Asymmetric Force Response Reveals Mechanical Role in Spindle Protein Localization

Abstract

The mitotic spindle is the self-organized, microtubule based structure which mechanically segregates chromosomes during cell division. The spindle ‘parts-list' includes microtubules, motor proteins, and non-motor microtubule-associated proteins (or MAPs), and the biochemical properties of many of these components have been well studied. By comparison, our understanding of the force-dependent behavior of many key interactions remains limited. In particular, we do not understand the role that cross-linking MAPs play in providing mechanical stability within the highly dynamic spindle, or how force regulates the function and localization of these proteins. To address this shortcoming, we examine the force-dependent response of NuMA, the major cross-linking MAP of minus-end focused parallel microtubules at the spindle pole. Combining data taken with single molecule TIRF-based imaging and optical trapping methods, we show that NuMA/microtubule interactions generate resistive, friction-like forces which approach ∼1pN when dragged at velocities in the micron/sec range. Unexpectedly, the mechanical response is asymmetric, with NuMA sliding more easily towards the minus ends of microtubules than the plus ends. For comparison, we show that PRC1, a dimeric protein which cross-links antiparallel microtubules at the spindle midzone in anaphase, does not possess such an asymmetric behavior under force. We further perform computer simulations on parallel microtubules cross-linked by ‘dimerized' NuMA (effectively a minimal structural unit of the spindle pole), and show that in the presence of small oscillatory perturbations, NuMA will migrate to the minus ends. These combined results suggests a mechanism for autonomous localization to the spindle poles, and may reveal a possible mechanical principle underlying spindle self-organization.