Cargo licenses maturation of stalled endocytic sites through a regulatory checkpoint

During Clathrin-mediated endocytosis (CME), over 50 different proteins assemble on the plasma membrane to reshape it into a cargo-laden vesicle. Studies of traced a detailed timeline of assembly of the CME machinery. Assembly of this machinery is coordinated with cargo capture; however, which CME steps are regulated, and which steps are deterministic, is unknown. In this study, we quantitatively and spatially describe progression through the CME pathway. We pinpointed a regulatory transition point that governs progression from the initiation phase of CME to the internalization phase. Proximity to endocytic cargo appears to influence maturation through this transition point, and direct evidence supports the conclusion that progress is regulated by cargo. While previous studies in yeast and mammalian cells suggested that cargo regulates rates of CME initiation or frequency of abortive events, our data instead identify maturation through a specific step in the CME pathway as the regulatory site.