Clathrin-mediated endocytosis is a major pathway for uptake of lipid and protein cargo at the plasma membrane. The lattices of clathrin-coated pits and vesicles are comprised of triskelions, each consisting of three oligomerized heavy chains (HC) bound by a light chain (LC). In addition to binding HC, LC interacts with members of the Hip1/R family of endocytic proteins, including the budding yeast homologue, Sla2p. Here, using in vivo analysis in yeast, we provide novel insight into the role of this interaction. We find that overexpression of LC partially restores endocytosis to cells lacking clathrin HC. This suppression is dependent on the Sla2p binding region of LC. Using live cell imaging techniques to visualize endocytic vesicle formation, we find that the N-terminal Sla2p binding region of LC promotes the progression of arrested Sla2p patches that form in the absence of HC. We propose that LC binding to Sla2p positively regulates Sla2p for efficient endocytic vesicle formation.

Myosins-I are conserved proteins that bear an N-terminal motor head followed by a Tail Homology 1 (TH1) lipidbinding domain. Some myosins-I have an additional C-terminal extension (C ext ) that promotes Arp2/3 complexdependent actin polymerization. The head and the tail are separated by a neck that binds calmodulin or calmodulinrelated light chains. Myosins-I are known to participate in actin-dependent membrane remodelling. However, the molecular mechanisms controlling their recruitment and their biochemical activities in vivo are far from being understood. In this study, we provided evidence suggesting the existence of an inhibitory interaction between the TH1 domain of the yeast myosin-I Myo5 and its C ext . The TH1 domain prevented binding of the Myo5 C ext to the yeast WIP homologue Vrp1, Myo5 C ext -induced actin polymerization and recruitment of the Myo5 C ext to endocytic sites. Our data also indicated that calmodulin dissociation from Myo5 weakened the interaction between the neck and TH1 domains and the C ext . Concomitantly, calmodulin dissociation triggered Myo5 binding to Vrp1, extended the myosin-I lifespan at endocytic sites and activated Myo5-induced actin polymerization.