Collagen IV NC1 domain

The Collagen IV NC1 domain is the noncollagenous protein domain at the C-terminus of a Collagen IV chain. NC1 regions participate in the end-to-end assembly of the network that supports a basement membrane. A three-chain Collagen IV protomer presents three NC1 regions at this end; association of two such ends forms a six-subunit Collagen IV NC1 hexamer.^[ref]

The counting is useful: a six-subunit junction joins two three-chain molecular units, rather than requiring each Collagen IV molecule to contain six chains. The Collagen IV NC1 hexamer describes a local meeting point in the larger scaffold.

The junction is reinforced by sulfilimine bonds between methionine and hydroxylysine residues of adjoining units. In 2009, mass spectrometry and nuclear magnetic resonance measurements on isolated linked peptides identified this bond. Chemical reduction separated the linked peptides, providing another test of the proposed structure. Up to six such links can reinforce the interface between the two NC1 ends.^[ref]

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.