Restricted homology between human alpha 1 type IV and other procollagen chains

Screening of a human fibroblast cDNA library with a mouse type IV procollagen clone resulted in one 1.05-kilobase isolate that was used to identify a 1.7-kilobase clone overlapping the former by less than 150 nucleotides. EcoRII digestion revealed that the larger clone exhibited the pattern characteristic of DNA coding for a collagenous sequence. Blot hybridization to RNA from mouse F9 stem cells and from these cells treated with retinoic acid and N6, O2'-dibutyryl-cAMP showed induction of type IV mRNA. DNA sequencing and comparison of the derived amino acids with the reported protein data demonstrated that the clones encode part of the alpha 1 chain of human type IV procollagen. Alignment of alpha 1 (IV) with other human procollagens showed minimal but detectable homology. A small cluster of charged residues in the alpha chain is partially shared by type IV. In close proximity is an interruption in the alpha 1 (IV) Gly-Xaa-Yaa region corresponding to the 3' end of a unique proline-free, and therefore also less rigid, area in other collagen triple helices. Analysis of the carboxyl-terminal alpha 1 (IV) peptide showed a repeat symmetry possibly dictated by the six cysteines in each half of the structure. The position of five cysteines in addition to four tyrosine/tryptophan groups allowed a correlation to be drawn between the 3' noncollagenous type IV region and the larger, highly conserved carboxyl propeptides of other human procollagens. Such similarities in the different chains may define functional domains conserved throughout evolution.