Figure 5.

Compression regulates spindle size. (A) Sketch of the response of a Ptk2 spindle to compression. Curved black arrows depict tubulin polymerization at the kinetochore and depolymerization at the poles (poleward sliding). In panel 2, new k-fibers are in focus (different color). Gray region depicts a photomarked tubulin population. Depolymerization at poles is inhibited during elongation; we predict it to increase during contraction but this has not been shown. For simplicity, active and passive responses are depicted in series, while they actually occur in parallel. (B) Mechanical coupling model for spindle length regulation, where the length of K-MTs is determined by force-dependent effects on microtubule dynamics at poles. Coupling between tension/compression on poles and K-MT dynamics is provided either by force-dependent regulation of the activity of a depolymerase, or by a direct effect of force on depolymerization. In either case, we postulate that the depolymerization rate at poles responds to the sum [43] of all the forces exerted on K-MTs (F), that we divide into pulling on plus-ends by kinetochores (red arrow), an outwards sliding force that is generated along the length of the K-MTs (green arrows), and pushing on minus-ends by other parts of the spindle or cell (gray arrow). K-MTs grow at the sliding rate until the original F (F_o) returns, when depolymerization resumes and a new spindle length steady-state is reached.