TI - Membrane-associated GAIP is a phosphoprotein and can be phosphorylated by clathrin-coated vesicles . AB - GAIP ( G alpha interacting protein ) is a member of the RGS ( regulators of G protein signaling ) family and accelerates the turnover of GTP bound to Galphai , Galphaq , and Galpha13 . There are two pools of GAIP-a soluble and a membrane-anchored pool . The membrane-anchored pool is found on clathrin-coated vesicles ( CCVs ) and pits in rat liver and AtT-20 pituitary cells . By treatment of a GAIP-enriched rat liver fraction with alkaline phosphatase , we found that membrane-bound GAIP is phosphorylated . By immunoprecipitation carried out on [ ( 32 ) P]orthophosphate-labeled AtT-20 pituitary cells stably expressing GAIP , ( 32 ) P-labeling was associated exclusively with the membrane pool of GAIP . Phosphoamino acid analysis revealed that phosphorylation of GAIP occurred largely on serine residues . Recombinant GAIP could be phosphorylated at its N terminus with purified casein kinase 2 ( CK2 ) . It could also be phosphorylated by isolated CCVs in vitro . Phosphorylation was Mn(2+) -dependent , using both purified CK2 and CCVs . Ser-24 was identified as one of the phosphorylation sites . Our results establish that GAIP is phosphorylated and that only the membrane pool is phosphorylated , suggesting that GAIP can be regulated by phosphorylation events taking place at the level of clathrin-coated pits and vesicles .