Characterization of the collagenous domain of tubular basement membrane.

The collagenous domain of rabbit renal tubular basement membrane was solubilized by controlled pepsin digestion. Complete solubilization occurred within 24 h at 4 degrees C, and the sodium chloride precipitable polypeptides remained resistant to further degradation for at least 72 h. These products, which represent 35 to 40% of the basement membrane, possess a composition similar to that od the collagenous domain of bovine glomerular basement membrane. Agarose gel electrophoresis in the presence of sodium dodecyl sulfate resolved 11 components, several of which comprise the majority of the protein in the system. Components I (Mr approximately 1,500,000) and II (Mr approximately 1,200,000), together with a minor component, III, comprise 45% of the Coomassie blue-staining material. An additional 25% is represented by component IV (Mr approximately 375,000) plus a minor component, V. The remaining components (VI through XI) span a molecular weight range from 34,000 to 200,000. Two-dimensional gel electrophoresis was used to establish which of these components contain disulfide cross-linked polypeptides and the number and molecular weight of such polypeptides. Components I, II, and IV each contained intensely staining polypeptides of Mr = 160,000, 140,000, and 80,000 plus lightly staining polypeptides of Mr = 43,000 and 34,000. Lightly staining polypeptides of Mr = 390,000, 180,000, and 125,000 were also detected among the reduction products of components I and II. In addition, evidence for the presence of aldehyde-derived cross-linked polypeptides within the structures of components I and II was observed in the form of reduction products whose molecular weight occurs in multiples of the smaller monomeric polypeptides.