Biochemistry and Diversity of the Gap Junction Protein: A Study of Liver, Heart and Lens

Year: 1983 DOI: 10.7907/bhjv-8053
Fractions highly enriched for gap junctions by morphological criteria have been isolated from rat liver, heart and eye lens, although some question exists as to the nature of the structures from lens. The junctions from each tissue are comprised of a single major protein of Mr 28,000 in the liver, Mr 30,000 in the heart, and Mr 26,000 (MIP 26) in the lens. The polypeptide profile of the liver fraction is complicated by endogenous proteolysis and aggregation in SDS of the gap junction protein and the presence of about 20% non-junctional material. Heart and lens junction proteins are also found to aggregate in SDS, while endogenous proteolysis typically reduces the cardiac gap junction protein to Mr 28,000 during isolation.

Comparisons of two-dimensional peptide maps of the junctional proteins from these tissues, and the use, where necessary, of a third dimension of resolution (HPLC), demonstrates the three proteins to be very different in terms of their primary structures. The protein of each tissue, however, seems well conserved between mammalian species. For liver and lens, this finding has been confirmed in amino acid analyses and partial NH₂-terminal sequences (to 58 and 33 residues, respectively). Cleavage products of these two proteins have also been produced to allow further sequence analysis in the future. In spite of the differences in primary structure, some conservation of the tertiary structures of these proteins is suggested by proteolysis of intact junctions (likely restricted to the cytoplasmic surfaces). Liver and heart gap junction proteins are reduced by trypsin to two fragments of Mr 10,000, while a single Mr 21,000 fragment is produced from lens MIP 26. Sequence analysis (liver and lens only) indicates that most of the protein removed by tryptic hydrolysis is from the carboxy-terminus, although an additional loop of 4,000 daltons is excised from the center of the liver polypeptide and five residues are lost from the NH₂-terminus of the lens protein.

The extent and possible significance of this surprising tissue specificity of the gap junction protein are discussed in the light of these findings.