Collagen IV NC1 hexamer

A Collagen IV NC1 hexamer is a six-subunit junction formed when the C-terminus ends of two Collagen IV protomers associate. Each three-chain protomer contributes three Collagen IV NC1 domains. The resulting arrangement is therefore two three-subunit ends joined together, one local connection in a basement membrane scaffold.^[ref]

Up to six sulfilimine bonds can reinforce the interface between the two ends. Analysis of linked peptides by mass spectrometry and nuclear magnetic resonance, followed by chemical reduction that separated the linked components, established the chemistry of this reinforcement.^[ref]

This provides a useful distinction between subunit counting and bond counting. Six subunits describes the intact junction's architecture; the number of reinforcing links is another property. Neither number implies that a single Collagen IV protomer has six polypeptide chains.