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General Comment |
Collagen has a triple-stranded ropelike coiled structure. The major collagen of skin, tendon, and bone is the same protein
containing 2 alpha-1 polypeptide chains and 1 alpha-2 chain. Sato K, et al reported that 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. The authors 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 2690 bp and 438 amino acids long, respectively. The protein contains two clusters of
Gly-X-Y collagenous repeats and three non-collagenous 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 and was designated as type XXVI collagen.
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