Jcb_201401091 1..12

During cell division, the mitotic spindle assembles itself from its constituent parts. Spindle microtubule minus ends are focused into two poles, and these poles dictate where duplicated chromatids are transported at anaphase. Forces that focus microtubules into poles are crucial to spindle organization and function. Cytoplasmic dynein, a minus end–directed microtubule motor, clusters parallel microtubules into spindle poles (Verde et al., 1991; Heald et al., 1996) and transports the microtubulebinding protein NuMA to build poles (Merdes et al., 2000). At poles, dynein and NuMA tether microtubules (Gaglio et al., 1995; Merdes et al., 1996; Heald et al., 1997; Dionne et al., 1999), and pole structure remains robust despite rapid microtubule turnover (Saxton et al., 1984) and opposing tension on kinetochore fibers (k-fibers) from kinetochore-based forces (Gordon et al., 2001; Manning and Compton, 2007; Silk et al., 2009). Thus, poles must both oppose force and be constantly rebuilt (Gaglio et al., 1997; Goshima et al., 2005). This engineering challenge highlights a long-standing paradox: how can the spindle maintain its structure and mechanical integrity and yet remain dynamic, flexible, and architecturally plastic, as its functions require? For the spindle to preserve its structural integrity, it must be able to continuously rebuild poles by recognizing and sorting new microtubule structures. Indeed, during spindle assembly, poles can integrate both new peripheral microtubules (Rusan et al., 2002; Tulu et al., 2003) and kinetochore-nucleated k-fibers (Khodjakov et al., 2003; Maiato et al., 2004). Established spindles can move short microtubule seeds to poles (Heald et al., 1996, 1997) and reincorporate k-fibers severed by ablation as microtubules grow back (Snyder et al., 1991; Chen and Zhang, 2004; Maiato et al., 2004), and poles from different spindles can fuse together (Gatlin et al., 2009). Although dynein and NuMA are either demonstrated or suspected to mediate these observations of dynamic microtubule integration into poles, it is not clear which microtubule structures serve as dynein cargo, where on them force is exerted, or how strong that force is. We do not know how forces that maintain poles compare to other spindle forces or on what timescale they contribute to spindle architecture. In large part, this is because the response of the established spindle to detached microtubules is challenging to study: k-fiber minus ends are already embedded in the spindle and free microtubules within the spindle body are difficult to image. The spindle is a dynamic self-assembling machine that coordinates mitosis. The spindle’s function depends on its ability to organize microtubules into poles and maintain pole structure despite mechanical challenges and component turnover. Although we know that dynein and NuMA mediate pole formation, our understanding of the forces dynamically maintaining poles is limited: we do not know where and how quickly they act or their strength and structural impact. Using laser ablation to cut spindle microtubules, we identify a force that rapidly and robustly pulls severed microtubules and chromosomes poleward, overpowering opposing forces and repairing spindle architecture. Molecular imaging and biophysical analysis suggest that transport is powered by dynein pulling on minus ends of severed microtubules. NuMA and dynein/dynactin are specifically enriched at new minus ends within seconds, reanchoring minus ends to the spindle and delivering them to poles. This force on minus ends represents a newly uncovered chromosome transport mechanism that is independent of plus end forces at kinetochores and is well suited to robustly maintain spindle mechanical integrity. Force on spindle microtubule minus ends moves chromosomes