Mto2 multisite phosphorylation inactivates non-spindle microtubule nucleation complexes during mitosis

Area of Science:

• Cell Biology
• Molecular Biology
• Cytoskeletal Dynamics

Background:

• Microtubule nucleation is crucial for cell cycle progression but its regulation remains poorly understood.
• In fission yeast (Schizosaccharomyces pombe), cytoplasmic microtubule nucleation is suppressed during mitosis.
• The Mto1/2 complex is essential for microtubule nucleation, activating the gamma-tubulin complex at various cellular sites.

Purpose of the Study:

• To elucidate the molecular mechanisms regulating microtubule nucleation during the cell cycle.
• To investigate the role of Mto2 phosphorylation in controlling Mto1/2 complex activity during mitosis.

Main Methods:

• Site-directed mutagenesis to generate mto2 phosphomutant strains.
• Analysis of Mto1/2 complex stability and interaction with the gamma-tubulin complex.
• Microscopy to assess microtubule nucleation activity in wild-type and mutant cells.

Main Results:

• The Mto1/2 complex disassembles during mitosis, coinciding with Mto2 hyperphosphorylation.
• Mutating Mto2 phosphorylation sites (e.g., mto2[24A]) enhances Mto1/2 complex stability and microtubule nucleation.
• The mto2[24A] mutant exhibits interphase-like microtubule nucleation activity during mitosis.

Conclusions:

• Phosphorylation of Mto2 acts as a 'switch' to inactivate microtubule nucleation complexes during mitosis.
• This study provides a molecular understanding of cell cycle-dependent regulation of microtubule nucleation.
• Findings illuminate mechanisms controlling both centrosomal and non-centrosomal microtubule nucleation.