Pivoting of microtubules around the spindle pole accelerates kinetochore capture

Abstract

During cell division, spindle microtubules attach to chromosomes through kinetochores, protein complexes on the chromosome. The central question is how microtubules find kinetochores. According to the pioneering idea termed search-and-capture, numerous microtubules grow from a centrosome in all directions and by chance capture kinetochores. The efficiency of search-and-capture can be improved by a bias in microtubule growth towards the kinetochores, by nucleation of microtubules at the kinetochores and at spindle microtubules, by kinetochore movement, or by a combination of these processes. Here we show in fission yeast that kinetochores are captured by microtubules pivoting around the spindle pole, instead of growing towards the kinetochores. This pivoting motion of microtubules is random and independent of ATP-driven motor activity. By introducing a theoretical model, we show that the measured random movement of microtubules and kinetochores is sufficient to explain the process of kinetochore capture. Our theory predicts that the speed of capture depends mainly on how fast microtubules pivot, which was confirmed experimentally by speeding up and slowing down microtubule pivoting. Thus, pivoting motion allows microtubules to explore space laterally, as they search for targets such as kinetochores.

Contributions of Microtubule Dynamic Instability and Rotational Diffusion to Kinetochore Capture

“…Kalinina et al (24) found that fission yeast mitotic MTs on average spent 75% of their time in a paused state and had lifetimes of 3 min, growth speed v g ¼ 2.7 mm min À1 , and shrinking speed v s ¼ 3.8 mm min À1 . These results differed from the results of Sagolla et al (32), who observed highly dynamic polar MTs in early mitosis (before spindle formation), with lifetimes of seconds and a shrinking speed v s ¼ 20 mm min À1 , and more recent work on meiotic MTs (27).…”

“…Kalinina et al (24) found that three polar MTs were typically visible during their KC capture experiments, and that the time of KC capture was sensitive to MT number. Here we determined capture times for a single MT.…”

“…However, Kalinina et al (24) found relatively little change in fission-yeast MT dynamic instability parameters between 24 and 32 C, suggesting that S. pombe mitotic MT dynamic instability may not vary markedly with temperature in this regime. Another possible complication is that MT dynamics might be altered in the monopolar spindle system where kinesin-5 is inactive.…”

“…S1 in the Supporting Material). Simultaneously, a spherical KC of radius a ¼ 100 nm diffuses in the nucleus (24). We assumed the fission yeast KC size remained constant during mitosis, in contrast to recent work on human KCs (20).…”

“…We modeled both relatively slow dynamic instability (using measured parameters (24)) and faster dynamic instability based on our measurements of single mitotic MTs in fission yeast. Rotational diffusion typically gave only a modest speedup of KC capture in our model.…”