Clathrin beta-light chain of rat liver coated vesicles is phosphorylated in vitro and in vivo.

Clathrin beta-light chain of rat liver coated vesicles is phosphorylated in vitro in the presence of poly(L-lysine) by an endogenous protein kinase which appears to be similar to casein kinase II. Clathrin beta-light chain is also phosphorylated in vivo. After injection of [32P]phosphate into rats and preparation of purified coated vesicles in the presence of phosphatase inhibitors, electrophoretic analysis showed the presence of several labeled polypeptides including clathrin beta-light chain. A polypeptide of 50 kDa, which may correspond to the major polypeptide phosphorylated in vitro of coated vesicles, is also labeled in vivo.

Phosphorylation/dephosphorylation of the beta light chain of clathrin from rat liver coated vesicles.

The phosphorylation in vitro, on serine residues by endogenous casein kinase 2, of the clathrin beta light chain (33 kDa) of rat liver coated vesicles requires the presence of poly(L-lysine) which acts through binding to the beta light chain. The phosphorylation of other proteins is also increased in the presence of poly(L-lysine) and casein kinase 2. In contrast, the phosphorylation of the upper band of the 50-kDa protein doublet from rat liver coated vesicles is inhibited. Rat liver coated vesicles display a protein phosphatase activity which preferentially dephosphorylates clathrin beta light chain. This activity is different from the protein phosphatase which dephosphorylates the 50-kDa protein. This enzyme seems to be unrelated to the ATP/Mg-dependent protein phosphatase, or the polycation-stimulated protein phosphatases, which dephosphorylate the 50-kDa protein and beta light chain very efficiently, but with a different specificity. After dissociation of coated vesicles the beta-light-chain phosphatase activity is recovered in the membrane fraction. This phosphatase activity is inhibited by 50 microM orthovanadate and 5 mM p-nitrophenyl phosphate but not by 10 mM EDTA.