Mechanical forces are integral to many cellular processes, including clathrin-mediated endocytosis, a principal membrane trafficking route into the cell. During endocytosis, forces provided by endocytic proteins and the polymerizing actin cytoskeleton reshape the plasma membrane into a vesicle. Assessing force requirements of endocytic membrane remodelling is essential for understanding endocytosis. Here, we determined forces applied during endocytosis using FRET-based tension sensors integrated into the major force-transmitting protein Sla2 in yeast. We measured force of approx. 10 pN transmitted over Sla2 molecule, hence a total force of 450-1300 pN required for endocytic vesicle formation. Importantly, decreasing cell turgor pressure and plasma membrane tension reduced force requirements of endocytosis. The measurements in hypotonic conditions and mutants lacking BAR-domain membrane scaffolds then showed the limits of the endocytic force-transmitting machinery. Our study provides force values and force profiles critical for understanding the mechanics of endocytosis and potentially other key cellular membrane-remodelling processes.
