CLONING AND EXPRESSION OF A MUTANT METHYLMALONYL COENZYME-A MUTASE WITH ALTERED COBALAMIN AFFINITY THAT CAUSES MUT- METHYLMALONIC ACIDURIA

Distinct genotypic and phenotypic forms of methylmalonyl CoA mutase (MCM) apoenzyme deficiency can be delineated by biochemical analysis of mutant fibroblasts. One form, designated mut-, expresses a phenotype in which residual enzyme activity is evident in cultured cells exposed to high concentrations of hydroxycobalamin. We describe cloning of an MCM cDNA from cells exhibiting a mut- phenotype and characterization of the mutant gene product overexpressed in primary mut(o) human fibroblasts and Saccharomyces cerevisiae. Three novel base changes were observed. Recombinant clones containing one of these base changes (G717V) express four characteristics of the mut- phenotype: failure to constitute [C-14]propionate incorporation activity in fibroblasts assayed under basal cell culture conditions, constitution of [C-14]propionate incorporation activity in fibroblasts stimulated with 0.1-1.0-mu-g/ml hydroxycobalamin, interallelic complementation with alleles bearing an R93H mutation, and an apparent K(m) (adenosylcobalamin) 1,000-fold higher than normal. These results demonstrate that the G717V mutation produces the mut- phenotype and localizes determinants for adenosylcobalamin binding near the carboxyl terminus of MCM.