Studies on the biosynthesis of laminin by murine parietal endoderm cells

The biosynthesis and processing of the polypeptides A (Mr = 450 x 10(3)), B1 (Mr = 240 x 10(3)), B2 (Mr = 230 x 10(3)) and C (Mr = 150 x 10(3)) of the extracellular matrix protein, laminin, were studied in murine parietal endoderm cells labelled with [35S]methionine. Various lines of evidence suggest that the A chains are not precursors to the smaller B chains. Firstly, the pulse-chase experiments, radioactivity in cytoplasmic A and (B1 + B2) chains declines with the same half-life of about 70 min. Secondly, peptide maps generated by digestion of A and B (B1 + B2) chains with Staphylococcus aureus V8 protease are different. Finally, rabbit antibodies to isolated, denatured (B1 + B2) chains do not cross-react with reduced and alkylated A chains. A, B1, B2 and C polypeptides are all glycosylated by an intracellular process involving the addition of tunicamycin and endo-beta-N-acetylglucosaminidase-H-sensitive N-linked oligosaccharide side chains. Further glycosylation probably occurs around the time of secretion. Disulphide bonding of some A and B chains can be observed in the cytoplasm within 10 min of adding [35S]methionine. However, it appears that some free A and B2 chains are present in the cytoplasm and that free A chains exist in the medium. The relationship between the 150 x 10(3)-Mr C glycoprotein and the A and B components is discussed. Although B and C chains generate different peptide maps after digestion with S. aureus V8 protease, antibodies raised against isolated, denatured C chains cross-react with reduced and alkylated B (but not A) chains. This suggests that B and C chains may share some antigenic determinant(s).