Rab Conversion as a Mechanism of Progression from Early to Late Endosomes

Abstract

The mechanisms of endosome biogenesis and maintenance are largely unknown. The small GTPases Rab 5 and Rab 7 are key determinants of early and late endosomes, organizing effector proteins into specific membrane subdomains. Whether such Rab machineries are indefinitely maintained on membranes or can disassemble in the course of cargo transport is an open question. Here, we combined novel image-analysis algorithms with fast live-cell imaging. We found that the level of Rab 5 dynamically fluctuates on individual early endosomes, linked by fusion and fission events into a network in time. Within it, degradative cargo concentrates in progressively fewer and larger endosomes that migrate from the cell periphery to the center where Rab 5 is rapidly replaced with Rab 7. The class C VPS/HOPS complex, an established GEF for Rab 7, interacts with Rab 5 and is required for Rab 5-to-Rab 7 conversion. Our results reveal unexpected dynamics of Rab domains and suggest Rab conversion as the mechanism of cargo progression between early and late endosomes.

A pathway for phagosome maturation during engulfment of apoptotic cells

“…Loss of the HOPS complex would then render RAB-7(+) phagosomes unable to mature into later lysosomal structures. Other studies have shown that the recruitment of Rab7 to endosomes is linked to Rab5 activity; hence, HOPS complex members can also be found in a complex 39 with Rab5 GTPÃS . We find that knockdown of one complex member, vps-41 resulted in an increase in the number of corpses in RAB-5(+) phagosomes, suggesting that in the nematode, VPS-41 may be involved in the recruitment of the HOPS complex to maturing RAB-5(+) phagosomes.…”

“…If this were the case, we would predict that RAB-5 and RAB-7 would co-localize only transiently on phagosomes, during ÃRab conversionà 39 . When we made double transgenic worms expressing both YFP::RAB-5 and CFP::RAB-7, we found minimal co-localization between these two markers ( Figure 1, iÃl, arrows vs. arrowheads).…”

“…EEA-1 and RABS-5 are hypothesized to function as a tethering complex during a Ãconcentratingà stage of endocytosis, where multiple vesicles may fuse into RAB-5 positive structures 39 . To address the importance of fusion events during corpse clearance, we scored worms deficient in the ATPase NSF-1, the nematode homologue of N-ethylmaleimidesensitive factor (NSF) and a key component of the vesicular fusion machinery on early endosomes 46 .…”

Virus trafficking – learning from single-virus tracking

“…Influenza viruses are preferentially delivered, by microtubule-dependent transport, to the dynamic early endosomes 27 . Viral fusion typically occurs in the midst of the endosomal maturation process with both early and late endosomal markers present on the endosomean intermediate state of endosomal maturation that has only recently been discovered [27][28][29] .…”

Drosophila Rab2 controls endosome-lysosome fusion and LAMP delivery to late endosomes

“…The upstream part of the pathway is characterized by endosomal maturation, whereby Rab5-containing early endosomal vesicles gradually mature into Rab7-containing late endosomes (LEs) through Rab conversion [8], mediated by the Rab7 guanine nucleotide exchange factor (GEF) complex Mon1-Ccz1 [9,10]. In contrast, the downstream delivery of material from LEs to lysosomes is thought to occur by transient and permanent fusion between LEs and lysosomes [11].…”