Collagen IV, also called type IV collagen, is the network-forming collagen that provides a major scaffold of basement membranes. Three polypeptide chains assemble into a molecular unit, and units associate through their ends into a sheet-like network. This organization suits a thin supporting layer beneath an epithelium, rather than a long cable made of parallel molecules.[ref]
A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others. Introduction and Figure3: protomers and basement-membrane network architecture
The Collagen IV NC1 domain is a noncollagenous end region where adjoining molecular units meet. A sulfilimine bond links a methionine side chain in one unit to a hydroxylysine side chain in another. Thus the scaffold combines molecular association with a chemically specific covalent bond.[ref]
A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others. Figures1–3: identification and placement of the crosslink
One can distinguish two questions about a basement membrane: which proteins form its scaffold, and how neighboring units are fastened together. Isolating linked peptides and comparing their mass spectrometry and nuclear magnetic resonance signals answered the second question. The investigation connected a tissue-scale network to the identities of individual amino acid residues.[ref]
A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others. Table1 and Figures1–2: crosslinked-peptide measurements
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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