NSF IS REQUIRED FOR TRANSPORT FROM EARLY TO LATE ENDOSOMES

Protein transport between early and late endosomes is a major membrane trafficking pathway in the cell followed by many proteins, including all down-regulated receptors, Yet, little is known at the molecular le vel about the mechanisms regulating membrane interactions in the endoc ytic pathway beyond early endosomes, In this study, we have used an in vitro transport assay to study the biochemical properties of endosome docking/fusion events, Our data demonstrate that N-ethylmaleimide (NE M) sensitive factor (NSF) and its soluble associated proteins (SNAPs) are required for transport from early to late endosomes, as well as at all other steps of endosomal membrane transport, We also find that th ese proteins are enriched on endosomal membranes. In addition, our stu dies suggest that besides NSF/SNAPs, another NEM-sensitive component m ay also be involved in docking/fusion at this late stage of the pathwa y. Finally, we find that, in contrast to Golgi membranes, NSF associat ion to both early and late endosomal membranes occurs via an ATP-indep endent mechanism, indicating that the binding properties of endosomal and biosynthetic NSF are different, Our data thus show that NSF/SNAPs, perhaps together with another NEM-sensitive factor, are part of the b asic molecular machinery which controls docking/fusion events during t ransport from early to late endosomes, along the lysosomal degradation pathway.