Simultaneous binding of PtdIns(4,5)P-2 and clathrin by AP180 in the nucleation of clathrin lattices on membranes

Adaptor protein 180 (AP180) and its homolog, clathrin assembly Lymphoid mye loid Leukemia protein (CALM), are closely related proteins that play import ant roles in clathrin-mediated endocytosis. Here, we present the structure of the NH2-terminal domain of CALM bound to phosphatidylinositol-4,5-bispho sphate [Ptdlns(4,5)P-2] via a lysine-rich motif. This motif is found in oth er proteins predicted to have domains of similar structure (for example, Hu ntingtin interacting protein 1). The structure is in part similar to the ep sin NH2-terminal (ENTH) domain, but epsin Lacks the Ptdlns(4,5) P-2-binding site. Because AP180 could bind to Ptdins(4,5)P-2 and clathrin simultaneous ly, it may serve to tether clathrin to the membrane. This was shown by usin g purified components and a budding assay on preformed Lipid monolayers. In the presence of AP180, clathrin lattices formed on the monolayer. When AP2 was also present, coated pits were formed.