Fig. 3.

RAB31 inhibits the lysosomal degradation of MVBs (A). The existence of a tyrosine kinase receptor namely EGFR on the late endosomal surface activates Rab31 via tyrosine phosphorylation. The phosphorylated Rab31 interacts with flotillin proteins. Rab31-flotillin acts as a scaffold heterodimer protein and forms a budding platform for sorting EGFR and other proteins into MVB lumen in collaboration with lipid rafts. On the other hand, Rab31 recruits TBC1D2B, and the Rab31-TBC1D2B complex inactivates Rab7, preventing the lysosomal degradation of MVBs. Besides the Syntenin-Alix-ESCRT-III, Rab31-flotillin complex acts as parallel sorting machinery that drives different cargoes such as CD63, CD81, and CD9 into exosomal pathways. CD63 is a membrane protein that can interact with PDZ domains of Syntenin. Interaction N-terminal of Syntenin with the V-domain of Alix induces the recruitment of ESCRT-III by the Bro domain of Alix and the formation of ILVs. HD-PTP acts as a scaffold for the binding of ESCRT subsets during the trafficking of ubiquitinated cargo into the MVB lumen (B). HRS and STAM are subunits of ESCRT-0 and can bind to the ubiquitinated cargos on the endosomal membrane. STAM is composed of STAM (GAT) and SH3 domains. STAM domain interacts with the Bro1 domain of HD-PTP and the SH3 domain is linked to the PPRPTAPKP motif in the PTP domain of HD-PTP. ESCRT-0 is dissociated from HD-PTP by the interaction of ESCRT-I TSG-101 with the PTAP motif of the PTP domain. Then, the ESCRT-I UBAP1 subunit can interact with the FYX2L motif in the CC region of the PTP domain. In the later phase, the interaction of ESCRT-III CHMP4 subunit with the Bro domain of HD-PTP enhances the dissociation of ESCRT-0 from HD-PTP, facilitates polymerization of ESCRT-III, and drives the ubiquitinated cargo into the MVB pathway