Significance of two desmosome plaque-associated polypeptides of molecular weights 75 000 and 83 000.

Isolated desmosomes from bovine epidermis contain two major polypeptides of mol. wts. 75 000 (D6) and 83 000 (D5) which, like the desmoplakins of mol. wt. greater than 200 000, are associated with the insoluble desmosomal plaque structure. We have characterized these two polypeptides and examined their significance by peptide map comparisons and translation of bovine epidermal mRNA in vitro. Polypeptide D5 is different from polypeptide D6 by its apparent mol. wt., its isoelectric pH (approximately 6.35, whereas D6 is a basic polypeptide isoelectric at pH approximately 8.5) and its peptide map. By all these criteria desmosomal polypeptides D5 and D6 are also different from cytokeratins, desmoplakins and the glycosylated desmosomal proteins. Both polypeptides are synthesized from different mRNAs separable by gel electrophoresis on agarose: mRNA coding for polypeptide D5 is approximately 3500 nucleotides long, that for D6 is significantly shorter (estimated to 3050 nucleotides), and both contain relatively large proportions of non-coding sequences. The translational products of these mRNAs co-migrate, on two-dimensional gel electrophoresis, with the specific polypeptides from bovine epidermis, indicating that they are genuine polypeptides and are not the result of considerable post-translational processing or modification of precursor molecules. The cell and tissue distribution of these two cytoskeletal proteins and possible functions are discussed.