The 160k α1(IV) Chain, a Short Form of a Type IV Collagen Polypeptide, of Bovine Lens Capsule Retains the NC1 Domain

We recently reported that the bovine lens capsule contained a shorter α1(IV) chain (160k) as a major polypeptide in addition to the 180k α1(IV) chain [J. Biochem. (1995) 117, 1298-1304]. Two experiments were performed to examine whether or not the 160k polypeptide retained the carboxyl-terminal NC1 domain. On immunoblotting analysis with a monoclonal antibody (H11) raised against the NC1 domain of the human α1(IV) chain [position 1643-1650; near the carboxyl-terminal end of the human α1(IV) chain], the 180k and 160k polypeptides showed identical immunoreactivity, suggesting that the two chains had the same human α1(IV) collagen NC1 domain sequence. Another monoclonal antibody (H21) specific for the NC1 domain of human α2(IV) did not react with these polypeptides, but with the bands corresponding to 175k and 155k. The 160k polypeptide was selectively solubilized from bovine lens capsules, leaving the other major polypeptides, 180k and 175k, insoluble. The 160k polypeptide was separated by preparative electrophoresis. Bacterial collagenase digestion of the separated 160k polypeptide produced collagenase-resistant segments of about 29k and 30k in size based on globular standards. These sizes corresponded well with those of the NC1 domains of type IV collagen α chains (25-30k). The results indicated that the 160k polypeptide retained the carboxyl-terminal NC1 domain of the α1(IV) chain. In turn, the 20k polypeptide of the amino-terminal region or the 7S domain of 180k α1(IV) would have been excised to yield 160k α1(IV), assuming that the 160k α1(IV) chain is a processed form of the 180k α1(IV) one and not an alternatively spliced chain of the α1(IV) gene.