Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.10.14.339671v1?rss=1 Authors: Morrison, A. J., Wonderlick, D. R., Harms, M. J. Abstract: Non-additivity between mutations--epistasis--profoundly shapes evolution. It can be difficult to understand its mechanistic origins. Here we show that "ensemble epistasis" is likely a universal feature of macromolecules. Using a simple analytical model, we found that epistasis arises when two conditions are met: 1) a macromolecule populates at least three structures and 2) mutations have differential effects on a least two of the inactive structures. To explore the relative magnitude of ensemble epistasis, we performed a virtual deep-mutational scan of the allosteric Ca2+ signaling protein S100A4. We found that 27% of mutation pairs gave ensemble epistasis with a magnitude on the order of thermal fluctuations, 1 kT. We observed many forms of epistasis: magnitude, sign, and reciprocal sign epistasis. Depending on the effector concentration, the same mutation pair could even exhibit different forms of epistasis. The ubiquity of ensembles in biology and its pervasiveness in our dataset suggests that ensemble epistasis may be a universal mechanism of epistasis. Copy rights belong to original authors. Visit the link for more info