ALLELE-SPECIFIC SUPPRESSION BY FORMATION OF NEW PROTEIN-PROTEIN INTERACTIONS IN YEAST

Yeast fimbrin is encoded by the SAC6 gene, mutations of which suppress temperature-sensitive mutations in the actin gene (ACT1). To examine the mechanism of suppression, we have conducted a biochemical analysis of the interaction between Various combinations of wild-type and muta nt actin and Sac6 proteins. Previously, we showed that actin mutations that are suppressed by sac6 mutations encode proteins with a reduced affinity for wild-type Sac6p. In the present study, we have found that mutant Sac6 proteins bind more tightly to mutant actin than does wild -type Sac6p, and thus compensate for weakened interactions caused by t he mutant actin. Remarkably, we have also found that mutant Sac6 prote ins bind more tightly to wild-type actin than does wild-type Sac6p. Th is result indicates that suppression does not occur through the restor ation of the original contact site, but rather through the formation o f a novel contact site. This finding argues against suppression occurr ing through a ''lock-and-key'' mechanism and suggests a mechanism invo lving more global increases in affinity between the two proteins. We p ropose that the most common kind of suppressors involving interacting proteins will likely occur through this less specific mechanism.