An endocytic myosin essential for plasma membrane invagination powers motility against resistance.

Assembling actin filaments work together with myosins to accomplish a wide array of biological processes. During clathrin-mediated endocytosis (CME), actin assembly and type I myosin cooperate to bend the plasma membrane into a pit that undergoes scission to internalize a cargo-bearing vesicle. How actin assembly and myosin work together in this process is a critical, unanswered question. Some type I myosins directly power motility, while others act as force-sensitive clamps. The Saccharomyces cerevisiae endocytic type I myosin Myo5 has been meticulously studied in vivo , yet whether this protein’s essential CME function is to power membrane invagination or to bind to force-bearing actin filaments to collect and organize them for optimal force production has not been established. We report that Myo5 is a low-duty-ratio motor with a working stroke that is rapid and force-insensitive compared to related myosins that act as force-sensitive anchors. We therefore propose that Myo5 generates power to augment actin assembly-based forces during endocytosis.