Clathrin β‐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 β‐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 β‐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.

The phosphorylation in vitro, on serine residues by endogenous casein kinase 2, of the clathrin p light chain (33 kDa) of rat liver coated vesicles requires the presence of poly(L-lysine) which acts through binding to the fl 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 /l 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 p light chain very efficiently, but with a different specificity. After dissociation of coated vesicles the P-light-chain phosphatase activity is recovered in the membrane fraction. This phosphatase activity is inhibited by 50 pM orthovanadate and 5 mM p-nitrophenyl phosphate but not by 10 mM EDTA.Coated vesicles are organelles specialized in the selective transport o f macromolecules between membrane-bound compartments of eukaryotic cells. Clathrin coats are formed from triskelions consisting of three heavy chains (180 kDa) and three light chains which fall into two classes, c( and p, in the molecular mass range 30 -36 kDa. Minor polypeptides of 50 kDa and I00 kDa are also associated with the clathrin coat. In native triskelions the two classes of light chains, whose primary structures and molecular variability are known, are in competition for the same binding site at the proximal segment of clathrin (for review see [l]).The importance of phosphorylation of proteins in the functioning of the cellular machinery is well known. A system of endogenous protein phosphorylation in coated vesicles has been described in different tissues.