Evidence that heart connexin43 is a phosphoprotein.

Cardiac gap junctions permit the conduction and propagation of the electrical impulses that are responsible for the synchronous contraction of the myocardium (Page and Manjunath, 1986). Cardiac gap junctions are formed by the association of connexin molecules within the plasma membrane of heart cells. They create aqueous channels that permit exchange of ions and other small molecules between adjacent cells. Intercellular communication via these channels may be regulated by phosphorylation. cAMP was shown to increase junctional conductance and stimulate phosphorylation of connexin32 in cultures of dissociated liver hepatocytes (Saez et al., 1986). Furthermore, a 47 kDa protein purified from dog heart gap junctions was phosphorylated in vitro by the catalytic subunit of protein kinase A (Pressler and Hathaway, 1987). Other studies have demonstrated that cAMP enhanced junctional conductance in intact heart and isolated heart cells (De Mello, 1986; De Mello and van Loon, 1987; Burt and Spray, 1988). This report provides direct evidence that the heart gap junction protein, connexin43, from unstimulated heart tissues and cultured myocytes is phosphorylated stoichiometrically in vivo. Phosphorylation of 45 and 47 kDa connexin43-related proteins occurred predominantly on serine. In addition, the 47 kDa protein contained a low-level of phosphothreonine.