A Collagen IV protomer is a three-chain molecular unit of Collagen IV that assembles with other units to form a basement membrane network. Much of its length has a collagen triple helix, while its C-terminus presents three Collagen IV NC1 domains. Association of two such ends forms a Collagen IV NC1 hexamer.[ref]
A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others. Introduction and Figure 3; triple-helical protomers and end-to-end network assembly
The organizational levels can be counted separately. Ten intact three-chain Collagen IV protomers contain thirty polypeptide chains, but this count does not describe how the ten units are connected. A six-subunit NC1 junction joins two protomers at one end; it does not convert either protomer into a six-chain molecule.
The term is therefore useful when moving between a protein's molecular composition and the architecture of the scaffold it builds. Identifying a three-chain unit answers a different question from identifying the contacts and covalent bonds between neighbouring units.
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