We used electron tomography to examine microtubules (MTs) growing from pure tubulin in vitro as well as two classes of MTs growing in cells from six species. The tips of all these growing MTs display bent protofilaments (PFs) that curve away from the MT axis, in contrast with previously reported MTs growing in vitro whose tips are either blunt or sheetlike. Neither high pressure nor freezing is responsible for the PF curvatures we see. The curvatures of PFs on growing and shortening MTs are similar; all are most curved at their tips, suggesting that guanosine triphosphate-tubulin in solution is bent and must straighten to be incorporated into the MT wall. Variations in curvature suggest that PFs are flexible in their plane of bending but rigid to bending out of that plane. Modeling by Brownian dynamics suggests that PF straightening for MT growth can be achieved by thermal motions, providing a simple mechanism with which to understand tubulin polymerization.

During vertebrate mitosis, the centromere-associated kinesin CENP-E transports misaligned chromosomes to the plus ends of spindle microtubules. Subsequently, the kinetochores that form at the centromeres establish stable associations with microtubule ends, which assemble and disassemble dynamically. Here we provide evidence that after chromosomes have congressed and bi-oriented, the CENP-E motor continues to play an active role at kinetochores, enhancing their links with dynamic microtubule ends. Using a combination of single molecule approaches and laser trapping in vitro we demonstrate that once reaching microtubule ends, CENP-E converts from a lateral transporter into a microtubule tip-tracker which maintains association with both assembling and disassembling microtubule tips. Computational modeling of this behavior supports our proposal that CENP-E tip-tracks bi-directionally via a “tethered motor” mechanism, which relies on both the motor and tail domains of CENP-E. Our results provide a molecular framework for CENP-E's contribution to the stability of attachments between kinetochores and dynamic microtubule ends.

Tubulin polymers, microtubules, can switch abruptly from the assembly to shortening. These infrequent transitions, termed "catastrophes", affect numerous cellular processes but the underlying mechanisms are elusive. We approached this complex stochastic system using advanced coarse-grained molecular dynamics modeling of tubulin-tubulin interactions. Unlike in previous simplified models of dynamic microtubules, the catastrophes in this model arise owing to fluctuations in the composition and conformation of a growing microtubule tip, most notably in the number of protofilament curls. In our model, dynamic evolution of the stochastic microtubule tip configurations over a long timescale, known as the system's "aging", gives rise to the nonexponential distribution of microtubule lifetimes, consistent with experiment. We show that aging takes place in the absence of visible changes in the microtubule wall or tip, as this complex molecular-mechanical system evolves slowly and asymptotically toward the steady-state level of the catastrophe-promoting configurations. This new, to our knowledge, theoretical basis will assist detailed mechanistic investigations of the mechanisms of action of different microtubule-binding proteins and drugs, thereby enabling accurate control over the microtubule dynamics to treat various pathologies.

No abstract

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.

Microtubule kinetochore attachments are essential for accurate mitosis, but how these force-generating connections move chromosomes remains poorly understood. Processive motion at shortening microtubule ends can be reconstituted in vitro using microbeads conjugated to the budding yeast kinetochore protein Dam1, which forms microtubule-encircling rings. Here, we report that, when Dam1 is linked to a bead cargo by elongated protein tethers, the maximum force transmitted from a disassembling microtubule increases sixfold compared with a short tether. We interpret this significant improvement with a theory that considers the geometry and mechanics of the microtubule-ring-bead system. Our results show the importance of fibrillar links in tethering microtubule ends to cargo: fibrils enable the cargo to align coaxially with the microtubule, thereby increasing the stability of attachment and the mechanical work that it can do. The force-transducing characteristics of fibril-tethered Dam1 are similar to the analogous properties of purified yeast kinetochores, suggesting that a tethered Dam1 ring comprises the main force-bearing unit of the native attachment.laser tweezers | mathematical modeling | microtubule depolymerization | anaphase | forced walk

No abstract

Microtubules are dynamic tubulin polymers responsible for many cellular processes, including the capture and segregation of chromosomes during mitosis. In contrast to textbook models of tubulin self-assembly, we have recently demonstrated that microtubules elongate by addition of bent guanosine triphosphate tubulin to the tips of curving protofilaments. Here we explore this mechanism of microtubule growth using Brownian dynamics modeling and electron cryotomography. The previously described flaring shapes of growing microtubule tips are remarkably consistent under various assembly conditions, including different tubulin concentrations, the presence or absence of a polymerization catalyst or tubulin-binding drugs. Simulations indicate that development of substantial forces during microtubule growth and shortening requires a high activation energy barrier in lateral tubulin-tubulin interactions. Modeling offers a mechanism to explain kinetochore coupling to growing microtubule tips under assisting force, and it predicts a load-dependent acceleration of microtubule assembly, providing a role for the flared morphology of growing microtubule ends.