Role of cyclin G-associated kinase in uncoating clathrin-coated vesicles from non-neuronal cells

Auxilin is a brain-specific DnaJ homolog that is required for Hsc70 to diss ociate 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 r equired 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, W estern blot analysis shows that GAK is present in rat liver, bovine testes, and bovine brain clathrin-coated vesicles. More importantly, liver clathri n-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 clat hrin polymerization into baskets and the combined clathrin-binding domain a nd 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, Immunolocaliza tion studies suggest that GAK is a cytosolic protein that is concentrated i n the perinuclear region; it appears to be highly associated with the trans -Golgi where the budding of clathrin-coated vesicles occurs. We propose tha t GAK is a required cofactor for the uncoating of clathrin-coated vesicles by Hsc70 in non-neuronal cells.