Mammalian Suppressor of Sec4 Modulates the Inhibitory Effect of Rab15 during Early Endocytosis

Abstract

Rab15 is a novel endocyticEndocytosis of cell surface receptors regulates both the intensity and duration of receptor signaling by controlling the location of signaling interactions and the desensitization and recycling of activated receptors (reviewed in Refs. 1 and 2). Accordingly, endocytic compartments are highly specialized both in terms of their organization and function. The early/ sorting endosome is a major trafficking compartment from which several trafficking pathways emerge. Recently, Rab GTPases have emerged as potent regulators of membrane trafficking through early/sorting endosomes. Rabs do not regulate membrane trafficking per se but function as regulatory throttles impacting the kinetics of membrane transport steps through the recruitment of specific effectors that in turn mediate membrane transport (reviewed in Refs. 3-5). For example, Rab5 mediates the internalization and fusion of incoming endocytic vesicles in vivo (6 -8) and the homotypic fusion of endosomes in vitro (8 -12). Overexpression of the constitutively active GTP-bound mutant Rab5-Q79L in baby hamster kidney cells results in a dramatic increase in fluid phase and receptormediated endocytosis and leads to formation of enlarged early/ sorting endosomes. Conversely, overexpression of GDP-bound Rab5 (S34N) reduces endocytic uptake and results in the formation of a diffuse network of small endocytic vesicles (6 -8).Following activation on endosome membranes, Rab5-GTP drives the organization of a specialized membrane domain with distinct functional characteristics (13-15). Rab5-GTP forms this domain by recruiting the phosphatidylinositol 3-kinase hVPs34, which catalyzes the local production of PI-3-phosphate (PI3P) 1 (16 -18). Rab5-GTP and hVPs34 activities are essential for the subsequent recruitment of rabenosyn-5 and the docking protein early endosome antigen (EEA1) to early endosomal membranes through PI3P (17-22). EEA1 also interacts directly with syntaxin 13, a SNARE implicated in the fusion of early endosomes (13,23). Thus a model is emerging in which Rab5-GTP functions as a regulatory protein, driving assembly of specific effector complexes on endosomal membranes leading to membrane fusion (24). Consistent with this model, the early endocytic GTPases, Rabs 4 and 11, have also been shown to organize into distinct domains on early endosomes through the local recruitment of effectors (14, 15). Moreover, Rab4 and Rab5 function are linked through the shared effectors rabaptin-5 (25) and rabenosyn-5 (15). Thus Rab-specific domains appear to coordinate endosomal trafficking directly by communicating via shared effector complexes.The early endocytic GTPase Rab15 exhibits distinct endocytic localization and activity. Rab15 distributes between two early endosomal compartments, colocalizing with Rabs 4 and 5 on early/sorting endosomes and with Rab11 on pericentriolar recycling endosomes (26). Overexpression of activated Rab15 (Rab15-GTP) inhibits both fluid phase and receptor-mediated endocytosis in vivo and the homotypic fusion of early e...

Rab-genome analysis reveals novel insights in Weibel-Palade body exocytosis

“…unexpected finding, since, this Rab is considered to be a Rab of the endocytic pathway (Strick et al, 2002;Zuk and Elferink, 1999;Zuk and Elferink, 2000). Consistently, we found here that, in HUVECs, the perinuclear compartment where Rab15 also localises is positive for internalized transferrin as well as for Rab11 (supplementary material Fig.…”

“…Additionally, knocking down of Rab15 results in reduced levels of endocytosed peroxidase (supplementary material Fig. S2D), further agreeing with the implication of this Rab in endocytosis (Strick et al, 2002;Zuk and Elferink, 1999;Zuk and Elferink, 2000).…”

“…The most striking difference between WPBs and other secretory vesicles, with regard to resident Rabs, is Rab15, a so far considered endocytic Rab (Strick et al, 2002;Zuk and Elferink, 1999;Zuk and Elferink, 2000). Interestingly, from the evolution point of view, this Rab is a very close neighbour of the branch of the secretory Rabs (supplementary material Fig.…”

“…S8) (Fukuda, 2008). Additionally, Rab15 has been found to interact with Mss4 (Strick et al, 2002), the mammalian homologue of Dss4 (Burton et al, 1993). The later has been shown to act as an exchange factor for Sec4 (Moya et al, 1993), a secretory Rab that controls fusion of secretory vesicles with the plasma membrane (Goud et al, 1988;Salminen and Novick, 1987).…”

Comparative bioinformatics analyses and profiling of lysosome-related organelle proteomes

“…Rab6B is also found in the Golgi complex [40] in addition to melanosomes, platelets and synaptosomes. Rab15 is detected in endosomes [41] and Rab38 in the ER [34], based on published information. In light of the distribution of Rab proteins among the LROs, one may postulate that some Rab proteins have common functions in many LROs, while others may have more specific functions for subsets of LROs.…”