Anti-GAK monoclonal antibody, clone FQS4537

Auxilin is a brain-specific DnaJ homolog that is required for Hsc70 to dissociate clathrin from bovine brain clathrin-coated vesicles. However, Hsc70 is also involved in uncoating clathrin-coated vesicles formed at the plasma membrane of non-neuronal cells suggesting that an auxilin homolog may be required for uncoating in these cells. One candidate is cyclin G-associated kinase (GAK), a 150-kDa protein expressed ubiquitously in various tissues. GAK has a C-terminal domain with high sequence similarity to auxilin; like auxilin this C-terminal domain consists of three subdomains, an N-terminal tensin-like domain, a clathrin-binding domain, and a C-terminal J-domain, Western blot analysis shows that GAK is present in rat liver, bovine testes, and bovine brain clathrin-coated vesicles. More importantly, liver clathrin-coated vesicles, which contain GAK but not auxilin, are uncoated by Hsc70, suggesting that GAK acts as an auxilin homolog in non-neuronal cells. In support of this view, the clathrin-binding domain of GAK alone induces clathrin polymerization into baskets and the combined clathrin-binding domain and J-domain of GAK supports uncoating of AP180-clathrin baskets by Hsc70 at pH 7 and induces Hsc70 binding to clathrin baskets at pH 6, Immunolocalization studies suggest that GAK is a cytosolic protein that is concentrated in the perinuclear region; it appears to be highly associated with the trans-Golgi where the budding of clathrin-coated vesicles occurs. We propose that GAK is a required cofactor for the uncoating of clathrin-coated vesicles by Hsc70 in non-neuronal cells.

Auxilin, a J-domain containing protein, recruits the Hsc70 uncoating ATPase to newly budded clathrin-coated vesicles. The timing of auxilin arrival determines that uncoating will commence only after the clathrin lattice has fully assembled and after membrane fission is complete. Auxilin has a region resembling PTEN, a PI3P phosphatase. We have determined the crystal structure of this region of bovine auxilin 1; it indeed resembles PTEN closely. A change in the structure of the P loop accounts for the lack of phosphatase activity. Inclusion of phosphatidylinositol phosphates substantially enhances liposome binding by wild-type auxilin, but not by various mutants bearing changes in loops of the C2 domain. Nearly all these mutations also prevent recruitment of auxilin to newly budded coated vesicles. We propose a specific geometry for auxilin association with a membrane bilayer and discuss implications of this model for the mechanism by which auxilin detects separation of a vesicle from its parent membrane.