Use of the polymerase chain reaction to clone and sequence a cDNA encoding the bovine α3 chain of type IV collagen

A novel type IV collagen, α3(IV), has previously been isolated from a collagenase digest of bovine and human glomerular and lens basement membranes. The cloning and sequencing of a cDNA encoding the α3(IV) chain is described here. Using the polymerase chain reaction, with primers derived from the known 27-residue bovine α3(IV) amino acid sequence, a 68-base pair bovine genomic fragment (KEM68) which encodes the known peptide sequence, was synthesized. KEM68 was then used to screen a bovine lens cDNA library and a 1.5-kilobase partial cDNA clone obtained, encoding 471 residues of the bovine α3(IV) chain: 238 residues from the triple helical collagenous domain and all 233 residues of the noncollagenous domain. The collagenous repeat sequence has three interruptions, coinciding with those in the α1(IV) chain. The noncollagenous domain has 12 cysteine residues in identical positions to those of other type IV collagens and 71, 61, and 70% overall similarity with the human α1(IV), α2(IV), and α5(IV) chains. The noncollagenous domain of α3(IV) is of particular interest as it appears to be the component of glomerular basement membrane that reacts maximally with the Goodpasture antibody. Furthermore, such antigenicity is absent from collagenase digests of the glomerular basement membrane of some patients with Alport syndrome. The α3(IV) cDNA clone described here now permits study of the molecular pathology of COL4A3 in Alport syndrome.