Characterization and putative post-translational regulation of α- and β-tubulin gene families in Salix arbutifolia

Microtubules, which are composed of heterodimers of alpha-tubulin (TUA) and beta-tubulin (TUB) proteins, are closely associated with cellulose microfibril deposition and play pivotal roles in plant secondary cell wall development. In the present study, we identified eight TUA and twenty TUB genes in willow (Salix arbutifolia). Quantitative real-time PCR analysis showed that the small number of TUA gene family members relative to that of TUBs was complemented by a higher transcript copy number for each TUA gene, which is essential to the maintenance of the tubulin 1:1 heterodimer assembly. In Salix, five of eight TUAs were determined to be unusual because these contained a C-terminal methionine acid, leucine acid, glutamic acid, and glutamine acid, instead of the more typical tyrosine residue, which in turn generated the hypothesis of post-translational modifications (PTMs) that included deleucylation, demethiolation, deglutamynation, and deaspartylation. These PTMs are responsible for the removal of additional amino acid residues from TUAs prior to detyrosination, which is the first step of C-terminal PTMs. The additional PTMs of the TUA gene family might be responsible for the formation of different tubulin heterodimers that may have diverse functions for the adaptation of the woody perennial growth for Salix.