Type XXVI collagen, a new member of the collagen family, is specifically expressed in the testis and ovary.

HSP47 is a collagen-specific molecular chaperone that specifically recognizes and binds to the triple helical domain of various types of collagens. Here we report the cloning of the entire coding region of a novel collagen-like protein by yeast two-hybrid screening of a 17.5-day whole mouse embryo cDNA library using HSP47 as a bait. The cDNA of this protein and its deduced amino acid sequence are 2,690 bp and 438 amino acids long, respectively. The protein contains two clusters of Gly-X-Y collagenous repeats and three noncollagenous domains. Northern blot analysis showed that its mRNA is specifically expressed in the testis and ovary in adult tissues and that expression in these tissues is highest in the neonate. Biochemical characterization of this protein revealed that its proline residues are hydroxylated, it undergoes N-linked glycosylation, it forms trimers, and it is secreted in vitro. Immunohistochemical studies showed that the myoid cells and the pre-theca cells synthesized it in the testis and ovary, respectively, resulting in the accumulation of this protein in the extracellular spaces of these organs. These observations suggest that this protein is a new member of the collagen protein family. We thus designated this protein as type XXVI collagen.