Polo-Like Kinase 1 Phosphorylation Tunes the Functional Viscoelastic Properties of the Centrosome Scaffold.

Cytoskeleton-organizing organelles often function while under mechanical load. The outer layer of centrosomes, called pericentriolar material (PCM), nucleates microtubules that move chromosomes during mitosis. How PCM resists microtubule-mediated forces is poorly understood at the material level. This study shows that PLK-1 phosphorylation of SPD-5 tunes the dynamics and material properties of the PCM scaffold in C. elegans embryos. Microrheology of reconstituted SPD-5 scaffolds reveals that PLK-1 phosphorylation decreases SPD-5 dynamics and increases scaffold viscoelasticity. Similarly, in embryos, phospho-mimetic SPD-5 is less dynamic than wild-type SPD-5, which itself is less dynamic than phospho-null SPD-5. PCM built with phospho-null SPD-5 is smaller than normal, but its assembly can be partially rescued by reducing microtubule-dependent forces. The same is true for PCM built with phospho-mimetic SPD-5, yet the underlying causes are distinct: under force, phospho-null SPD-5 fails to assemble, while phospho-mimetic SPD-5 forms hyper-stable foci that fail to cohere into a uniform, spherical mass. Both mutants have defects with chromosome segregation and viability. Thus, tuning of SPD-5 phosphorylation optimizes PCM material properties to achieve correct PCM size, integrity, and function. These results demonstrate how regulated chemical modification of a scaffolding protein tunes the material properties and function of a microtubule-organizing organelle.