A membrane-associated protein complex with selective binding to the clathrin coat adaptor AP1.

Adaptors are the membrane-binding components of clathrin-coated vesicles. The interaction of the trans-Golgi coat adaptor AP1 with membrane-associated proteins was analyzed by affinity chromatography. Proteins of 83 and 52 kDa bound specifically to the core domain of AP1 and showed no interaction with AP2 or other clathrin-coated vesicle proteins. The AP1-binding proteins were tightly membrane-associated, though behaved as peripheral membrane proteins. They were detected in membranes depleted of clathrin-coated vesicles and not in coated vesicles, suggesting that the interaction of these proteins with AP1 may precede coated vesicle budding. Co-fractionation of the AP1-binding proteins with trans-Golgi network membrane was also observed. Upon gel filtration, both AP1-binding proteins eluted in a high molecular mass complex which was labile at high concentrations of Tris. The 83 kDa protein bound to AP1 affinity resin in the absence of the 52 kDa protein. In contrast, the separated 52 kDa protein did not bind AP1, suggesting that the 83 kDa protein is the AP1-binding component of the complex. Characterization of this protein complex defines a novel membrane-associated component that specifically interacts with AP1 and may contribute to its function in forming clathrin-coated vesicles.