Evolution Of Oligomeric State Through Allosteric Pathways That Mimic Ligand Binding

INTRODUCTION: Evolution and design of protein complexes are frequently viewed through the lens of amino acid mutations at protein interfaces, but we showed previously that residues distant from interfaces are also commonly involved in the evolution of alternative quaternary structures. We hypothesized that in these protein families, the difference in oligomeric state is due to a change in intersubunit geometry. The indirect mutations would act by changing protein conformation and dynamics, similar to the way in which allosteric small molecules introduce functional conformational change. We refer to these substitutions as "allosteric mutations."RATIONALE: In this work, we investigate the mechanism of action of allosteric mutations on oligomeric state in the PyrR family of pyrimidine operon attenuators. In this family, an entirely sequence-conserved helix that forms a tetrameric interface in the thermophilic ortholog (BcPyrR) switches to being solvent-exposed in the mesophilic ortholog (BsPyrR). This results in a homodimeric structure in which the two subunits are clearly rotated relative to their orientation in the tetramer. What is the origin of this rotation and the change in quaternary structure? To dissect the role of the 49 substitutions between BsPyrR and BcPyrR, we used ancestral sequence reconstruction in combination with structural and biophysical methods to identify a set of allosteric mutations that are responsible for this shift in conformation. We compared the conformational changes introduced by the mutations to the protein motion during allosteric regulation by guanosine monophosphate (GMP).RESULTS: We identified 11 key mutations controlling oligomeric state, all distant from the interfaces and outside ligand-binding pockets. We confirmed the role of these allosteric mutations by engineering a shift in oligomeric state in an inferred ancestral PyrR protein (intermediate in sequence between the extant orthologs). We further used the inferred ancestral states and their mutants to show that the allosteric mutations are part of a downhill adaptation of the PyrR proteins to lower temperatures. We compared the x-ray crystal structures of ancestral and engineered PyrR proteins to the free and GMP-bound structure of the mesophilic BsPyrR, which shifts its equilibrium from dimer to tetramer upon ligand binding. Binding of the allosteric molecule introduces a change in intersubunit geometry that is equivalent to the evolutionary difference in intersubunit geometry between the dimeric and tetrameric homologs. We further find that the difference in oligomeric state is coupled to the difference in intrinsic dynamics of the dimers. Finally, we used the residue-residue contact network approach to show that the residues corresponding to the allosteric mutations undergo large contact rewiring when the intersubunit geometry and, in turn, oligomeric state change, either by GMP binding or by the introduction of allosteric mutations.CONCLUSION: We show that evolution employs the intrinsic dynamics of this protein to toggle a conformational switch in a manner similar to that of small molecules. Shifting the relative populations of different states by subtle modifications is a process central to protein function and, as shown here, also to protein evolution. This suggests that we can learn from evolution and design proteins with multiple conformational states.