gtpase of the immunity - associated protein family ( gimaps ) , also termed immune - associated nucleotide - binding proteins ( ians ) , are a relatively recently described , uncharacterized protein family of putative small gtpases conserved among vertebrates and higher plants [ 14 ] .
the original publications describing the different human , mouse , and rat gimap family members used a variety of different names for the genes generating confusion .
thus , the gimap nomenclature followed also in this minireview was introduced by the hugo gene nomenclature committee and includes the human , mouse , and rat orthologs   ( table 1 ) .  
small gtp ( guanosine triphosphate ) binding proteins , also known as small gtpases , ras - like gtpases , or ras superfamily of gtp binding proteins , regulate key cellular functions in virtually all living organisms .
they are involved in signal transduction events and regulation of gene expression in almost all cell types , including the cells of the immune system [ 2628 ] .
the ras superfamily can be subclassified into ras , rho , rab , and arf families , and the closely related g family of the heterotrimeric g proteins , which sometimes are excluded from the ras superfamily .
the ras proteins induce signaling pathways that include a variety of second messengers , such as calcium and camp .
the ras superfamily proteins play key roles in a variety of cellular functions in the immune system , such as cell migration , t - cell anergy [ 31 , 32 ] , antigen presentation ,   and radical formation .  
the gimap family members have unique primary structures and , thus , they define a new family of g proteins distinct from the ras superfamily and the heterotrimeric g proteins .
the expression of gimaps in vertebrates has been shown to be highest in the cells of the immune system , although a more ubiquitous expression has also been suggested .
several studies have associated gimaps with immunological functions , such as thymocyte development and apoptosis regulation in lymphocytes .
all vertebrate species examined so far have gimap genes in tight clusters in their genome [ 3 , 4 , 12 ] .
the seven functional human gimap genes and one pseudogene are clustered on chromosome 7q36.1   and there are eight functional mouse gimaps clustered on chromosome 6 and seven functional genes in rat chromosome 4 [ 13 , 25 ] .
the ongoing sequencing project of the genome of danio rerio ( zebrafish ) has revealed the existence of gimap orthologs also in a lower vertebrate .
the genomic organization of human , mouse and rat gimap genes is depicted in figure 1 .    
homolog searches in available corn , soybean , and tobacco genomes by liu et al .   came up with one to two homologs of gimap / ian genes in each genome .
however , searches within the well - characterized genomes of the unicellular organisms saccharomyces cerevisiae ( baker 's yeast ) and schizosaccharomyces pombe ( fission yeast ) , or invertebrates , such as caenorhabditis elegans ( free - living roundworm ) and drosophila melanogaster ( fruit fly ) did not reveal any homologs of the gimap gene family .
thus , gimap genes exist only in vertebrates and angiosperm ( i.e. , flowering ) plants and the yet poorly characterized cellular functions of the gimap proteins are specific for vertebrates and higher plants .
gimap / ian proteins emerged before plants and animals split into their own evolutionary paths .
phylogenetic analyses of both protein and genomic sequences [ 3 , 4 ] showed that human and mouse gimaps 1 , 4 , 5 , 6 , 7 , and 8 form highly orthologous pairs , and , thus , suggest that a gene duplication event in a common ancestor of rodents and primates gave rise to these genes . the phylogenetic analyses by liu et al .  
place the arabidopsis and rice ians to a clade distinct from the mouse and human gimap proteins , thus indicating that the gene duplication events have taken place after the divergence of vertebrates and plants .
gimap8 makes an exception by having three gtpase domains , which is extremely unusual not only for gimaps , but for small gtpases in general , too .
thus , its molecular size is 74.9  kda , making it by far the largest gimap protein .   the gtpase domain with the five motifs g1-g5 characteristic for all small gtpases is included in the aig1 domain , named after the prototype gene aig1 found in arabidopsis thaliana ( avrrpt2-induced gene ) .
the aig1 domain is found in all gimap and ian proteins and besides the gtpase motifs , it contains a conserved box , which is characteristic for all aig1 domain gtpases . all human gimaps also contain putative coiled coil domains which suggest protein - protein interactions
. some gimaps , namely , gimap1 , 2 , 4 , and 5 , contain putative transmembrane domains in their cooh - terminal ends and gimap7 , gimap6 , gimap1 , and gimap2 have basic amino acids in their nh2- or cooh - terminus with weak similarity to endoplasmic reticulum- ( er)-localization signals . however , localization studies found gimap4 mainly in cytosol [ 12 , 14 ] but also in er and golgi   and in membrane fraction of fractionated cd4 t - lymphocytes .
gimap1 has been reported to localize in er   and gimap7 in er and golgi .  
human gimap4 was shown to be able to bind guanosine nucleotides gtp and gdp but not gmp or other nucleotide triphosphates atp , ctp or ttp .
it has a carboxy - terminal iq domain , which is known to bind calmodulin , a second messenger molecule involved in , among other cellular processes , tcr signaling , and activation of nfat in t - cells [ 14 , 37 ] .
the iq motif usually consists of ~23 residues and contains the consensus sequence hydrophobqxxxrxxxxrxxxr / k , where hydrophob is a hydrophobic residue .
this motif was proven to be functional in murine gimap4 and , furthermore , seems to be absent from other gimap protein family members .
also the putative phosphoylation sites of murine gimap4 were shown to be functional , since mouse gimap4 was phosphrorylated rapidly after 10 minutes of pma / ionomycin stimulation in primary splenocytes .
the phosphorylation was mediated by pkc since it was inhibited by a pkc inhibitor , rottlerin .
four of the six putative phosphorylation sites of mouse gimap4 are also found in human gimap4 .
cell surface expression of leukocyte markers cd4 and cd8 defines the stages of t - cell development in the thymus . during the development process ,
thymocytes mature from cd4cd8 double negative ( dn ) cells into cd4cd8 double positive ( dp ) cells and finally into single positive ( sp ) cd4 or cd8 cells which exit the thymus and enter the periphery ( figure 2 ) .
the dn cells are further categorized according to their cd44 and cd25 cell surface expression : ( 1 ) dn1 ( cd44cd25 ) , ( 2 ) dn2 ( cd44cd25 ) , ( 3 ) dn3 ( cd44cd25 ) , and ( 4 ) dn4 ( cd44cd25 ) . furthermore , the dn4 cells pass through a cd4cd8 immature single positive ( isp ) stage before developing into dp cells . the different maturation stages are phenotypically and functionally distinct .   the expression of gimap genes is strictly regulated during t - cell development in the thymus .
dion et al .   studied the expression of rat gimaps in different thymic subpopulations and observed a significant elevation in expression of all rat gimaps but gimap 4 genes between the dn and dp stages .
the expression patterns for gimap1 , 6 , 8 , and 9 during thymocyte development were very similar showing the highest expression in the dp stage .
mouse gimap 3 , 4 , 5 , and 7 expression was increased during the transition from cd4cd8 dp thymocytes into cd4 or cd8 sp thymocytes . while shrna mediated knockdown of mouse gimap4 hade no effect on thymocyte development in fetal thymic organ culture model shrna mediated knockdown of mouse gimap3 and gimap5 disturbed thymocyte development .
.   showed that gimap3 was needed for optimal positive selection of sp cd4 and cd8 t - cells and gimap5 was important for optimal development of dp thymocytes .
it has also been shown that the proportion of cd4cd8 sp thymocytes was somewhat reduced in gimap5   mice .
expression of mouse gimap4 protein during thymocyte development has been studied in more detail by poirier et al .   and schnell et al . .
it was not detectable in dn1 , dn2 , dn3 , or dp thymocytes whereas high levels of gimap4 were expressed in sp cd4 cells and dn4 cells [ 1 , 14 ] . the development of thymocytes in rag mice is blocked at the dn3 stage but in vivo administration of anti - cd3 antibody allows a low number of thymocytes to proceed up to the dp - stage .
this system allows for the enrichment of the low - frequency cell subsets from the different developmental stages , namely , dn3 , dn4 , and isp , as well as dp cells .
using the rag2 model , schnell et al .   demonstrated that mouse gimap4 transcript was absent in differentiation arrested rag2 dn3 thymocytes while it was present in high amounts in dn4 , in lower amounts in isp , and absent in dp cell subsets of anti - cd3 treated rag2 mice .
similar results were obtained by poirier et al .   with a rag fetal thymic organ culture ( ftoc ) model where thymocytes from rag mice blocked at the dn3 stage were cultured in vitro with anti - cd3 antibodies .
the antibody treatment resulted in the transient expression of gimap4 protein between the dn3 and dp developmental stages .
the transitions both from dn3 to dn4 stages and from dp to sp stages require tcr - mediated signals , and , thus , the results described above indicate that gimap4 expression is directly or indirectly induced by tcr - mediated signals during thymocyte development .
however , gimap4 mice showed no significant role for gimap4 in t - cell development in thymus .
thus , gimap4 seems to be dispensable for t - cell development , although its expression is tightly regulated during the development process .
gimaps have been studied in most detail in apoptotic functions and in t - helper ( th ) cell differentiation and these processes are depicted in figures 3 and 4 and discussed below .
mouse gimap1 was first identified by krcken et al .   as a gene expressed in the spleens of mice infected with plasmodium chabaudi malaria .
c57bl/10 mice are capable of self - healing the p. chabaudi malaria infection and this immunity is suppressed by testosterone . in the absence of testosterone
the immunity progresses to a long - lasting protective immunity upon rechallenge . once acquired , the immunity is testosterone independent and mediated by spleen cells .
the high expression of gimap1 in the spleen cells of p. chabaudi infected mice was strongly associated with the acquisition of the testosterone - resistant , immunity - mediating phenotype and was observed to be highest in macrophages with lower expression in b cells and t - cells .
later it was shown that the expression of gimap1 in the spleens of the immune mice remains constitutively high for at least 13 weeks postinfection . surprisingly , however , a later study detected no upregulation of mouse gimap1 mrna or protein in spleen cell lysates from mice infected with p. chabaudi   and this phenomenon could not be explained .
mouse gimap1 has also been reported to be upregulated by p53 during apoptosis in a mouse myeloid leukemia cell line ltr6 .
a conditional knockout of gimap1 in murine lymphocytes led to severe reduction of mature t and b cells but there was little effect on immature thymocytes .
both human and mouse gimap1 orthologs are preferentially expressed in the spleen with some expression in the lymph nodes .
more detailed analysis of gimap1 protein expression in mouse splenic , thymic , and bone marrow cell populations revealed gimap1 expression in t ( cd3 + ) and b ( b220 + ) cells , dn , dp , and cd4 + and cd8 + sp thymocytes , as well as in nk / nkt ( nk1.1 + ) cells .
interestingly , gimap1 was expressed at significant levels in 5 out of 11 tested lymphoid cancer cell lines , namely c1498 , tk-1 , a20 and p815 .  
its expression was induced by th1 promoting cytokine il-12 whereas its expression was downregulated by il-4 , a th2-inducing cytokine .
promoter analysis of human gimap1 gene revealed the existence of multiple silencer elements in the promoter .
stamm et al .   studied a large 6.2  kb fragment containing also the first intron ( from 3760 to + 2419 ) .
deletion of sequences from the first intron and from the 5 flanking area resulted in inducible promoter activity indicating the removal of silencers .
the smallest promoter clone of 0.8  kb ( from 760 to + 76 ) was highly active in jurkat cells thus suggesting that it contained all the elements necessary for active transcription . 

gimap3-ps is a pseudogene in humans   and gimap3 is not annotated in the rat genome .
mouse gimap3 gene expression was detected at low levels in spleen , but no expression was detected in thymus , liver , or kidney . however
bcr / abl is a chimeric oncogenic protein generated from translocation between chromosomes 9 and 22 resulting in the so - called philadelphia chromosome .
bcr / abl has constitutive tyrosine kinase activity and has been shown to activate several signal transduction pathways , such as ras , raf , myc , and stat .
gimap4 protein expression is highly regulated at the post - transcriptional level [ 14 , 15 ] . during t- and
b - cell activation by anti - cd3/anti - cd28 and cd40-ligand / il-4 stimulation , respectively , the mrna level expression of gimap4 was shown to be quite stable .
however , gimap4 protein level started to decrease after 4 days of t - cell activation and was undetectable at day 6 . similarly , yet more rapidly , b - cell activation resulted in the reduction of gimap4 protein already after two days of activation .
the rapid activation induced phosphorylation / dephosphorylation / degradation of mouse gimap4 observed by schnell et al
the expression of rat gimap4 protein was approximately 10-fold greater in wild type lymph node ( ln ) t - cells compared to ln t - cells from lyp / lyp ( gimap5 ) rat .
it is not known whether this is due to gimap4 and gimap5 interaction in some shared biochemical pathway or a result of the altered activation status of t - cells from lyp / lyp animals .
the latter seems plausible in the light of the findings by cambot et al .   and schnell et al .  
while the majority of the peripheral t - cells of wt animals are resting g0 cells , the t - cells in lyp / lyp rats are a mixture of semiactivated cells and recent thymic emigrants ( rte ) [ 43 , 44 ] .  
although gimap4 knockout seemed to be redundant for t - cell development as well as selection and activation of t - cells in vivo , the knockout had a significant effect on t - cell apoptosis .
a convenient method for measuring apoptosis is the staining of exposed phosphatidylserine ( ps ) on plasma membrane with fluorochrome labeled annexin v and measuring the accumulation of propidium iodide ( pi ) in the nucleus after alterations in plasma membrane permeability .
when mature splenic t - cells from wild - type and gimap4 mice were induced to apoptosis , the number of apoptotic ( ps / pi ) cells was greater among the gimap4 t - cells than the wt t - cells and the number of dead cells were reduced accordingly , that is , the cells executed apoptosis with slower kinetics .
furthermore , the accumulation of apoptotic cells could be inhibited by caspase inhibitors , and there were no changes in caspase-3 activation between apoptotic ko and wt t - cells .
this indicated that gimap4 acts downstream of caspase-3 and plays a role rather in the execution than the induction phase of apoptosis . a similar , but
less marked effect was found in t - cells of the inbred brown norway ( bn ) rat , which carries a natural hypomorphic variant of the gimap4 gene .
further support for the proapoptotic function of gimap4 came also from the findings of nitta et al . , who showed that ectopic expression of gimap4 in immature mouse thymocytes led to increased apoptosis .
furthermore , gimap4 protein was shown to associate with the proapoptotic bcl-2 family protein bax but not other bcl-2 family members [ 12 , 45 ] .   although schnell et al .   and carter et al
.   showed that gimap4 is largely a cytosolic protein filn et al .   found it to be expressed in the membrane fraction of human cd4 + t - helper cells .
gimap4 expression was furthermore shown to be tightly regulated during early human cd4 + t - helper cell differentiation towards th1 and th2 .
proteomic analysis of il-4 regulated membrane proteins revealed downregulation of gimap4 during early th2 differentiation and it was further shown that il-12 upregulated gimap4 expression during th1 differentiation .
the expression of two distinct isoforms of gimap4 in response to il-12 and il-4 was also studied and it was shown that they followed a similar expression pattern in response to the studied cytokines , although the short isoform was more highly abundant .
rnai studies further showed that gimap4 protein expression was negatively regulated by stat6 in response to il-4r signaling during th2 differentiation .
human gimap5 mrna expression showed very wide tissue distribution among most human tissues outside the central nervous system [ 17 , 18 ] .
it seemed to localize to the mitochondrial membrane   although localization to the centrosome / er / plasma membrane   or to a distinct sedimentable subcellular fraction outside er and mitochondria   has also been detected .
the regulation of gimap5 gene expression remains largely unknown but it has been shown that gimap5 is a transcriptional target of notch signaling in jurkat cells .   an extensively studied genetic defect of lymphopenia ( i.e. , lyp , also called iddm2 ) in diabetes - prone biobreeding rat ( bbdp ) was shown to be caused by a frameshift mutation in the gimap5 gene [ 2023 ] .
this frameshift deletion of 1  bp introduces a premature stop codon leading to a truncated protein product .
this deletion was absent in the diabetes resistant bb ( bbdr ) rat [ 20 , 21 ] .
it closely resembles human type 1 diabetes and is a consequence of th1-mediated destruction of pancreatic islet  cells characterized by high levels of ifn- and il-12 [ 48 , 49 ] .
the bbdp rat has severe , life - long t - cell lymphopenia [ 43 , 50 ] caused by increased apoptosis and shortened life - span of recent thymic emigrants and peripheral t - cells [ 5153 ] .
furthermore , peripheral lyp / lyp t - cells display normal ca signaling and proliferation in response to tcr crosslinking .
interestingly , the in vitro life - span of lyp / lyp b cells is not affected by the mutation .
the lymphopenic phenotype of the bbdp rat argues for anti - apoptotic function for gimap5 . the studies with the mutated gimap5 in bbdp rat by pandarpurkar et al .
, keita et al . , and pino et al .   and with human gimap5 by sandal et al .  
mitochondrial membrane potential was found to be lower in bbdp t - cells than in bbdr t - cells .
the lack of full - length gimap5 in bbdp rat t - cells lead to er stress signaling and c / ebp homologous protein-(chop)-mediated apoptosis .
the bbdp rat t - cells also display an impaired tcr - stimulated ca response which suggests that gimap5 is a regulator of calcium responses in t - lymphocytes .
moreover , knockdown of gimap5 by rnai in jurkat t - cells resulted in increased apoptosis .
gimap5   mice exhibit peripheral t - cell lymphopenia , especially of cd8 t - cells .
overexpression of human gimap5 in jurkat t - cell line resulted in increased resistance to okadaic acid or -irradiation induced apoptosis .
the anti - apoptotic function of gimap5 was shown to be upstream of caspase-3 . however , more recent studies by dalberg et al .   were contradictory to the earlier findings and showed a proapoptotic function for human gimap5 .
they showed that gimap5 knockdown by rnai did not affect the number of apoptotic cells but overexpression of gimap5 in both jurkat t - cells and nave t - cells led to increased apoptosis .
furthermore , they showed that while expression of the wt rat gimap5 led to increased apoptosis of the cells , the expression of the truncated gimap5-lyp in c58 , a rat t - cell line , led to a very rapid death of the transfected cells .
albeit the results obtained by different research groups seem contradictory , they may only indicate that the role of gimap5 in apoptosis is dependent on the activation status of the cells and the availability of growth factors .  
when the rat gimap5-lyp mutation was backcrossed into pvg - rt1 rat strain the lyp / lyp rats developed a spontaneous , progressive , inflammatory bowel disease with th2 characteristics .
indeed , the purified lyp / lyp cd4cd45rc cells produced increased amounts of il-4 but similar amounts of ifn- compared to control wt cells . also the expression of il-4 and il-13 was higher in the lyp / lyp cells compared to wt .
accordingly , chemical mutation of mouse gimap5 led to severe intestinal inflammation and wasting disease .   
a common polyadenylation polymorphism in the human gimap5 gene was associated with the risk to autoimmune systemic lupus erythematosus ( sle )   and with the increased prevalence of ia-2 autoantibodies in patients with type i diabetes .
this polymorphism produces an inefficient polyadenylation signal to the 3 part of the gimap5 mrna and leads to increased proportion of nonterminated mrna .
there are a growing number of reports describing a functional role for gimap family proteins in lymphocyte biology and we have discussed the role of gimaps in t - lymphocyte functions .
determining , for example , the interaction partners , site of action , and signaling pathways will strengthen our knowledge of the function of this highly interesting novel family of gtpases .
 ( gimaps ) gtpase of the immunity associated protein family are a novel protein family of putative small gtpases . 
 gimaps are mainly expressed in the cells of the immune system and have been associated with immunological functions , such as thymocyte development , apoptosis of peripheral lymphocytes and t helper cell differentiation . 
 gimaps have also been linked to immunological diseases , such as t cell lymphopenia , leukemia and autoimmune diseases . in this review 
 we examine the role of gimap proteins in t - lymphocyte biology . 
1. GTPase of the Immunity-Associated Protein Family (GIMAP)
2. Genomic Organization of 
3. Features of GIMAP Proteins
4. GIMAPs in T-lymphocyte Development and Function
5. Concluding Remarks