Pivoting of microtubules driven by minus-end-directed motors leads to spindle assembly

Abstract

Background At the beginning of mitosis, the cell forms a spindle made of microtubules and associated proteins to segregate chromosomes. An important part of spindle architecture is a set of antiparallel microtubule bundles connecting the spindle poles. A key question is how microtubules extending at arbitrary angles form an antiparallel interpolar bundle. Results Here, we show in fission yeast that microtubules meet at an oblique angle and subsequently rotate into antiparallel alignment. Our live-cell imaging approach provides a direct observation of interpolar bundle formation. By combining experiments with theory, we show that microtubules from each pole search for those from the opposite pole by performing random angular movement. Upon contact, two microtubules slide sideways along each other in a directed manner towards the antiparallel configuration. We introduce the contour length of microtubules as a measure of activity of motors that drive microtubule sliding, which we used together with observation of Cut7/kinesin-5 motors and our theory to reveal the minus-end-directed motility of this motor in vivo. Conclusion Random rotational motion helps microtubules from the opposite poles to find each other and subsequent accumulation of motors allows them to generate forces that drive interpolar bundle formation. Electronic supplementary material The online version of this article (10.1186/s12915-019-0656-2) contains supplementary material, which is available to authorized users.

Mechanisms of chromosome biorientation and bipolar spindle assembly analyzed by computational modeling

“…Computational modeling has been used previously to study the mitotic spindle [3,4,67]. Recent work on spindle and MT organization includes studies of spindle elongation and force balance [59,68], the formation and maintenance of antiparallel MT overlaps [69,70], MT bundling and sliding [15], spindle movements and positioning [71,72], spindle length and shape [15,51,52,73,74], MT organization [75], and spindle assembly from a bipolar initial condition [32,76]. Models of kinetochore-MT attachment and biorientation have examined capture of lost kinetochores [63,77], chromosome reorientation after MT attachment [31], attachment error correction [33,39,78,79], and chromosome movement on the spindle [52,61,[80][81][82].…”

Mechanisms of chromosome biorientation and bipolar spindle assembly analyzed by computational modeling

“…Computational modeling has been used previously to study the mitotic spindle [3,4,67]. Recent work on spindle and MT organization includes studies of spindle elongation and force balance [59,68], the formation and maintenance of antiparallel MT overlaps [69,70], MT bundling and sliding [15], spindle movements and positioning [71,72], spindle length and shape [15,51,52,73,74], MT organization [75], and spindle assembly from a bipolar initial condition [32,76]. Models of kinetochore-MT attachment and biorientation have examined capture of lost kinetochores [63,77], chromosome reorientation after MT attachment [31], attachment error correction [33,39,78,79], and chromosome movement on the spindle [52,61,[80][81][82].…”

Comparison of mechanisms of kinetochore capture with varying number of spindle microtubules

“…A second motivation is related to the spindle assembly in fission yeast [6,8,[40][41][42][43][44], and more specifically, to an experiment [8] which reported an exception to the usual S&C mechanism in Schizosaccharomyces pombe (fission yeast). Here a diffusing KC inside the nucleus is captured by MTs executing rotational diffusion (RD) [8,40,42], being pivoted at the spindle pole body (SPB) [see Figs. 1(a) and 1(b)].…”