Falling in Love with Genetics: ESCRTing Proteins into the Lysosome
Scott D. Emr
Cornell University, Ithaca, NY

Our lab has focused its efforts on uncovering the molecular machinery responsible for the biogenesis of the lysosome, an organelle which functions in the degradation and recycling of cellular proteins, lipids, carbohydrates and nucleic acids. We have used a single-cell yeast, Saccharomyces cerevisiae, as a genetic model system to discover and isolate the complex machinery that sorts and delivers proteins and enzymes to the lysosome-like vacuole (isolation of vacuolar protein sorting, vps, mutants). One set of machines, the Endosomal Sorting Complexes Required for Transport (ESCRT-0, -I, -II, -III, and the Vps4 AAA-ATPase), sort growth factor receptors, and other membrane proteins, along the endocytic pathway into vesicles that invaginate and bud into the lumen of the endosome (forming multi-vesicular bodies, MVBs). ESCRT-0, ESCRT-I and ESCRT-II were found to bind ubiquitin, a tag specifically attached to membrane proteins destined for packaging into ESCRT-generated MVB vesicles. As a result, ESCRT-0, ESCRT-I, and ESCRT-II are recruited in succession and assemble on the endosome surface, where they nucleate recruitment of the ESCRT-III components (Vps20, Vps32, Vps24 and Vps2). The ESCRT-III proteins co-assemble into flat spirals and helical filaments on the cytoplasmic face of the endosome where they corral the cargo captured by the other ESCRTs. ESCRT-III deforms the endosome membrane, bending it away from the cytoplasm generating vesicles that invaginate into the endosome. The Vps4 ATPase catalyzes late events in the process, driving scission and release of the vesicles into the lumen of the endosome. Vps4 also disassembles the ESCRT-III filaments, thereby recycling the ESCRT machinery back into the cytoplasm for new rounds of protein sorting. The ESCRT machinery has been found to not only mediate MVB formation but also directs other critical cellular processes, including the budding of enveloped viruses like HIV, cytokinesis, plasma membrane repair, and nuclear envelope reformation. Consequently, defects in ESCRT function have been linked to many human diseases ranging from cancer to neurodegeneration.