MUTATIONS IN THE ALPHA-2(IV) BASEMENT-MEMBRANE COLLAGEN GENE OF CAENORHABDITIS-ELEGANS PRODUCE PHENOTYPES OF DIFFERING SEVERITIES

Type IV collagen forms a network that provides the major structural su pport of basement membranes. We have determined the nucleotide alterat ions and phenotypes of 17 mutant alleles of the Caenorhabditis elegans alpha 2(IV) collagen gene let-2. All 17 mutations are within the trip le helical (Gly-X-Y) repeat domain of the molecule. Fifteen of the mut ations are replacements of Gly-X-Y repeat glycines with aspartate, glu tamate or arginine, and they cause a wide range of phenotypes. The mil dest alleles are nearly,wild-type at 15 and 20 degrees C but embryonic lethal at 25 degrees C, while the most severe allele is embryonic let hal at all three temperatures, Mutations resulting in severe phenotype s are generally located in areas of lower calculated thermal stability of the type IV collagen molecule. An alanine to threonine substitutio n at position X of a Gly-X-Y triplet immediately following an interrup tion results in a severe phenotype. This mutation is unusual because s ubstitutions at positions X or Y have not generally been found to caus e strong phenotypes in C.elegans or human collagens. An intron splice acceptor mutation causes a strict embryonic lethal phenotype, but does not completely abolish gene function. Pairs of independent mutations affect each of three glycines, indicating a non-random distribution of mutations in the molecule. It is suggested that this clustering resul ts because many glycine substitutions may cause dominant lethal or ste rile phenotypes.