Immunochemical characterization of the gap junction protein connexin45 in mouse kidney and transfected human HeLa cells.

Antibodies to the gap junction protein connexin45 (Cx45) were obtained by immunizing rabbits with fusion protein consisting of glutathione S-transferase and 138 carboxy-terminal amino acids of mouse Cx45. As shown by immunoblotting and immunofluorescence, the affinity-purified antibodies recognized Cx45 protein in transfected human HeLa cells as well as in the kidney-derived human and hamster cell lines 293 and BHK21, respectively. In Cx45-transfected HeLa cells, this protein is phosphorylated as demonstrated by immunoprecipitation after metabolic labeling. The phosphate label could be removed by treatment with alkaline phosphatase. A weak phosphorylation of Cx45 protein was also detected in the cell lines 293 and BHK21. Treatment with dibutyryl cyclic adenosine- or guanosine monophosphate (cAMP, cGMP) did not alter the level of Cx45 phosphorylation, in either Cx45 transfectants or in 293 or BHK21 cells. The addition of the tumor-promoting agent phorbol 12-myristate 13-acetate (TPA) led to an increased 32P phosphate incorporation into the Cx45 protein in transfected cells. The Cx45 protein was found in homogenates of embryonic brain, kidney, and skin, as well as of adult lung. In kidney of four-day-old mice, Cx45 was detected in glomeruli and distal tubules, whereas connexin32 and -26 were coexpressed in proximal tubules. No connexin43 protein was detected in proximal tubules. No connexin43 protein was detected in renal tubules and glomeruli at this stage of development. Our results suggest that cells in proximal and distal tubules are interconnected by gap junction channels made of different connexin proteins. The Cx45 antibodies characterized in this paper should be useful for investigations of Cx45 in renal gap junctional communication.