Vps68 cooperates with ESCRT-III in intraluminal vesicle formation

The endosomal sorting complex required for transport (ESCRT)-III mediates budding and abscission of intraluminal vesicles (ILVs) into multivesicular endosomes. To further define the role of the ESCRT-III associated protein Mos10/Vps60 in ILV formation, we screened for new interaction partners by SILAC/MS. Here, we focused on the newly identified interaction partner Vps68. Our data suggest that Vps68 cooperates with ESCRT-III in ILV formation. The deletion of VPS68 caused a sorting defect similar to the SNF7 deletion, when the cargo load was high. The composition of ESCRT-III was altered, the level of core components was higher and the level of associated proteins was lower in the deletion strain. This suggests that a shift occurs from an active complex to a disassembly competent complex and that this shift is blocked in the{Delta} vps68 strain. We present evidence that during this shift Snf7 is replaced by Mos10. Vps68 has an unusual membrane topology. Two of its potential membrane helices are amphipathic helices localized to the luminal side of the endosomal membrane. Based on this membrane topology we propose that Vps68 and ESCRT-III cooperate in the abscission step by weakening the luminal and cytosolic leaflets of the bilayer at the abscission site.