Episodic evolution of coadapted sets of amino acid sites in mitochondrial proteins

The rate of evolution differs between protein sites and changes with time. However, the link between these two phenomena remains poorly understood. Here, we design a phylogenetic approach for distinguishing pairs of amino acid sites that undergo coordinated evolution, i.e., such that substitutions at one site trigger subsequent substitutions at the other; and also pairs of sites that undergo discordant evolution, so that substitutions at one site impede subsequent substitutions at the other. We distinguish groups of amino acid sites that undergo coordinated evolution and evolve discordantly from other such groups. In mitochondrially encoded proteins of metazoans and fungi, we show that concordantly evolving sites are clustered in protein structures. Moreover, the substitution rates within individual concordant groups themselves change in the course of evolution, and phylogenetic positions of these changes are consistent between proteins, suggesting common selective forces underlying them. Author summaryEvolution in most protein sites is constrained by alleles in other sites, this phenomena is called epistasis. Generally, newly arisen allele, if it would not be lost, increases its fitness with time due to adaptive mutations occurred in other sites in the process called the entrenchment. Thus, we expect that the evolution of sites would be coordinated, such that mutations in sites rapidly follow each other on phylogenetic lineages. Indeed, we have observed such coordinated evolution of sites in five proteins encoded in the mitochondria genomes of metazoan and fungi. Unexpectedly, coordinated evolution is observed only for nearby sites on protein structures, such that each protein could be partitioned into several groups of concordantly evolving sites. Evolution of sites from different groups is discordant, i.e. their mutations repel each other into different phylogenetic lineages or clades. Thus, the proteins encoded in mitochondrial genome consist of the sort of structural blocks like elements of the LEGO kit. Some of them have functional specialization, e.g. some blocks are associated with interfaces between proteins composing respiratory complexes.