , that are too big to fit into conventional transport vesicles .
as is well established for the endocytic pathway , rab gtpases would define specific subdomains and retain specific golgi enzyme subsets there .
compartments would be defined by their distinct rab gtpases , and adjacent cisternae might fuse at some frequency that allows cargoes to encounter sequentially acting , processing enzymes . in a mixed compartment , rab gefs and gaps would segregate individual rabs into separate regions that would be segregated upon the simple action of a membrane - associated , cytoskeletal motor protein to drive fission .
order within the stack would be maintained by the relationship between specific rabs and their cognate activators ( gefs ) and inactivators ( gaps ) , which are designed to permit rabs to function in a sequential cascade .
indeed , the proteins that stack the cisternae may use a rab cascade to achieve their position in the stack . at the trans golgi
, rabs would also initiate the process by which specific cargoes are collected into distinct transport carriers and delivered to their final destinations .
validation and clarification of this model will require defining which rabs build which specific golgi enzyme microdomains and determining the specific molecular interactions that permit fission , fusion , and enzyme organization .
vesicles are likely to be involved in golgi transport : we know that cop - i - coated vesicles collect kdel receptors for delivery back to the endoplasmic reticulum ; in this case , we can postulate that such vesicles bud from a rab gtpase - organized , functional membrane microdomain .
the same proteins that drive vesicle targeting and fusion may also participate in cisternal docking and fusion to permit protein transport across this central cellular compartment .
the mechanism by which depletion of any one of at least 10 different golgin proteins leads to mini - stack formation will likely tell us much about how proteins move through the mammalian golgi stack .
 the golgi complex is a central processing station for proteins traversing the secretory pathway , yet we are still learning how this compartment is constructed and how cargo moves through it . 
 recent experiments suggest a key role for ras - like rab gtpases and provide important new ideas for how the golgi may function . 