Figure 8. Model for regulation of Mto1/2 complex assembly and activity by phosphorylation.

(a) Schematic diagram showing how the activity of wild-type and phosphomutant Mto1/2 complex is regulated by phosphorylation. Diagram depicts negative relationship between Mto2 phosphorylation (red) and Mto1/2-dependent microtubule (MT) nucleation (blue), aligned with range of Mto2 phosphorylation states in interphase (I) and mitosis (M) in wild-type Mto2 and Mto2 phosphomutants (grey bars). In wild-type cells, interphase levels of Mto2 phosphorylation allow efficient interaction with Mto1 and γ-TuC binding, leading to Mto1/2 complex puncta formation and microtubule (MT) nucleation; mitotic hyperphosphorylation of Mto2 abolishes these interactions, leading to suppression of microtubule nucleation. In mto2-phosphomutant cells, increasing the number of mutated phosphosites leads to a graded decrease in Mto2 phosphorylation and a corresponding graded increase in Mto1/2 activity in interphase and mitosis. (b) Role of Mto2 phosphorylation in context of model for Mto1/2 complex assembly proposed by Lynch et al.³². Several different aspects of Mto1/2 assembly and activity may be regulated by Mto2 phosphorylation in both interphase and mitosis, with higher levels of phosphorylation in mitosis. Interaction between the Mto1/2 multimer and γ-TuSCs might be regulated either directly, by altering affinity between Mto2 and γ-TuSCs, or indirectly, by altering the multimeric state of the Mto1/2 complex, thereby affecting cooperative binding to γ-TuSCs. MT, microtubule.