Studies on the subunit composition of the renal glomerular basement membrane

The bovine glomerular basement membrane in its S-carboxymethylated form has been fractionated into a large number of polypeptide components ranging from 25,800 to 205,000 in apparent molecular weight by performing polyacrylamide gel electrophoresis in sodium dodecyl sulfate on fractions obtained by DEAE- and CM-cellulose chromatography. While only 20 electrophoretic bands were recognized in the unfractionated S-carboxymethylated membrane the DEAE-cellulose column fractions yielded 43 distinct polypeptide components and an additional 15 were obtained from the CM-cellulose chromatography. Analyses of the purified subunits indicated pronounced compositional diversity even in polypeptide components of the same molecular weight. Great differences in the content of the amino acids characteristic of collagen, namely, hydroxyproline, hydroxylysine, and glycine were evident and these three amino acids varied in a parallel manner. In contrast a reciprocal relationship between the number of lysine and hydroxylysine residues was observed so that the sum of these two remained fairly constant. While the collagen-like polypeptide components had a relatively low content of S-carboxymethylcysteine, tyrosine, aspartic acid, and heteropolysaccharides the more polar subunits were enriched in there constituents. These results are consistent with a structural model for the membrane in which the peptide chains vary greatly in the proportion of helical segments and polar regions which they contain. It is likely that the subunit polydispersity is nor primarily a biosynthetic function, but is due to limited in vivo proteolysis.