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Crystal Structure of an Ancient Protein: Evolution by Conformational Epistasis
Eric A. Ortlund,1*Jamie T. Bridgham,2*Matthew R. Redinbo,1Joseph W. Thornton2
The structural mechanisms by which proteins have evolved newfunctions are known only indirectly. We report x-ray crystalstructures of a resurrected ancestral protein—the 450million-year-old precursor of vertebrate glucocorticoid (GR)and mineralocorticoid (MR) receptors. Using structural, phylogenetic,and functional analysis, we identify the specific set of historicalmutations that recapitulate the evolution of GR's hormone specificityfrom an MR-like ancestor. These substitutions repositioned crucialresidues to create new receptor-ligand and intraprotein contacts.Strong epistatic interactions occur because one substitutionchanges the conformational position of another site. "Permissive"mutations—substitutions of no immediate consequence, whichstabilize specific elements of the protein and allow it to toleratesubsequent function-switching changes—played a major rolein determining GR's evolutionary trajectory.
1 Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599, USA. 2 Center for Ecology and Evolutionary Biology, University of Oregon, Eugene, OR 97403, USA.
* These authors contributed equally to this work.
To whom correspondence should be addressed. E-mail: joet{at}uoregon.edu
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