James Hurley, Ph.D.
National Institutes of Diabetes and Digestive and Kidney Diseases, NIH
Nipped in the Bud: The Strange Ways that ESCRTs Sever Membranes
The ESCRT complexes catalyze one of the most unusual membrane remodeling events in cell biology. ESCRT-mediated membrane neck cleavage is critical for the biogenesis of multivesicular bodies (MVBs), key intermediates in lysosomal sorting; cytokinesis; and the detachment of HIV-1 and other enveloped viruses from the plasma membrane of infected cells. Moreover, ESCRTs are responsible for ubiquitinated cargo sorting and membrane budding into multi-vesicular bodies. The biogenesis of MVBs was reconstituted and visualized using giant unilamellar vesicles, fluorescent ESCRT-0, I, II, and III complexes, and a membrane-tethered fluorescent ubiquitin fusion as a model cargo. ESCRT-0 forms domains of clustered cargo but does not deform membranes. ESCRT-I and II in combination deform the membrane into buds, in which cargo is confined. ESCRT-I and II localize to the bud necks, and recruit ESCRT-0-ubiquitin domains to the buds. ESCRT-III subunits localize to the bud neck and efficiently cleave the buds to form intralumenal vesicles. Intralumenal vesicles produced in this reaction contain the model cargo but are devoid of ESCRTs. The observations explain how the ESCRTs direct membrane budding and scission from the cytoplasmic side of the bud without being consumed in the reaction. Among the upstream human ESCRT complexes 0-II, ESCRT-I is unique in its ability to function at both endosomes and the plasma membrane. We have characterized a novel domain in ESCRT-I responsible for binding acidic lipids in both endosomes and the plasma membrane. This domain appears to confer on ESCRT-I the ability to localize to and function at distinct membrane with different lipid compositions.