ATPase-deficient hVPS4 impairs formation of internal endosomal vesicles and stabilizes bilayered clathrin coats on endosomal vacuoles

Epidermal growth factor receptors (EGFRs) destined for lysosomal degradation are sorted in the early endosomal vacuole into small, lumenal vesicles that arise by inward budding of the limiting membrane. We have previously shown that, before their incorporation into internal vesicles, EGFRs are concentrated in flat bilayered-clathrin coats on the endosomal vacuole. Here, we show that an ATPase-deficient mutant of hVPS4 (hVPS4(EQ)) increases the association of bilayered coats with endosomal vacuoles. In addition, hVPS4(EQ) leads to a reduction in the number of internal vesicles in early and late endosomal vacuoles, and retention of EGFRs at the limiting membrane. Interestingly, hVPS4EQ was predominantly found on non-coated regions of endosomal vacuoles, often at the rim of a coated area. In line with published data on Vps4p function in yeast, these results suggest that hVPS4 is involved in the release of components of the bilayered coat from the endosomal membrane. Moreover, our data suggest that disassembly of the coat is required for the formation of internal vesicles.