LAUSR

Chaperones facilitate the folding of other ("client") proteins, and can thus affect the adaptive evolution of these clients. Specifically, chaperones affect the phenotype of proteins via two opposing mechanisms. On the one hand, they can buffer the effects of mutations in proteins and thus help preserve an ancestral, pre-mutation phenotype. On the other hand, they can potentiate the effects of mutations and thus enhance the phenotypic changes caused by a mutation. We study how the bacterial Hsp90 chaperone (HtpG) affects the evolution of green fluorescent protein (GFP). To this end, we performed directed evolution of GFP under low and high cellular concentrations of Hsp90. Specifically, we evolved GFP under both stabilizing selection for its ancestral (green) phenotype, and directional selection towards a new (cyan) phenotype. While Hsp90 did only affect the rate of adaptive evolution transiently, it did affect the phenotypic effects of mutations that occurred during adaptive evolution. Specifically, Hsp90 allowed strongly deleterious mutations to accumulate in evolving populations by buffering their effects. Our observations show that the role of a chaperone for adaptive evolution depends on the organism and the trait being studied.