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 mutant 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 the mutant actin. Remarkably, we have also found that mutant Sac6 proteins bind more tightly to wild-type actin than does wild-type Sac6p. This result indicates that suppression does not occur through the restoration of the original contact site, but rather through the formation of a novel contact site. This finding argues against suppression occurring through a "lock-and-key" mechanism and suggests a mechanism involving more global increases in affinity between the two proteins. We propose that the most common kind of suppressors involving interacting proteins will likely occur through this less specific mechanism.