Size heterogeneity, phosphorylation and transmembrane organisation of desmosomal glycoproteins 2 and 3 (desmocollins) in MDCK cells

Abstract

Metabolic labelling with [35S]methionine and immunoprecipitation with specific antibodies to bovine desmosomal glycoproteins 2 and 3 (dg2 and dg3: desmocollins) reveals a triplet of polypeptides of Mr 115,000, 107,000 and 104,000 in MDCK cells. Tunicamycin treatment shows that this heterogeneity does not arise through differential N-linked glycosylation. Under conditions in which cells are actively forming desmosomes, the largest polypeptide, dg2, becomes phosphorylated on serine, but the two smaller polypeptides, dg3a and 3b, do not. Controlled trypsinisation of intact cells yields three membrane-protected fragments (Mr 28,000, 24,000 and 23,000) derived from these glycoproteins. The largest of these fragments is phosphorylated but the two smaller fragments are not. A monoclonal antibody to bovine dg2 and dg3 stains MDCK cells cytoplasmically. In immunoblotting of MDCK cells the monoclonal antibody recognises dg2 strongly and shows a weaker reaction with a band of lower Mr corresponding to dg3a. It also recognises the immunoprecipitated 28,000 Mr fragment from trypsinised cells and a smaller fragment of 24,000 Mr. The simplest interpretation of these data is that all three glycoproteins have a transmembrane configuration with a single membrane-spanning domain, and show heterogeneity of size and phosphorylation in their cytoplasmic domains. The data are discussed in relation to the known structures of some cell adhesion molecules. Questions about the relative roles and distributions of the different polypeptides in desmosomal organisation are raised.