Trapping an intermediate form of connexin43 in the Golgi

We have used the metabolic inhibitor monensin to study the biosynthesis and translocation of connexin43 (Cx43) gap junction protein in rat cardiac myocytes. Immunocytochemical labeling of monensin-treated cells show that Cx43 enters and accumulates in the Golgi apparatus. Immunoprecipitation of Cx43 from monensin-treated cells results in the isolation of 40- and 41-kDa forms of the protein. Our results demonstrate that the 41-kDa form of Cx43 is sensitive to alkaline phosphatase, suggesting that it represents a phosphorylated state of the nascent 40-kDa form of Cx43. These results further suggest that Cx43 is initially phosphorylated early in the secretory pathway. It is likely that more extensive phosphorylation of Cx43 to the higher forms (42 and 44 kDa) occur at locations distal to the site of monensin blockage in the Golgi.