Autoregulation clamps the synaptic membrane-remodeling machinery and promotes productive actin-dependent endocytosis

Synaptic membrane-remodeling events such as endocytosis require force-generating actin assembly. The endocytic machinery that regulates these actin and membrane dynamics localizes at high concentrations in a micron-scale synaptic membrane domain called the "periactive zone" (PAZ). Paradoxically, endocytic events occur only sparsely in space and time within the PAZ. Here we describe a mechanism whereby autoinhibition clamps the PAZ machinery to limit actin assembly to discrete functional events. We found that collective interactions between the Drosophila PAZ proteins Nwk/FCHSD2, Dap160/Intersectin, and WASp relieve Nwk autoinhibition and promote robust membrane-coupled actin assembly in vitro. Using automated particle tracking to quantify synaptic actin dynamics in vivo, we discovered that Nwk-Dap160 interactions constrain spurious assembly of WASp-dependent actin structures. These interactions also promote synaptic endocytosis, suggesting that autoinhibition both clamps and primes the synaptic endocytic machinery, thereby constraining actin assembly to drive productive membrane remodeling in response to physiological cues.