A central question in protein molecular evolution is whether an amino acid that occurs at a given site makes an independent contribution to fitness or whether its contribution depends on other amino acid sites in the protein sequence, a phenomenon known as intragenic epistasis. In the absence of intragenic epistasis, natural selection acts on a protein mutation independent of its genetic background, and the experimentally determined fitness for a mutation should be the same across all genetic backgrounds. We tested this hypothesis by using site-directed mutagenesis to introduce a well-characterized deleterious single amino acid substitution in 56 different hepatitis C virus NS3 protease variants. The catalytic efficiency of the mutated proteases was determined and compared with the corresponding wild-type protein. Fitness effects ranged from lethality to significantly beneficial. Although primarily deleterious and lethal effects were observed (41 and 5 out of 56 tested variants, respectively), deleterious effects ranged from near neutral (-26.7 reduction of fitness) to near lethal (-98.5). Our findings demonstrate that the introduced amino acid substitution has different fitness effects in different protein variants and provide independent support for the relevant role of intragenic epistasis in protein evolution.
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