Functional NC1 domains control collagen IV protomer and network assembly. (A) Collagen IV NC1 domains nucleate protomer assembly by controlling chain stoichiometry, specificity, and registration. (B) Extracellular Cl⁻ concentrations signal protomers to oligomerize into networks, forming NC1 hexamers at the protomer–protomer junctions. At the NC1 junctions, hexamers are reinforced with sulfilimine cross-links by PXDN and Br⁻. (C) Assembled collagen IV networks function as a scaffold for binding other macromolecules and growth factors in BMs.