The NCI domain of collagen IV encodes a novel network composed of the alpha 1, alpha 2, alpha 5, and alpha 6 chains in smooth muscle basement membranes

Type IV collagen, the major component of basement membranes (BMs), is a fam ily of six homologous chains (alpha1-alpha6) that have a tissue-specific di stribution. The chains assemble into supramolecular networks that differ in the chain composition. In this study, a novel network was identified and c haracterized in the smooth muscle BMs of aorta and bladder. The noncollagen ous (NC1) hexamers solubilized by collagenase digestion were fractionated b y affinity chromatography using monoclonal antibodies against the alpha5 an d alpha6 NC1 domains and then characterized by two-dimensional gel electrop horesis and Western blotting. Both BMs were found to contain a novel alpha1 .alpha2.alpha5.alpha6 network besides the classical alpha1.alpha2 network. The alpha1.alpha2.alpha5.alpha6 network represents a new arrangement in whi ch a protomer (triple-helical isoform) containing the alpha5 and alpha6 cha ins is linked through NC1-NC1 interactions to an adjoining protomer compose d of the alpha1 and alpha2 chains. Re-association studies revealed that the NCI domains contain recognition sequences sufficient to encode the assembl y of both networks. These findings, together with previous ones, indicate t hat the six chains of type IV collagen are distributed in three major netwo rks (alpha1.alpha2 alpha3.alpha4.alpha5, and alpha1.alpha2.alpha5.alpha6) w hose chain composition is encoded by the NCI domains. The existence of the alpha1.alpha2.alpha5.alpha6 network provides a molecular explanation for th e concomitant loss of alpha5 and alpha6 chains from the BMs of patients wit h X-linked Alport's syndrome.