γ Subunit of the AP-1 Adaptor Complex Binds Clathrin: Implications for Cooperative Binding in Coated Vesicle Assembly

The heterotetrameric AP-1 adaptor complex is involved in the assembly of clathrin-coated vesicles originating from the trans-Golgi network (TGN). The β1 subunit of AP-1 is known to contain a consensus clathrin binding sequence, LLNLD (the so-called clathrin box motif), in its hinge segment through which the β chain interacts with the N-terminal domains of clathrin trimers. Here, we report that the hinge region of the γ subunit of human and mouse AP-1 contains two copies of a new variant, LLDLL, of the clathrin box motif that also bind to the terminal domain of the clathrin heavy chain. High-affinity binding of the γ hinge to clathrin trimers requires both LLDLL sequences to be present and the spacing between them to be maintained. We also identify an independent clathrin-binding site within the appendage domain of the γ subunit that interacts with a region of clathrin other than the N-terminal domain. Clathrin polymerization is promoted by glutathione S-transferase (GST)-γ hinge, but not by GST-γ appendage. However, the hinge and appendage domains of γ function in a cooperative manner to recruit and polymerize clathrin, suggesting that clathrin lattice assembly at the TGN involves multivalent binding of clathrin by the γ and β1 subunits of AP-1.