Fetal membrane collagens: identification of two new collagen alpha chains.

Human fetal membranes contain two new genetically distinct collagen polypeptide chains which are subunits of one (or two) new molecular species of collagen. These new polypeptide chains, which we have tentatively named alphaA and alphaB, have been directly compared with the polypeptide chain subunits of Types I, II, and III human collagen and Type IV collagen from bovine lens capsule. Both alphaA and alphaB exhibit characteristic profiles on carboxymethyl-cellulose chromatography and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The distribution of methionine residues along both new chains is different from known collagen chains as manifest by distinctly different cyanogen bromide peptide profiles on carboxymethyl-cellulose chromatography and/or sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Both alphaA and alphaB exhibit contents of amino acids and glycine typical of collagens, and comparison with the observed and reported compositions of collagen chains of Types I-IV collagens reveals notable differences, particularly in the content of alanine, leucine, isoleucine, and the basic amino acids, lysine, hydroxylysine, and arginine. The new collagen species containing both alphaA and alphaB may be separated in the native (triple-helical) state from other native collagen species by differential salt precipitation. The observations that both chains coprecipitate in the same narrow NaCl range, and that the ratio of alphaA:alphaB is constant, suggest the possibility of a single new species of collagen with a subunit structure alphaA alphaB2.