Asymmetric Microtubule Pushing Forces in Nuclear Centering

Abstract

Dynamic properties of microtubules contribute to the establishment of spatial order within cells. In the fission yeast Schizosaccharomyces pombe, interphase cytoplasmic microtubules are organized into antiparallel bundles that attach to the nuclear envelope and are needed to position the nucleus at the geometric center of the cell. Here, we show that after the nucleus is displaced by cell centrifugation, these microtubule bundles efficiently push the nucleus back to the center. Asymmetry in microtubule number, length, and dynamics contributes to the generation of force responsible for this unidirectional movement. Notably, microtubules facing the distal cell tip are destabilized when the microtubules in the same bundle are pushing from the proximal cell tip. The CLIP-170-like protein tip1p and the microtubule-bundling protein ase1p are required for this asymmetric regulation of microtubule dynamics, indicating contributions of factors both at microtubule plus ends and within the microtubule bundle. Mutants in these factors are defective in nuclear movement. Thus, cells possess an efficient microtubule-based engine that produces and senses forces for centering the nucleus. These studies may provide insights into mechanisms of asymmetric microtubule behaviors and force sensing in other processes such as chromosome segregation and cell polarization.

Interphase microtubule bundles use global cell shape to guide spindle alignment in fission yeast

“…We therefore analyzed spindle orientation in a mutant with altered interphase MT organization. Deletion of the S. pombe CLIP170 homologue tip1 results in cells with short MTs that often do not extend the entire length of the cell, and therefore are only partially aligned with the long axis of the cells (Brunner and Nurse, 2000;Carazo-Salas and Nurse, 2006;Daga et al, 2006b;Grallert et al, 2006). We observed in tip1Δ cells, which have short, poorly aligned MTs, that the frequency of spindle misalignment increased significantly, consistent with our hypothesis that interphase MTs aligned along the axis of the cell influence spindle orientation (Fig.…”

“…During this time MTs continue polymerizing so that the minus end of the MT bundle and the nucleus, which is attached to it, are pushed back in the opposite direction. Since MTs extend from sites of perinuclear nucleation towards each cell tip with equal number and frequency, and dwelling time is similar at both cell tips, the net pushing forces generated by polymerization are similar at both cell ends and the nucleus is positioned in a medial position between the cell tips (Daga et al, 2006b;Tran et al, 2001).…”

The Spindle Midzone Microtubule-Associated Proteins Ase1p and Cin8p Affect the Number and Orientation of Astral Microtubules in Saccharomyces cerevisiae

“…24,25 The fission yeast Ase1p is involved in nuclear positioning through its ability to bundle anti-parallel cytoplasmic microtubules in the overlap region of the microtubule arrays. 24,[26][27][28][29] A role in nuclear positioning for the S. cerevisiae Ase1p, which is thought to be exclusively nuclear, has yet to be described.…”

Dynamic instability-driven centering/segregating mechanism in bacteria

“…Microtubules, which exhibit dynamic instability, have been shown to play a role in both cell motility, and intracellular positioning via experimental and theoretical methods (3)(4)(5)(6)(7)(8). Intracellularly, microtubules typically polymerize radially outward from their microtubule organizing centers (centrosomes), make contact with their enclosure from multiple directions, and consequently push their attached microtubule organizing center radially away from the cell membrane and toward the cell center (4,5,(9)(10)(11). This microtubule-driven mechanical centering has been observed in vivo in cylindrical Schizosaccharomyces pombe cells (3,6), alone in vitro (12), and in conjunction with motor proteins in eukaryotic cells with complex shapes (13).…”