Clathrin Regulates the Association of PIPKIγ661 with the AP-2 Adaptor β2 Appendage

The AP-2 clathrin adaptor differs fundamentally from the related AP-1, AP-3, and AP-4 sorting complexes because membrane deposition does not depend directly on an Arf family GTPase. Instead phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P-2) appears to act as the principal compartmental cue for AP-2 placement at the plasma membrane as well as for the docking of numerous other important clathrin coat components at the nascent bud site. This PtdIns(4,5)P-2 dependence makes type I phosphatidylinositol 4-phosphate 5-kinases (PIPKIs) lynchpin enzymes in the assembly of clathrin-coated structures at the cell surface. PIPKI gamma is the chief 5-kinase at nerve terminals, and here we show that the 26-amino acid, alternatively spliced C terminus of PIPKI gamma 661 is an intrinsically unstructured polypeptide that binds directly to the sandwich subdomain of the AP-2 beta 2 subunit appendage. An aromatic side chain-based, extended interaction motif that also includes the two bulky C-terminal residues of the short PIPKI gamma 635 variant is necessary for beta 2 appendage engagement. The clathrin heavy chain accesses the same contact surface on the AP-2 beta 2 appendage, but because of additional clathrin binding sites located within the unstructured hinge segment of the beta 2 subunit, clathrin binds the beta 2 chain with a higher apparent affinity than PIPKI gamma 661. A clathrin-regulated interaction with AP-2 could allow PIPKI gamma 661 to be strategically positioned for regional PtdIns(4,5)P-2 generation during clathrin-coated vesicle assembly at the synapse.