Department of Food Science and Technology, Tokyo University of Fisheries, Minato, Tokyo 108-8477, Japan
Fish type I collagen is widely distributed in skin, bone, scale, muscle and other internal organs. In addition to α1(I) and α2(I), many bony fish are characterized by having a third subunit, α3(I), which is not found in any other vertebrates. This unique α3(I) chain forms α1(I)α2(I)α3(I) heterotrimers. Recently, cloning of the cDNAs from rainbow trout fibroblasts has revealed the complete primary structure of three distinct collagen proα(I) chains. The proα1(I) and proα2(I) chains share a common structure with those of higher vertebrates. From the high level of sequence homology between proα1(I) and proα3(I), we deduce that the proα3(I) gene appears to have arisen from a duplication of the proα1(I) gene near the time of the adaptive radiation of bony fish. Furthermore, the internal organs of cyclostomes, which belong to the lowest class of vertebrates, contain an unusual fibrillar collagen. This body collagen is genetically distinct from the skin type I collagen and is similar in its amino acid composition to major fibrillar collagens from some invertebrates such as octopus, squid and sea urchin. Thus, we speculate that type I collagen might have appeared first in the skin of primitive vertebrates.