PHOSPHORYLATION OF CONNEXIN-32 BY PROTEIN-KINASE-C PREVENTS ITS PROTEOLYSIS BY MU-CALPAIN AND M-CALPAIN

Isolated connexin-32s from rat and mouse liver are proteolyzed in vitr o by the intracellular Ca2+-dependent neutral proteases, mu-calpain an d m-calpain, producing a major fragment of 26 kDa. Connexin-26 is not proteolyzed by calpain. Calpain cleaves connexin-32 at its C-terminal end as shown by I-125-calmodulin binding experiments. Connexin-32, but not connexin-26, is phosphorylated by both protein kinase A and prote in kinase C in serine residues and the sites of phosphorylation by bot h kinases remain in the major 26-kDa fragment resulting from calpain p roteolysis. Phosphorylation of connexin-32 by protein kinase C, but no t by protein kinase A, prevents the proteolytic attack of mu-calpain a nd m-calpain. Phosphorylation of connexin-32 by protein kinase A and p rotein kinase C does not prevent its proteolysis by papain, alpha-chym otrypsin, proteinase K, and trypsin.