PROJECT SUMMARY Proposed are complementary studies on the mechanisms and regulation of clathrin-mediated endocytosis (CME) and actin force generation during CME in budding yeast and human stem cells. CME is responsible for uptake of molecules from a cell's environment through the permeability barrier of the plasma membrane and for selective removal of plasma membrane proteins. It is also one of the main routes for COVID-19 to enter cells. Therefore, this process is crucial for determining how cells respond to their surroundings and has heightened translational significance. Many proteins and lipids that mediate CME have been identified and their functions determined biochemically and in living cells. Imaging of fluorescently labeled CME proteins in live cells has revealed the intricate recruitment timing and order for some 60 CME proteins. However, how cargo capture is coordinated with vesicle formation, how correct protein recruitment order and timing are achieved, which events and molecules play critical roles in the pathway, and how forces curve the membrane and drive vesicle scission, are not fully understood. The following key questions will be addressed in budding yeast and human stem cells: 1) How does membrane curvature affect biochemical reaction rates? 2) How does CME become specialized for different cell types during differentiation? 3) How does a checkpoint sense cargo and regulate CME progress? and, 4) How does actin assemble at CME sites and how does its ultrastructure contribute to CME force production and adapt to increased membrane tension? Yeast studies will be empowered by a rich legacy in the lab of elucidating actin assembly and force production mechanisms. Human cell studies will be empowered by over 120 stable human tissue culture and stem cell lines generated using genome editing to express CME and actin cytoskeleton proteins as fluorescent protein fusions at native, endogenous levels. Because CME proteins are highly conserved in structure and function, principles learned from studies of yeast and humans will complement and inform each other. Together, these studies will provide a comprehensive mechanistic understanding that could not be achieved by studies in only one cell type. Because the actin cytoskeleton has been adapted by evolution for diverse, essential activities including cell motility, organelle transport, adhesion, and cell polarity development, what is learned will apply broadly for many cellular processes and will join the growing armamentarium of possible defensive measures against the pandemic.