title: Stability-Mediated Epistasis Restricts Accessible Mutational Pathways in the Functional Evolution of Avian Hemoglobin.
abstract: If the fitness effects of amino acid mutations are conditional on genetic background, then mutations can have different effects depending on the sequential order in which they occur during evolutionary transitions in protein function. A key question concerns the fraction of possible mutational pathways connecting alternative functional states that involve transient reductions in fitness. Here we examine the functional effects of multiple amino acid substitutions that contributed to an evolutionary transition in the oxygenation properties of avian hemoglobin (Hb). The set of causative changes included mutations at intradimer interfaces of the Hb tetramer. Replacements at such sites may be especially likely to have epistatic effects on Hb function since residues at intersubunit interfaces are enmeshed in networks of salt bridges and hydrogen bonds between like and unlike subunits; mutational reconfigurations of these atomic contacts can affect allosteric transitions in quaternary structure and the propensity for tetramer-dimer dissociation. We used ancestral protein resurrection in conjunction with a combinatorial protein engineering approach to synthesize genotypes representing the complete set of mutational intermediates in all possible forward pathways that connect functionally distinct ancestral and descendent genotypes. The experiments revealed that 1/2 of all possible forward pathways included mutational intermediates with aberrant functional properties because particular combinations of mutations promoted tetramer-dimer dissociation. The subset of mutational pathways with unstable intermediates may be selectively inaccessible, representing evolutionary roads not taken. The experimental results also demonstrate how epistasis for particular functional properties of proteins may be mediated indirectly by mutational effects on quaternary structural stability.
authors.affiliation: School of Biological Sciences, University of Nebraska, Lincoln, NE.
authors.forename: Amit
authors.initials: A
authors.lastname: Kumar
authors.affiliation: School of Biological Sciences, University of Nebraska, Lincoln, NE.
authors.forename: Chandrasekhar
authors.initials: C
authors.lastname: Natarajan
authors.affiliation: School of Biological Sciences, University of Nebraska, Lincoln, NE.
authors.forename: Hideaki
authors.initials: H
authors.lastname: Moriyama
authors.affiliation: Department of Biology, University of New Mexico, Albuquerque, NM.|Museum of Southwestern Biology, University of New Mexico, Albuquerque, NM.
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Witt
authors.affiliation: Zoophysiology, Department of Bioscience, Aarhus University, Aarhus, Denmark.
authors.forename: Roy E
authors.initials: RE
authors.lastname: Weber
authors.affiliation: Zoophysiology, Department of Bioscience, Aarhus University, Aarhus, Denmark.
authors.forename: Angela
authors.initials: A
authors.lastname: Fago
authors.affiliation: School of Biological Sciences, University of Nebraska, Lincoln, NE.
authors.forename: Jay F
authors.initials: JF
authors.lastname: Storz
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D005075:Biological Evolution; D001717:Birds / Q000235:genetics*; D003198:Computer Simulation; D004843:Epistasis, Genetic / Q000235:genetics*; D019143:Evolution, Molecular; D056084:Genetic Fitness / Q000235:genetics; D058685:Genetic Pleiotropy / Q000235:genetics; D005838:Genotype; D006454:Hemoglobins / Q000235:genetics* / Q000378:metabolism; D009154:Mutation
chemical_list: D006454:Hemoglobins
keywords: Caprimulgidae; epistasis; fitness landscape; hemoglobin; nightjars; pleiotropy