Figure 2. Comparative analysis of evolutionary divergence for mitotic and flagellate tubulins.

(A) Plots of the normalized divergence score (STAR Methods) as a function of amino acid position for α-tubulin (top) and β-tubulin (bottom). Lower scores indicate positions where mitotic tubulins show increased divergence relative to flagellate tubulins. The analysis was performed on three species: N. gruberi (lavender diamonds), N. fowleri (navy circles), and A. kona (teal squares). The horizontal gray line indicates the two standard deviation cutoff we used to identify especially divergent sites.

(B) Structural context of the sites with increased divergence in the mitotic tubulins. Side chain positions for the N. gruberi amino acids identified in (A) are represented as sticks (blue) on a model of αβ-tubulin in the microtubule lattice (α-tubulin, pink; β-tubulin, lime). “Outside” and “inside” views of the lattice are shown, and longitudinal (labeled 1) and lateral (labeled 2) microtubule lattice contacts are indicated, as is the luminal (internal) surface of α-tubulin (labeled 3).

(C) Table summarizing the proportion of positions with elevated divergence near microtubule lattice interfaces. For all three species, there are more divergent positions in α-tubulin compared with β-tubulin, and the divergence seems to be particularly enriched at the lateral interfaces.

See also Figures S2 and S3, Data S5 and S6, and Table S2.