Three novel collagen VI chains, α4(VI), α5(VI), and α6(VI)

We report the identification of three new collagen VI genes at a single locus on human chromosome 3q22.1. The three new genes are COL6A4, COL6A5, and COL6A6 that encode the alpha 4(VI), alpha 5(VI), and alpha 6(VI) chains. In humans, the COL6A4 gene has been disrupted by a chromosome break. Each of the three new collagen chains contains a 336-amino acid triple helix flanked by seven N-terminal von Willebrand factor A-like domains and two (alpha 4 and alpha 6 chains) or three (alpha 5 chain) C-terminal von Willebrand factor A-like domains. In humans, mRNA expression of COL6A5 is restricted to a few tissues, including lung, testis, and colon. In contrast, the COL6A6 gene is expressed in a wide range of fetal and adult tissues, including lung, kidney, liver, spleen, thymus, heart, and skeletal muscle. Antibodies to the alpha 6(VI) chain stained the extracellular matrix of human skeletal and cardiac muscle, lung, and the territorial matrix of articular cartilage. In cell transfection and immuno-precipitation experiments, mouse alpha 4(VI) N6-C2 chain co-assembled with endogenous alpha 1(VI) and alpha 2(VI) chains to form trimeric collagen VI molecules that were secreted from the cell. In contrast, alpha 5(VI) N5-C1 and alpha 6(VI) N6-C2 chains did not assemble with alpha 1(VI) and alpha 2(VI) chains and accumulated intracellularly. We conclude that the alpha 4(VI) N6-C2 chain contains all the elements necessary for trimerization with alpha 1(VI) and alpha 2(VI). In summary, the discovery of three additional collagen VI chains doubles the collagen VI family and adds a layer of complexity to collagen VI assembly and function in the extracellular matrix.