Acquired functional redundancy of an invariant amino acid residue in an evolutionarily conserved protein family

Abstract

AbstractMany amino acid residues at specific locations in a protein family are conserved across evolution. Does a specific substitution in a conserved amino acid residue of a protein influence its function differently across evolution? To understand such functional evolvability of an amino acid, we identified an evolutionarily conserved arginine residue at the C-terminus of an essential fungus-specific outer kinetochore protein Dad2 across three phylogenetically diverged species: Saccharomyces cerevisiae, Candida albicans and Cryptococcus neoformans. The R126 residue of Dad2 is essential for viability in S. cerevisiae carrying a single nucleosome-length long point centromere. The corresponding arginine residues are functionally important but not essential for viability of C. albicans and Cryptococcus neoformans both of which possess regional centromeres on several kilobases of DNA sequence. Functional analysis further revealed a progressive loss in the requirement of the conserved arginine residue for chromosome biorientation and mitotic progression with increasing centromere length. We propose that the mutational tolerance of the conserved arginine of Dad2 in organisms with regional centromeres is imparted by their ability to conditionally elevate the kinetochore protein levels to enable multiple kMT binding to each chromosome. The capacity of a chromosome to bind multiple kMTs may act as a failsafe mechanism enabling tolerance towards such lethal mutations.