Distinct non-collagen based cartilages comprising the endoskeleton of the Atlantic hagfish, Myxine glutinosa

Previous evidence from our laboratories showed that collagen is not the major matrix protein of the cartilaginous endoskeleton of the lamprey (Petromyzon marinus). Here we have characterized the cartilage matrix proteins of the only other extant agnathan, the hagfish (Myxine glutinosa). Using morphological, immunochemical and biochemical methods, we show that the structural proteins of the cartilaginous endoskeleton of the hagfish are also non-collagenous in nature. Although these hagfish cartilage proteins share properties both with each other and with lamprey cartilage proteins, including resistance to solubilization with cyanogen bromide and an usual amino acid composition rich in glycine and non-polar amino acids, it is clear that at least two and probably more hagfish cartilage proteins can be distinguished, with distinct distributions in different cartilage structures. Furthermore, in spite of their similarities, matrix proteins from hagfish cartilage are not identical to the proteins we have previously characterized in lamprey cartilage. These results suggest the existence of a larger family of similar but not identical proteins that form the major structural elements of cartilage tissues of agnathans. These data also support our previous conclusion that type II collagen became the predominant structural protein of cartilage only after the divergence of the agnathans from the ancestral line of the vertebrates.