Asymmetric requirement for α-tubulin over β-tubulin

How cells regulate the supply of - and {beta}-tubulin monomers to meet the demand for {beta}- 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 - or {beta}-tubulin genes impacts tubulin proteostasis and microtubule function. We find that - tubulin gene copy number is important for maintaining an excess -tubulin protein compared to {beta}-tubulin protein and preventing accumulation of super-stoichiometric {beta}-tubulin. Super- stoichiometric {beta}-tubulin is toxic to cells, leading to loss of microtubules, formation of non- microtubule assemblies of tubulin, and disrupted cell proliferation. In contrast, decreased {beta}- tubulin or increased -tubulin has minor effects. We provide evidence that cells rapidly equilibrate the concentration of -tubulin protein during shifts in -tubulin isotype expression to maintain a ratio in excess of {beta}-tubulin. We propose an asymmetric relationship between - and {beta}-tubulins, where -tubulins are maintained in excess to supply {beta}-heterodimers and limit the accumulation of {beta}-tubulin monomers.