Cyclin G-associated kinase (GAK), the ubiquitously-expressed J-domain protein, is essential for the Hsc70-dependent chaperoning and uncoating of clathrin. Adjacent to the C-terminal J-domain that binds Hsc70, GAK has a clathrin-binding domain that is linked to an N-terminal kinase domain via a PTEN-like domain. Knocking out GAK in fibroblasts caused inhibition of clathrin-dependent trafficking, which was rescued by expressing a 62-kDa fragment of GAK consisting of just the clathrin-binding and J-domains. Expressing this fragment as a transgene in mice rescued the lethality and the histological defects caused by knocking out GAK in the liver or in the brain. Furthermore, when both GAK and auxilin, the neuronal-specific homolog of GAK, were knocked out in the brain, mice expressing the 62-kDa GAK fragment were viable, lived a normal life-span, and had no major behavior abnormalities. However, these mice were about half the size of wild-type mice. Therefore, the PTEN-like domains of GAK and auxilin are not essential for Hsc70-dependent chaperoning and uncoating of clathrin, but depending on the tissue, these domains appear to increase the efficiency of these cochaperones.

Read more »