Characterization of Selachian Egg Case Collagen

The egg case of the dogfishScyliorhinus caniculais a remarkable collagenous structure that combines mechanical strength and toughness with high permeability to small molecules and ions. The collagenous lamellae that form over 80% of the thickness of the case wall are secreted by the D-zone of the nidamental (oviducal gland). An acid-soluble collagen extracted from this zone and partially purified ran as a single band on a native gel at pH 4.3. A single band of identical mobility was extracted from egg cases removed from the oviducal gland. SDS–PAGE of both extracts revealed a major component with an apparent molecular weight of 35 kDa and a minor component at 34 kDa. Neither of these components appeared to be glycosylated. Amino acid analysis of the partially purified collagen extracted from the oviducal gland revealed a composition similar to that of the collagenous lamellae of the egg case with glycine accounting for 16% and imino acids for 10% of the total residues. Partial N-terminal and internal sequences were obtained by Edman degradation for peptides extracted from the D-zone of the nidamental gland. Four of the internal sequence fragments showed repeated G-X-Y triplets showing them to be collagenous. These four fragments were novel but showed similarity to the triple-helical domains of mammalian type IV, X, and VI collagens. The noncollagenous N-terminal and pepsin-resistant sequences were unique, showing no significant similarity to known proteins in the database. Several possible N-myristoylation and phosphorylation sites were identified in the noncollagenous sequences.