AP2 controls clathrin polymerization with a membrane-activated switch

Clathrin-mediated endocytosis (CME) is vital for the internalization of most cell-surface proteins. In CME, plasma membrane-binding clathrin adaptors recruit and polymerize clathrin to form clathrin-coated ‘pits’ into which cargo is sorted. AP2 is the most abundant adaptor, and is pivotal to CME. By determining a new structure of AP2 that includes the clathrin-binding β2-hinge and developing an AP2-dependent budding assay, we reveal the existence of an autoinhibitory mechanism that prevents clathrin recruitment by cytosolic AP2. A large-scale conformational change driven by the plasma membrane phosphoinositide PtdIns(4,5)P2 and cargo relieves this autoinhibition, so triggering clathrin recruitment and hence clathrin-coated bud formation. This molecular switching mechanism constitutes an unsuspected layer of regulation that couples AP2’s membrane recruitment to its key functions of cargo and clathrin binding.