Contributions of Microtubule Dynamic Instability and Rotational Diffusion to Kinetochore Capture

Abstract

Microtubule dynamic instability allows search and capture of kinetochores during spindle formation, an important process for accurate chromosome segregation during cell division. Recent work has found that microtubule rotational diffusion about minus-end attachment points contributes to kinetochore capture in fission yeast, but the relative contributions of dynamic instability and rotational diffusion are not well understood. We have developed a biophysical model of kinetochore capture in small fission-yeast nuclei using hybrid Brownian dynamics/kinetic Monte Carlo simulation techniques. With this model, we have studied the importance of dynamic instability and microtubule rotational diffusion for kinetochore capture, both to the lateral surface of a microtubule and at or near its end. Over a range of biologically relevant parameters, microtubule rotational diffusion decreased capture time, but made a relatively small contribution compared to dynamic instability. At most, rotational diffusion reduced capture time by 25%. Our results suggest that while microtubule rotational diffusion can speed up kinetochore capture, it is unlikely to be the dominant physical mechanism for typical conditions in fission yeast. In addition, we found that when microtubules undergo dynamic instability, lateral captures predominate even in the absence of rotational diffusion. Counterintuitively, adding rotational diffusion to a dynamic microtubule increases the probability of endon capture.

Physical determinants of bipolar mitotic spindle assembly and stability in fission yeast

“…The MTs diffuse both rotationally and translationally, constrained by the tethering of their minus ends to the SPB ( 124 , 125 ). MT rotational diffusion about a pivot at the SPB was measured ( 91 ), allowing us to correctly compute MT diffusion for any length ( 119 ). MT plus-end dynamic instability is described by growing and shrinking speeds and catastrophe and rescue frequencies, which have been measured or estimated ( 82 , 91 , 119 ).…”

Physical Determinants of Bipolar Mitotic Spindle Assembly and Stability in Fission Yeast

“…The images of spindle elongation shown in Fig. 1 were taken using cell preparation and confocal imaging, as previously described (71,119). Images are maximum-intensity projections.…”

“…MT rotational diffusion about a pivot at the SPB was measured (91), allowing us to correctly compute MT diffusion for any length (119). MT plus-end dynamic instability is described by growing and shrinking speeds and catastrophe and rescue frequencies, which have been measured or estimated (82,91,119). MTs switch stochastically between growing and shrinking states.…”

“…Kinetic Monte Carlo methods model stochastic events that change the state of molecules in the system, including motor/cross-linker binding/unbinding and MT dynamic instability. We previously used these techniques to model MT-motor mixtures and kinetochore capture (115)(116)(117)(118)(119).…”

“…The effects of kinetochore-MT attachments on MT dynamics (23,25,114) are implicitly modeled through our optimization of MT dynamics for spindle stability. We use a hybrid Brownian dynamics-kinetic Monte Carlo simulation scheme, as previously described (115)(116)(117)(118)(119). Bipolar spindles robustly form (Fig.…”