Proposed model for α-tubulin regulation of β-tubulin. (A) Updated model for how wild-type cells express tubulin from mRNA to protein and undergo folding and assembly into heterodimers. Our data suggests that cells hold an excess of α-tubulin that could exist as TBC-bound or soluble monomers. (B) In conditions where there is super-stoichiometric α-tubulin in cells, surplus α-tubulin drives monomerization and the formation of a transient α-tubulin assemblies. Our data shows that cells then down regulate the alternative isotype to restore endogenous expression and at the same time the assemblies dissipate. (C) In conditions where there is super-stoichiometric β-tubulin in cells, surplus β-tubulin drives the monomerization and formation of dead-end β-tubulin assemblies. Our data suggests these assemblies disrupt microtubule architecture and +TIP localization.