Cardiac myocyte gap junctions: evidence for a major connexon protein with an apparent relative molecular mass of 70,000.

It is widely accepted that there is a family of gap junction connexon proteins, their distribution appearing to vary with tissue type and species. In cardiac tissues the major junctional channel component identified is a 43K (K = 10(3) Mr) polypeptide. Using a gap junction isolation protocol in which low temperatures are maintained, and which is detergent-free, we have identified a second gap junction-related protein in cardiac tissues with an apparent relative molecular mass of 70,000. Antibodies raised to three synthetic peptides matching portions of the 43K gap junction protein cDNA sequence cross-react with the 70K protein, but biochemical studies indicate that these proteins are distinct from one another. The structures that contain the 70K protein are susceptible to fragmentation at warm temperatures, and by electron microscopy this can be correlated with loss of 'minidomains' within the junctional plaque. Using a gap junction enriched-fraction prepared from purified isolated adult myocytes we show that both the 43K and 70K gap junction proteins are present in ventricular cardiac myocytes. In such preparations, and those from whole heart, the 70K protein appears to be the major gap junction-related protein present.