Biochemical characterization of desmosomal proteins isolated from bovine muzzle epidermis: amino acid and carbohydrate composition.

The seven major desmosomal polypeptides from isolated bovine muzzle desmosomes ranging from Mr 75 000 to 250 000 were separated by gel electrophoresis, isolated and characterized with respect to their amino acid composition and sugar content. The two largest polypeptides (bands 1 and 2), i.e. desmoplakins I and II, are similar in their amino acid composition, confirming our previous immunological and biochemical data, and display a relatively high glycine content. In contrast, the other two cytoplasmic components also believed to be associated with the desmosomal plaque, i.e. polypeptides of bands 5 (Mr 83 000) and 6 (Mr 75 000), differ significantly in their amino acid composition from the desmoplakins and from each other. All four candidate polypeptides for plaque association, i.e. bands 1, 2, 5, and 6, show no significant glycosylation. The glycoproteins 4a and 4b (Mr 115 000 and 130 000) are similar in their amino acid composition, peptide analysis and immunological reactivity. Both are relatively rich in mannose and galactose but also contain sialic acid. Our determinations also indicate that the two polypeptides differ significantly in their N-acetylglucosamine and mannose content. Most, if not all, of the sugar residues are associated with a water-soluble fragment of Mr 15 500 obtained after limited digestion with V8 protease. The glycopolypeptides obtained in band 3 (Mr 164 000-175 000) are distinct from the glycopolypeptides 4a and 4b in amino acid composition, sugar content, isoelectric pH values, certain antigenic determinants and in their pattern of cleavage products obtained by treatment with proteases or cyanogen bromide. The results identify polypeptides of bands 3, 4a and 4b as glycosylated with characteristic sugar compositions. It is suggested that the major glycoproteins (bands 3, 4a, 4b) of the desmosome are integral membrane components arranged in a special way conferring resistance to detergent treatment. The possible roles of these glycoproteins in cell recognition and in adhesive functions of the desmosome are discussed.