Anillin propels myosin-independent constriction of actin rings.

Constriction of the cytokinetic ring, a circular structure of actin filaments, is an essential step during cell division. Mechanical forces driving the constriction are attributed to myosin motor proteins, which slide actin filaments along each other. However, in multiple organisms, ring constriction has been reported to be myosin independent. How actin rings constrict in the absence of motor activity remains unclear. Here, we demonstrate that anillin, a non-motor actin crosslinker, indispensable during cytokinesis, autonomously propels the contractility of actin bundles. Anillin generates contractile forces of tens of pico-Newtons to maximise the lengths of overlaps between bundled actin filaments. The contractility is enhanced by actin disassembly. When multiple actin filaments are arranged into a ring, this contractility leads to ring constriction. Our results indicate that passive actin crosslinkers can substitute for the activity of molecular motors to generate contractile forces in a variety of actin networks, including the cytokinetic ring.

Intrinsically Disordered Domain of Kinesin-3 Kif14 Enables Unique Functional Diversity.

In addition to their force-generating motor domains, kinesin motor proteins feature various accessory domains enabling them to fulfill a variety of functions in the cell. Human kinesin-3, Kif14, localizes to the midbody of the mitotic spindle and is involved in the progression of cytokinesis. The specific motor properties enabling Kif14's cellular functions, however, remain unknown. Here, we show in vitro that the intrinsically disordered N-terminal domain of Kif14 enables unique functional diversity of the kinesin. Using single molecule TIRF microscopy, we found that Kif14 exists either as a diffusible monomer or as processive dimer and that the disordered domain (1) enables diffusibility of the monomeric Kif14, (2) renders the dimeric Kif14 super-processive and enables the kinesin to pass through highly crowded areas, (3) enables robust, autonomous Kif14 tracking of growing microtubule tips, independent of microtubule end-binding (EB) proteins, and (4) is sufficient to enable crosslinking of parallel microtubules and necessary to enable Kif14-driven sliding of antiparallel ones. We explain these features of Kif14 by the observed diffusible interaction of the disordered domain with the microtubule lattice and the observed increased affinity of the disordered domain for GTP-bound tubulin. We suggest that the disordered domain tethers the motor domain to the microtubule providing a diffusible foothold and a regulatory hub, tuning the kinesin's interaction with microtubules. Our findings thus exemplify pliable protein tethering as a fundamental mechanism of molecular motor regulation.

Mitochondria-adaptor TRAK1 promotes kinesin-1 driven transport in crowded environments.

Intracellular trafficking of organelles, driven by kinesin-1 stepping along microtubules, underpins essential cellular processes. In absence of other proteins on the microtubule surface, kinesin-1 performs micron-long runs. Under crowding conditions, however, kinesin-1 motility is drastically impeded. It is thus unclear how kinesin-1 acts as an efficient transporter in intracellular environments. Here, we demonstrate that TRAK1 (Milton), an adaptor protein essential for mitochondrial trafficking, activates kinesin-1 and increases robustness of kinesin-1 stepping on crowded microtubule surfaces. Interaction with TRAK1 i) facilitates kinesin-1 navigation around obstacles, ii) increases the probability of kinesin-1 passing through cohesive islands of tau and iii) increases the run length of kinesin-1 in cell lysate. We explain the enhanced motility by the observed direct interaction of TRAK1 with microtubules, providing an additional anchor for the kinesin-1-TRAK1 complex. Furthermore, TRAK1 enables mitochondrial transport in vitro. We propose adaptor-mediated tethering as a mechanism regulating kinesin-1 motility in various cellular environments.

Kinetically distinct phases of tau on microtubules regulate kinesin motors and severing enzymes.

Tau is an intrinsically disordered protein, which diffuses on microtubules1. In neurodegenerative diseases, collectively termed tauopathies, malfunction of tau and its detachment from axonal microtubules are correlated with axonal degeneration2. Tau can protect microtubules from microtubule-degrading enzymes such as katanin3. However, how tau carries out this regulatory function is still unclear. Here, using in vitro reconstitution, we show that tau molecules on microtubules cooperatively form cohesive islands that are kinetically distinct from tau molecules that individually diffuse on microtubules. Dependent on the tau concentration in solution, the islands reversibly grow or shrink by addition or release of tau molecules at their boundaries. Shielding microtubules from kinesin-1 motors and katanin, the islands exhibit regulatory qualities distinct from a comparably dense layer of diffusible tau. Superprocessive kinesin-8 motors penetrate the islands and cause their disassembly. Our results reveal a microtubule-dependent phase of tau that constitutes an adaptable protective layer on the microtubule surface. We anticipate that other intrinsically disordered axonal proteins display a similar cooperative behaviour and potentially compete with tau in regulating access to the microtubule surface.

The kinesin-14 Klp2 organizes microtubules into parallel bundles by an ATP-dependent sorting mechanism.

The dynamic organization of microtubules into parallel arrays allows interphase cells to set up multi-lane highways for intracellular transport and M-phase cells to build the mitotic and meiotic spindles. Here we show that a minimally reconstituted system composed of Klp2, a kinesin-14 from the fission yeast Schizosaccharomyces pombe, together with microtubules assembled from purified S. pombe tubulin, autonomously assembles bundles of parallel microtubules. Bundles form by an ATP-dependent sorting mechanism that requires the full-length Klp2 motor. By this mechanism, antiparallel-overlapped microtubules slide over one another until they dissociate from the bundles, whereas parallel-overlapped microtubules are selectively trapped by an energy-dissipating force-balance mechanism. Klp2-driven microtubule sorting provides a robust pathway for the organization of microtubules into parallel arrays. In vivo evidence indicates that Klp2 is required for the proper organization of S. pombe interphase microtubules into bipolar arrays of parallel-overlapped microtubules, suggesting that kinesin-14-dependent microtubule sorting may have wide biological importance.

The effect of tamoxifen or exemestane on bone mineral density during the first 2 years of adjuvant treatment of postmenopausal women with early breast cancer.

BACKGROUND: Adjuvant therapy with aromatase inhibitors is associated with increased bone loss in postmenopausal women with breast cancer. We assessed changes in bone mineral density (BMD) from baseline to 24 months in patients receiving either tamoxifen (T) or exemestane (E). PATIENTS AND METHODS: A total of 578 women randomly assigned to T 20 mg per day orally or E 25 mg/day orally enrolled in this substudy; baseline, 12-month, and 24-month BMD measurements of the femur and lumbar spine by dual-energy x-ray absorptiometry were planned. Women receiving bone antiresorptive agents were excluded. Mean BMD changes from baseline to 12 and 24 months were tested between the treatment groups using 2-sample t tests and both g/cm2 (as percent changes) and T scores (as differences from baseline). RESULTS: A total of 167 women with all 3 imaging studies were evaluable and form the basis of this report (T=89, E=78). Using T scores, the mean difference from baseline was significant between the 2 groups at 12 months at both the spine (P=.0002) and the hip (P=.0004), and at 24 months only at the hip (P=.02). CONCLUSION: More bone loss occurred during the first 12 months of treatment with E compared with T, but by 2 years the differences were less apparent and bone loss with E had slowed.