epistasis in an a-helix of ß-lactamase TEM-1

The effect of mutations in a protein may depend on the presence of others—a phenomenon known as epistasis. Epistasis plays a key role in protein evolution and complicates predictions of mutational effects, as effects can be context-dependent. Yet, despite its importance, the mechanistic basis of epistasis remains poorly understood. To better characterize epistasis, we focused on an 11-residue a-helix in TEM-1 ß-lactamase and constructed a comprehensive library of over 15,000 double mutants. Fitness and minimum inhibitory concentration assays, two contrasted measure of protein efficiency, revealed consistent widespread epistasis. A non-linear two-state protein stability model in which inactivating, destabilizing, neutral, or stabilizing mutations contribute additively to the stability phenotype, largely explained the data. Most epistatic effects were consequently predictable from single-mutation effects. However, systematic deviations from the model occurred when both mutated residues directly interacted in the 3D structure—a fold conserved across distant TEM-1 homologues. We therefore investigated the predictive power of statistical models trained on distant homologous sequences and found that they could partially recover the observed epistatic interactions. Our results, built on a short structural element of a protein, shed light on multiple determinants of the epistatic landscape that have shaped the evolutionary trajectory of ß-lactamase proteins over long timescales.