ALIX- and ESCRT-III–dependent sorting of tetraspanins to exosomes

TLDR

Intraluminal vesicles of endosomes mediate protein degradation and intercellular communication via exosomes, and their formation depends on ESCRT‑0, ‑I, ‑II for cargo sorting and ESCRT‑III for membrane deformation and fission. The study shows that ALIX recruits ESCRT‑III to endosomes independently of other ESCRTs, a process requiring lysobisphosphatidic acid, and that this ALIX‑ESCRT‑III pathway promotes tetraspanin sorting into exosomes, providing an alternative ILV formation route that regulates exosomal protein targeting.

Abstract

The intraluminal vesicles (ILVs) of endosomes mediate the delivery of activated signaling receptors and other proteins to lysosomes for degradation, but they also modulate intercellular communication when secreted as exosomes. The formation of ILVs requires four complexes, ESCRT-0, -I, -II, and -III, with ESCRT-0, -I, and -II presumably involved in cargo sorting and ESCRT-III in membrane deformation and fission. Here, we report that an active form of the ESCRT-associated protein ALIX efficiently recruits ESCRT-III proteins to endosomes. This recruitment occurs independently of other ESCRTs but requires lysobisphosphatidic acid (LBPA) in vivo, and can be reconstituted on supported bilayers in vitro. Our data indicate that this ALIX- and ESCRT-III-dependent pathway promotes the sorting and delivery of tetraspanins to exosomes. We conclude that ALIX provides an additional pathway of ILV formation, secondary to the canonical pathway, and that this pathway controls the targeting of exosomal proteins.