Mitotic chromosomes segregate at the ends of shortening spindle microtubules (MTs). In budding yeast, the Dam1 multiprotein complex supports this dynamic attachment, thereby contributing to accurate chromosome segregation. Purified Dam1 will track the end of a depolymerizing MT and can couple it to microbead transport in vitro. The processivity of such motions has been thought to depend on rings that the Dam1 complex can form around MTs, but the possibility that alternative coupling geometries contribute to these motilities has not been considered. Here, we demonstrate that both rings and nonencircling Dam1 oligomers can track MT ends and enable processive cargo movement in vitro. The coupling properties of these two assemblies are, however, quite different, so each may make a distinct contribution to chromosome motility.chromosome motions ͉ kinetochore-microtubule interactions ͉ microtubule end tracking M itotic chromosomes are attached to spindle fibers by a multiprotein complex, the kinetochore (1-3). Kinetochores bind strongly to the plus ends of spindle microtubules (MTs), but tubulin subunits can still be added or lost from these MT ends as the chromosomes move. Kinetochores are also sites where forces can be generated to move chromosomes either toward or away from the pole that a kinetochore faces, so this interface provides multiple important mitotic functions. Kinetochores include MT-dependent motors and several protein complexes that lack motor activity but can still bind to a MT wall or end. These nonmotor complexes too may be important for chromosome motion, because depolymerizing MTs can generate enough force to move chromosomes without the help of motors, both in vivo and in vitro (4-7).The mechanism by which a kinetochore is coupled to the end of a shortening or lengthening MT is still unknown, but a 10-subunit protein complex from budding yeast kinetochores, called Dam1 or DASH, is important for chromosome-spindle attachments and spindle integrity in this organism (3). Purified Dam1 heterodecamers assemble around MTs into rings of 16-25 subunits (8, 9). At higher protein concentrations, both multiple rings and helices of Dam1 will form (10, 11).In yeast spindles, Dam1 is found primarily at kinetochores, although some of it associates with spindle MTs (12). A careful estimate of the number of Dam1 subunits at metaphase kinetochores in budding yeast gave 16-20 heterodecamers (12), enough to form a single ring. The behavior of kinetochore-associated Dam1 fluorescence suggests that these subunits are stably attached to kinetochore MTs, showing little or no turnover. However, a second population of Dam1, which is dimly localized to spindle MTs, shows faster turnover, indicating the presence of a dynamic pool of complexes (12).Oligomers of Dam1 can travel with a shortening MT in vitro (8), suggesting that this complex may be the primary MT-chromosome coupler in yeast. A bead coated with Dam1 will also associate with a MT in vitro and follow its end during either assembly or disassembly (13, 14), proper...

Accurate chromosome segregation during mitotic division of budding yeast depends on the multiprotein kinetochore complex, Dam1 (also known as DASH). Purified Dam1 heterodecamers encircle microtubules (MTs) to form rings that can function as “couplers,” molecular devices that transduce energy from MT disassembly into the motion of a cargo. Here we show that MT depolymerization develops a force against a Dam1 ring that is sixfold larger than the force exerted on a coupler that binds only one side of an MT. Wild-type rings slow depolymerization fourfold, but rings that include a mutant Dam1p with truncated C terminus slow depolymerization less, consistent with the idea that this tail is part of a strong bond between rings and MTs. A molecular-mechanical model for Dam1-MT interaction predicts that binding between this flexible tail and the MT wall should cause a Dam1 ring to wobble, and Fourier analysis of moving, ring-attached beads corroborates this prediction. Comparison of the forces generated against wild-type and mutant complexes confirms the importance of tight Dam1-MT association for processive cargo movement under load.

Chromosome biorientation, the attachment of sister kinetochores to sister spindle poles, is vitally important for accurate chromosome segregation. We have studied this process by following the congression of pole-proximal kinetochores and their subsequent anaphase segregation in fission yeast cells that carry deletions in any or all of this organism's minus end-directed, microtubule-dependent motors: two related kinesin 14s (Pkl1p and Klp2p) and dynein. None of these deletions abolished biorientation, but fewer chromosomes segregated normally without Pkl1p, and to a lesser degree without dynein, than in wild-type cells. In the absence of Pkl1p, which normally localizes to the spindle and its poles, the checkpoint that monitors chromosome biorientation was defective, leading to frequent precocious anaphase. Ultrastructural analysis of mutant mitotic spindles suggests that Pkl1p contributes to error-free biorientation by promoting normal spindle pole organization, whereas dynein helps to anchor a focused bundle of spindle microtubules at the pole.