Interactions between adaptor protein-1 of the clathrin coat and microtubules via type 1a microtubule-associated proteins

The classical view suggests that adaptor proteins of the clathrin coat medi ate the sorting of cargo protein passengers into clathrin-coated pits and t he recruitment of clathrin into budding areas in the donor membrane. In the present study, we provide biochemical and morphological evidence that the adaptor protein 1 (AP-1) adaptor of the trans-Golgi network clathrin intera cts with microtubules. AP-1 in cytosolic extracts interacted with in vitro assembled microtubules, and these interactions were inhibited by ATP deplet ion of the extracts or in the presence of 5 ' -adenylylimidodiphosphate. An overexpressed gamma -subunit of the AP-1 complex associated with microtubu les, suggesting that this subunit may mediate the interaction of AP-1 with the cytoskeleton. Purified AP-1 did not interact with purified microtubules , but interaction occurred when an isolated microtubule-associated protein fraction was added to the reaction mix. The gamma -adaptin subunit of AP-1 specifically co-immunoprecipitated with a microtubule-associated protein of type la from rat brain cytosol. This suggests that type la microtubule-ass ociated protein may mediate the association of AP-1 with microtubules in th e cytoplasm. The microtubule binding activity of AP-1 was markedly inhibite d in cytosol of mitotic cells. By means of its interaction with microtubule -associated proteins, we propose novel roles for AP-1 adaptors in modulatin g the dynamics of the cytoskeleton, the stability and shape of coated organ elles, and the loading of nascent AP-1-coated vesicles onto appropriate mic rotubular tracks.