Epistasis among adaptive mutations in deer mouse hemoglobin

Abstract

Epistatic interactions between mutant sites in the same protein can exert a strong influence on pathways of molecular evolution. We performed protein engineering experiments that revealed pervasive epistasis among segregating amino acid variants that contribute to adaptive functional variation in deer mouse hemoglobin (Hb). Amino acid mutations increased or decreased Hb-O2 affinity depending on the allelic state of other sites. Structural analysis revealed that epistasis for Hb-O2 affinity and allosteric regulatory control is attributable to indirect interactions between structurally remote sites. The prevalence of sign epistasis for fitness-related biochemical phenotypes has important implications for the evolutionary dynamics of protein polymorphism in natural populations.

Fig. 1. Structural and functional variation among recombinant deer mouse Hbs (rHbs)

(A) rHbs representing all combinatorial permutations of allelic variants at α-globin exon 2, α-globin exon 3, and β-globin. Shaded regions represent the products of low-altitude L-type alleles (red), and unshaded regions represent products of high-altitude H-type alleles (blue). (B) Variation in the allosteric regulation of Hb-O₂ affinity by DPG. Sensitivity to DPG is indexed by the difference in log-transformed P₅₀ values between stripped Hb in the presence and absence of DPG.

Fig. 2. Difference in the network of hydrogen bonds between high- and low-altitude Hb variants, HH-H and LL-L, respectively

The α1 and β1 subunits of HH-H (light blue) and LL-L (light red) are superimposed, and van der Waals radii are shown for α-chain residues that are in atomic contact with β-chain residues of the opposing α2β2 dimer.