rab - type small gtpases are conserved membrane trafficking proteins in all eukaryotes , and they mediate various steps in membrane trafficking , including vesicle budding , vesicle movement along cytoskeletons , vesicle docking to specific membranes , and vesicle fusion . rabs function as a molecular switch by cycling between two nucleotide - bound states , a gdp - bound inactive state (  off  state ) and a gtp - bound active state (  on  state ) . rabs are activated by specific guanine nucleotide exchange factors ( gefs ) , which promote the release of gdp from rab and binding of gtp to rab , and the activated rabs are then inactivated by gtpase - activating proteins ( gaps ) or spontaneously inactivated by their intrinsic gtpase activity . thus , investigation of rab - gefs is crucial to understanding the spatio - temporal regulation of rab gtpase activation . although a large number of putative rab - gefs , including denn - domain - containing proteins , have recently been reported , very little is known about their in vivo roles in rab targeting to specific membrane compartments . rab17 was originally described as an epithelial cell - specific rab isoform that regulates polarized trafficking but was subsequently found to be expressed in melanocytes and human breast adenocarcinoma , and more recently we demonstrated that rab17 is also expressed in mouse brain . localization of rab17 in mouse hippocampal neurons is unique , because rab17 is the only rab isoform that is specifically targeted to the dendrites and is not targeted to the axon . the targeting of rab17 to the dendrites is neuronal differentiation stage - dependent : at an early stage ( 3 d of in vitro culture ) it localizes only in the cell body , whereas at a later stage ( 11 d of in vitro culture ) some of the rab17 is translocated from the cell body to the dendrites . by contrast , other rab isoforms are targeted to the axon alone ( e.g. , rab3a ) or to both the axon and dendrites ( e.g. , rab5a ) . targeting of rab17 to the dendrites is known to be crucial for dendrite morphogenesis and subsequent postsynapse formation , because knockdown of rab17 has been found to result in a marked reduction in both total dendrite length and the number of dendrite branches without affecting axon morphogenesis ( i.e. , total axon length and the number of axon branches ) . although several mammalian rab isoforms ( e.g. , rab7 and rab11 ) have been shown to regulate dendrite morphogenesis , rab17 is the first reported rab isoform that specifically regulates dendrite morphogenesis , but not axon morphogenesis , in mammalian neurons . we therefore thought that rab17 would be an ideal rab isoform to use to analyze a rab targeting mechanism at the cellular level . however , until recently the molecular basis of the specific rab17 targeting to dendrites had completely remained unknown and no physiological rab17-gefs that function in hippocampal neurons had been identified . in our latest study , we screened for rab17-gefs by performing yeast two - hybrid assays with a constitutive negative mutant of rab17 as bait and succeeded in identifying rabex-5 and als2 , both of which were originally described as rab5-gefs , as plausible candidate rab17-gefs in mouse hippocampal neurons . it is noteworthy that overexpression of rabex-5 , but not of als2 , increased the proportion of rab17 in the dendrites , whereas knockdown of rabex-5 caused a dramatic reduction in the proportion of rab17 in the dendrites . importantly , overexpression of a gef - activity - deficient mutant of rabex-5 ( rabex-5-d313a ) failed to increase translocation of rab17 from the cell body to the dendrites . based on these findings , activation of rab17 by rabex-5 is responsible for the stage - dependent movement of rab17 from the cell body to the dendrites of hippocampal neurons . actually , forced activation of rab17 , i.e. , expression of a constitutive active mutant of rab17 ( rab17-q77l ) , increased the dendrite localization of rab17 even in early stage neurons . more importantly , rabex-5 knockdown was found to cause a significant reduction in total dendrite length , the same as rab17 knockdown did , and the reduction was partially rescued by co - expression with rab17-q77l . these findings indicated that rabex-5 functions as an upstream activator of rab17 in developing hippocampal neurons ( fig . proposed model of rabex-5-mediated translocation of its downstream rabs to the neurites of developing hippocampal neurons . rabex-5 functions not only as a rab17-gef ( a ) but also as a rab5/21-gef ( b ) , and it determines dendrite targeting of rab17 and axon / dendrite targeting of rab5/21 , respectively ( dotted arrows ) . the solid arrows indicate the gef function of rabex-5 that promotes release of bound gdp from rab5/17/21 in exchange for gtp . because of the different sorting functions of rabex-5 , rab17 specifically regulates dendrite morphogenesis , whereas rabex-5 and rab5 are involved in neurite morphogenesis in general . factor x  in ( a ) is a putative rab17-specific effector that may also interact with rabex-5 and support dendrite targeting of rab17 . rabaptin-5 is known to interact with both rab5/21 and rabex-5 at early endosomes , but whether rabaptin-5 is involved in neurite outgrowth remains to be determined ( b ) . determining the molecular mechanism by which active rab17 is translocated from the cell body to the dendrites is an important task that has yet to be achieved . active rab17 itself is unlikely to have the ability to target dendrites , because unlike endogenous rab17 , which specifically localizes in the dendrites , rab17-q77l also localizes in the axon . interestingly , overexpression of a gef domain ( i.e. , vps9 domain ) of rabex-5 alone similarly increased the translocation of rab17 to both the axon and the dendrites , whereas overexpression of full - length rabex-5 increased endogenous rab17 translocation to the dendrites alone . thus , some additional domains of rabex-5 besides its gef domain ( e.g. , a zinc finger domain and/or a coiled - coil domain ) must also be involved in rab17 targeting . we speculate that rabex-5 functionally links to dendrite - directed motors and that rab17 activated by rabex-5 is captured by certain motor proteins that transport it to the dendrites . since some rab effector molecules ( or rabs themselves ) are known to directly associate with motor proteins , in the future it would be interesting to search for rab17-specific effectors that form a link between rabex-5 and motor proteins . our finding that rabex-5 determines the dendrite localization of its downstream target rab17 is highly consistent with a recent report showing that rab - gefs ( e.g. , rabex-5 and rabin8 ) are major determinants of specific rab membrane targeting . however , one puzzling result of our research is that rab5 and rab21 , two other downstream targets of rabex-5 , are translocated  both to the axon and to the dendrites  of developing hippocampal neurons in a rabex-5-dependent manner ( fig . 1b ) , meaning that rabex-5 determines not only the dendrite targeting of rab17 but the axon / dendrite targeting of rab5 and rab21 as well . because of the multiple roles of rabex-5 in rab targeting to neurites , rabex-5 is involved in both the axon morphogenesis and dendrite morphogenesis of developing hippocampal neurons by activating at least two downstream targets , rab5 and rab17 . however , the molecular mechanism by which rabex-5 sorts different downstream rab proteins into the axon and/or dendrites is completely unknown . since rabex-5 constitutes a gef cascade by recruiting a rab5 effector , rabaptin-5 , to early endosomes , rabex-5 may also recruit an as yet unidentified rab17-specific effector(s ) ( factor  x  in fig . 1a ) that does not bind rab5 or rab21 and support dendrite targeting of rab17 . thus , identifying rab17-specific effectors will be one of the most important tasks in future rab17 research designed to understand the molecular mechanism by which rabex-5 determines the targeting of its downstream rab proteins to neurites at the cellular level . since rab17 is also expressed in epithelial cells and melanocytes , it would be interesting to investigate whether rabex-5 also contributes to the polarized trafficking of rab17 in other cell types .
rab family small gtpases function as molecular switches in the regulation of membrane traffic , and their activity is thought to be controlled by guanine nucleotide exchange factors ( gefs ) . 
 however , the role of gefs in targeting rab proteins to specific membrane compartments is poorly understood . 
 we have recently reported finding that rabex-5 , originally described as a rab5-gef , also functions as a rab17-gef in mouse hippocampal neurons . 
 the rab17 in developing hippocampal neurons is specifically targeted to their dendrites and not to their axons , and the gef activity of rabex-5 is required for translocation of rab17 from the cell body to the dendrites . 
 interestingly , rabex-5 is also required for the axon and dendrite localization of rab5 and rab21 in hippocampal neurons . 
 our findings indicate that rabex-5 determines the targeting of its downstream rab proteins to the dendrites ( rab17 ) or to both the axon and dendrites ( rab5 and rab21 ) .