Mutations in the regulatory domain of cystathionine beta-synthase can functionally suppress patient-derived mutations in cis

Human cystathionine P-synthase (CBS) is an S- adenosylmethionine-regulated enzyme that plays a key role in the metabolism of homocysteine. Mutations i n CBS are known to cause homocystinuria, an inborn error in metabolism. We previously developed a yeast functional assay for CBS and used it to charac terize mutations found in homocystinuric patients. We discovered that many patient-derived mutations are functionally suppressed by deletion of the C- terminal 142 amino acids, which contain a 53 amino acid motif known as the CBS domain. This domain is found in a wide variety of proteins of diverse b iological function. Here we have used a genetic screen to identify missense mutations in the C-terminal region of CBS that can suppress the most commo n patient mutation, 1278T. Seven suppressor mutations were identified, four of which map to the CBS domain. When combined in cis with another pathogen ic mutation, V168M, six of seven of the suppressor mutations rescued the ye ast phenotype, Enzyme activity analyses indicate that the suppressors resto re activity from <2% to 17-64% of the wildtype levels. Analysis of the supp ressor mutations in the absence of the pathogenic mutation shows that six o f the seven suppressor alleles have lost enzymatic responsiveness to Sadeno sylmethionine. Using homology modeling, we show that the suppressor mutatio ns appear to map on one face of the CBS domain. Our results indicate that s ubtle changes to the C-terminus of CBS can restore activity to mutant prote ins and provide a rationale for screening for compounds that can activate m utant CBS alleles.