Multiple endocytic pathways of C protein-coupled receptors delineated by GIT1 sensitivity

Recently, we identified a GTPase-activating protein for the ADP ribosylatio n factor family of small GTP-binding proteins that we call GIT1, This prote in initially was identified as an interacting partner for the G protein-cou pled receptor kinases, and its overexpression was found to affect signaling and internalization of the prototypical beta(2)-adrenergic receptor. Here, we report that GIT1 overexpression regulates internalization of numerous, but not all, G protein-coupled receptors. The specificity of the GIT1 effec t is not related to the type of C protein to which a receptor couples, but rather to the endocytic route it uses. GIT1 only affects the function of G protein-coupled receptors that are internalized through the clathrin-coated pit pathway in a beta-arrestin- and dynamin-sensitive manner. Furthermore, the GIT1 effect is not limited to G protein-coupled receptors because over expression of this protein also affects internalization of the epidermal gr owth factor receptor. However, constitutive agonist-independent internaliza tion is not regulated by GIT1, because transferrin uptake is not affected b y GIT1 overexpression. Thus, GIT1 is a protein involved in regulating the f unction of signaling receptors internalized through the clathrin pathway an d can be used as a diagnostic tool for defining the endocytic: pathway of a receptor.