Family permutation profiling identifies a dynamic protein domain as functionally tolerant to increased conformational entropy

To investigate whether adenylate kinase (AK) homologs differ in their functional tolerance to mutational lesions that alter dynamics, we subjected three homologs having a range of thermostabilities to random circular permutation and evaluated where new protein termini were non-disruptive to activity using a cellular selection and deep mutational scanning. Analysis of the positional tolerance to new termini, which increase local conformational entropy by breaking peptide bonds, showed that bonds were either functionally sensitive to cleavage across all three homologs, differentially sensitive, or uniformly tolerant. The mobile AMP binding domain, which displays the highest calculated contact energies (frustration), presented the greatest tolerance to new termini across all AKs. In contrast, retention of function in the lid and core domains was more dependent upon AK melting temperature. Thus, regions of high energetic frustration tolerated increases in conformational entropy in a manner that was less dependent on thermostability than regions of lower frustration. Our results suggest that family permutation profiling identifies primary structure that has been selected by evolution for high frustration that is critical to enzymatic activity. They also illustrate how deep mutational scanning can be applied to protein homologs in parallel to learn how topology and function govern mutational tolerance.