= Collagen IV
{c}
<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.<A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others>[Introduction and Figure3: protomers and basement-membrane network architecture]{ref}
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>.<A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others>[Figures1–3: identification and placement of the crosslink]{ref}
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.<A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others>[Table1 and Figures1–2: crosslinked-peptide measurements]{ref}
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