Reductive cleavage and reformation of the interchain and intrachain disulfide bonds in the globular hexameric domain NC1 involved in network assembly of basement membrane collagen (type IV)

The formation of collagen IV dimers in the extracellular space requires the association of two C-terminal globular domains giving rise to a large hexameric structure NC1 (Mr = 170,000). NC1 hexamer was purified from collagenase digests of a mouse tumor and several human tissues. It was shown by electrophoresis to consist of two kinds of cross-linked, dimeric segments, Da and Db (Mr about 50,000), and monomeric segments in a molar ratio of about 3:1. In the native hexamers free SH groups were detectable by N-14C]ethylmaleimide and other sulfhydryl reagents. They account for 4-11% of the total number of cysteine residues with some variations between preparations from different sources and in the distribution between monomers and dimers. Reduction with 10 mM dithioerythritol under non-denaturing condition completely converted dimers into monomers and allowed the alkylation of all twelve cysteine residues present in each monomeric NC1 segment. A monomeric intermediate with four to six free SH groups and a higher electrophoretic mobility than the final product was observed. Generation of this intermediate from dimers Da and Db follows apparently different routes proceeding either directly or through a dimeric intermediate respectively. The time course of conversion is best described by a mechanism consisting of two (Db) or three (Da) consecutive steps with pseudo-first-order rate constants ranging from 0.14 ms-1 to 0.5 ms-1. Glutathione-catalyzed reoxidation of completely reduced NC1 in the presence of 2 M urea results in a product indistinguishable from native material by ultracentrifugation and electrophoresis pattern. The data suggest that in situ formation of NC1 structures is catalyzed by a small fraction (5-10%) of intrinsic SH groups leading to the formation and stabilization of dimers by rearrangement of disulfide bonds.