Division of Biology, California Institute of Technology, Pasadena 91125.
Gap junctions consist of closely packed pairs of transmembrane channels, the connexons, through which materials of low relative molecular mass diffuse from the cell to neighbouring cells. In liver, connexons consist of six protein subunits which, until now, were believed to be identical. However, besides the major polypeptide of relative molecular mass (Mr) 28,000 (and see refs 4 and 6), a component of Mr 21,000 (21K) has been repeatedly observed in liver. The amino-terminal sequence (18 residues) of this less abundant protein shows that it is related to, but distinct from, the Mr 28K protein. Immuno-staining and immuno-precipitation show both proteins to be in the same gap junctional plaques. Thus, it seems that hepatic gap junction channels (and by extension possibly others) are composed of two (or more) homologous proteins.