The NC1 Domain of Collagen IV Encodes a Novel Network Composed of the α1, α2, α5, and α6 Chains in Smooth Muscle Basement Membranes

Abstract

Type IV collagen, the major component of basement membranes (BMs), is a family of six homologous chains (α1-α6) that have a tissue-specific distribution. The chains assemble into supramolecular networks that differ in the chain composition. In this study, a novel network was identified and characterized in the smooth muscle BMs of aorta and bladder. The noncollagenous (NC1) hexamers solubilized by collagenase digestion were fractionated by affinity chromatography using monoclonal antibodies against the α5 and α6 NC1 domains and then characterized by two-dimensional gel electrophoresis and Western blotting. Both BMs were found to contain a novel α1·α2·α5·α6 network besides the classical α1·α2 network. The α1·α2· α5·α6 network represents a new arrangement in which a protomer (triple-helical isoform) containing the α5 and α6 chains is linked through NC1-NC1 interactions to an adjoining protomer composed of the α1 and α2 chains. Re-association studies revealed that the NC1 domains contain recognition sequences sufficient to encode the assembly of both networks. These findings, together with previous ones, indicate that the six chains of type IV collagen are distributed in three major networks (α1·α2, α3·α4·α5, and α1·α2·α5·α6) whose chain composition is encoded by the NC1 domains. The existence of the α1·α2·α5·α6 network provides a molecular explanation for the concomitant loss of α5 and α6 chains from the BMs of patients with X-linked Alport's syndrome.