The occurrence of two types of collagen proα‐chain in the abalone Haliotis discus muscle

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Abstract
Acid‐soluble collagens were prepared from connective tissues in the abalone Haliotis discus foot and adductor muscles with limited proteolysis using pepsin. Collagen preparation solubilized with 1% pepsin contained two types of α‐chains which were different in their N‐terminal amino acid sequences. Accordingly, two types of full‐length cDNAs coding for collagen proα‐chains were isolated from the foot muscle of the same animal and these proteins were named Hdcols (Haliotis discus collagens) 1α and 2α. The two N‐terminal amino acid sequences of the abalone pepsin‐solubilized collagen preparation corresponded to either of the two sequences deduced from the cDNA clones. In addition, several tryptic peptides prepared from the pepsin‐solubilized collagen and fractionated by HPLC showed N‐terminal amino acid sequences identical to those deduced from the two cDNA clones. Hdcols 1α and 2α consisted of 1378 and 1439 amino acids, respectively, showing the primary structure typical to those of fibril‐forming collagens. The N‐terminal propeptides of the two collagen proα‐chains contained cysteine‐rich globular domains. It is of note that Hdcol 1α completely lacked a short Gly‐X‐Y triplet repeat sequence in its propeptide. An unusual structure such as this has never before been reported for any fibril‐forming collagen. The main triple‐helical domains for both chains consisted of 1014 amino acids, where a supposed glycine residue in the triplet at the 598th position from the N‐terminus was replaced by alanine in Hdcol 1α and by serine in Hdcol 2α. Both proα‐chains of abalone collagens contained six cysteine residues in the carboxyl‐terminal propeptide, lacking two cysteine residues usually found in vertebrate collagens. Northern blot analysis demonstrated that the mRNA levels of Hdcols 1α and 2α in various tissues including muscles were similar to each other.