Endosomal compartments undergo a wide variety of remodeling events to efficiently sort cargoes to multiple, unique destinations. Membrane tubulation at endosomes, mediated by BAR domain proteins and members of the EHD family of ATPases, has been shown to promote endocytic recycling to the cell surface. In contrast, components of the ESCRT (Endosomal Sorting Complex Required for Transport) machinery have been implicated in the formation of intralumenal vesicles (ILVs), which exhibit negative membrane curvature
and bud away from the cytoplasm toward the endosome interior. In topologically similar processes, the ESCRT machinery also participates in membrane abscission during cytokinesis, the formation of retroviral particles that bud from the cell surface during infection, plasma membrane repair, and nuclear envelope resealing after mitosis.
The ESCRT machinery is composed of five multi-subunit complexes (ESCRT-0, ESCRT-I, ESCRT-II, ESCRT-III, and the Vps4 complex) that each exhibits unique membrane binding properties. At the endosome, early acting components of the machinery function to select and concentrate cargo molecules for deposition into ILVs. This sorting event depends on the presence of ubiquitin, which is conjugated to membrane proteins destined for turnover. Subsequently, downstream ESCRT complexes maintain cargo within endosomal subdomains, while simultaneously initiating inward membrane bending. Ultimately, assembly of ESCRT-III filaments at the vesicle bud neck drives vesicle scission, thereby sequestering cargo molecules within the lumen of the endosome. The ATPase activity of the Vps4 complex is necessary to remodel and disassemble the ESCRT-III complex and recycle its
components for future rounds of vesicle biogenesis. Using a wide range of biochemical, biophysical, genetic, and structural approaches, we aim to define mechanisms by which the ESCRT machinery regulates cargo trafficking and degradation in the endosomal system. Furthermore, we are extending our studies to determine whether similar principles apply to ESCRT function during cytokinesis, nuclear envelope sealing, and viral budding.
In a particularly exciting line of investigation, we identified the first curvature sensitive component of the ESCRT machinery, which may play a key role in targeting ESCRT function during lumenal vesicle formation at endosomes. Specifically, in collaboration with investigators in the UK, we have shown that a complex of ESCRT-II bound to the ESCRT-III component Vps20 binds selectively to membranes of high curvature, similar to that found at a vesicle bud neck. Our data further suggest that the ESCRT-II/Vps20 complex is mechanosensitive, binding more tightly to membranes as they become increasingly bent, which may stabilize curvature sensitive ESCRT-III filaments within the vesicle bud neck as the membrane deforms. Ongoing studies aim to define the contributions of individual ESCRT-III subunits to ILV biogenesis.