Two New Collagen Subgroups: Membrane-associated Collagens and Types XV and XVIII

Publisher Summary This chapter discusses the two new subgroups among the large collagen gene family, the membrane-associated collagens, types XIII and XVII, and the subgroup formed, by collagen types XV and XVIII. Variant transcripts of collagens are discussed, as this phenomenon is pertinent for the members of both the new subgroups. The categorization of all known members of the collagen family into subgroups is also discussed in the chapter. The collagen types are numbered, by Roman numerals, in order of their discovery and the α- chains found in each collagen type are identified with Arabic numerals. Because some collagens consist of two or even more genetically distinct α-chains, over 30 genes exist to code for the constituent chains. In addition, several other proteins contain triple-helical Gly-X-Y sequences but are not formally classified as collagens, as they are not the structural components of the extracellular matrix. The significance of collagen types XIII, XV, and XVIII, identified solely on the grounds of their repetitive Gly-X-Y sequence, can only be fully assessed from a genetic analysis of mutations in the respective genes. It is, thus, hoped that gene “knockout” experiments in transgenic mice and the introduction of dominant-negative collagen mutations into mice will result in increased understanding of the function of these collagens. Furthermore, much information is needed on the chemical and physical nature of the proteins and their interactions in their native environment.