Mammalian CORVET Is Required for Fusion and Conversion of Distinct Early Endosome Subpopulations

Abstract

Early endosomes are organized in a network of vesicles shaped by cycles of fusion, fission, and conversion to late endosomes. In yeast, endosome fusion and conversion are regulated, among others, by CORVET, a hexameric protein complex. In the mammalian endocytic system, distinct subpopulations of early endosomes labelled by the Rab5 effectors APPL1and EEA1 are present. Here, the function of mammalian CORVET with respect to these endosomal subpopulations was investigated. Tgfbrap1 as CORVET-specific subunit and functional ortholog of Vps3p was identified, demonstrating that it is differentially distributed between APPL1 and EEA1 endosomes. Surprisingly, depletion of CORVET-specific subunits caused fragmentation of APPL1-positive endosomes but not EEA1 endosomes in vivo. These and in vitro data suggest that CORVET plays a role in endosome fusion independently of EEA1. Depletion of CORVET subunits caused accumulation of large EEA1 endosomes indicative of another role in the conversion of EEA1 endosomes into late endosomes.In addition, depletion of CORVET-specific subunits caused alterations in transport depending on both the type of cargo and the specific endosomal subpopulation. These results demonstrate that CORVET plays distinct roles at multiple stages in the mammalian endocytic pathway.

Characterization of the Mammalian CORVET and HOPS Complexes and Their Modular Restructuring for Endosome Specificity

“…As in yeast, the mammalian HOPS complex consists of VPS16, VPS11, VPS33A, VPS18, VPS41, and VPS39. We confirm that TGFBRAP1 and VPS8 are the mammalian CORVET-specific subunits (with TGFBRAP1 as the yeast vps3 ortholog) (11,35). In contrast to previous data (16), however, our data indicate that VIPAS39 and VPS33B, two CORVET/HOPS subunit homologues not present in yeast, are not part of CORVET/HOPS as also suggested by others (12,21,36 interactions of VPS33B and/or VIPAS39 with CORVET/HOPS or the existence of VPS33B-VIPAS39-HOPS interactions in other cellular systems.…”

Vaccinia Virus Infection Requires Maturation of Macropinosomes

“…The association of macropinosomes with EEA1 was less prominent than with Rab5 reaching a maximum of 8.0%. That this Rab5 effector and tethering factor is only found on a subset of EEs 43, 57 may explain the low level of colocalization we observed. Perhaps, early macropinosomes, such as classical endosomes, exist as several distinct subsets.…”

Rab5 Isoforms Specifically Regulate Different Modes of Endocytosis in Leishmania

“…Recent studies have shown that APPL1, a Rab5 effector, regulates the differential sorting of EGF and transferrin from endosomes (38). Moreover, cargo sorting might occur at the plasma membrane depending on the mode of entry as clathrin-coated vesicles are also differentially tethered to the two types of endosomes (39). As all Rab5 isoforms are located on EE, it is tempting to speculate that Rab5 isoforms might have a differential role in regulating various modes of endocytosis.…”