Polymerization Forces Exerted by Microtubules and the Position of Cellular Organelles

Jonathon Howard

Department of Molecular Biophysics and Biochemistry

Yale University




Actin filaments and microtubules are active polymers that couple chemical energy derived from ATP or GTP hydrolysis to their growth and shrinkage. As a consequence of this coupling, filaments can do mechanical work in cells, to move the boundary of the cell or to organize spatially the contents of the cell. Our laboratory is interested in the GTP-dependent assembly and disassembly of microtubules, and its regulation by motor proteins and other microtubule associated proteins. A key role of microtubule dynamics is in the centering of the mitotic spindle, the microtubule-based structure that segregates the duplicated chromosomes of dividing cells. The spindle is orientated along the anterior-posterior axis, thereby defining the plane of cell division and determining the location of the two daughters relative to the mother. We are studying this process in the one-cell embryo of the nematode worm C. elegans. Using very precise tracking of the spindle, we have found that centering is highly accurate, precise and stable, with the standard deviation of the fluctuations from the mean position of only 1% of cell diameter. Using magnetic fields to exert forces on small magnetic particles injected into dividing cells, we have discovered that the spindle behaves as a spring: the larger the applied force, the greater the displacement of the spindle from the center, and, when the force is turned off, the spindle returns to the center. Approximately 20 pN is required to deflect the spindle on micrometer from the anterior-posterior axis. We attribute the high stability of centration to a large number of microtubules, approximately 10,000, which grow out from the spindle poles and push against the inner surface of the cell. When the buckling of microtubules is included, our models account quantitatively both for the stability and the forces associated with centering.