Asymmetric requirement for α-tubulin over β-tubulin

How cells regulate the supply of α- and β-tubulin monomers to meet the demand for αβ- heterodimers while avoiding consequences of monomer imbalance is not understood. We investigate the role of gene copy number in tubulin regulation and how shifting the expression of α- or β-tubulin genes impacts tubulin proteostasis and microtubule function. We find that α- tubulin gene copy number is important for maintaining an excess α-tubulin protein compared to β-tubulin protein and preventing accumulation of super-stoichiometric β-tubulin. Super- stoichiometric β-tubulin is toxic to cells, leading to loss of microtubules, formation of non- microtubule assemblies of tubulin, and disrupted cell proliferation. In contrast, decreased β- tubulin or increased α-tubulin has minor effects. We provide evidence that cells rapidly equilibrate the concentration of α-tubulin protein during shifts in α-tubulin isotype expression to maintain a ratio in excess of β-tubulin. We propose an asymmetric relationship between α- and β-tubulins, where α-tubulins are maintained in excess to supply αβ-heterodimers and limit the accumulation of β-tubulin monomers.