The rate of microtubule breaking increases exponentially with curvature

Multi-kinesin clusters impart mechanical stress that reveals mechanisms of microtubule breakage in cells

Microtubules are cytoskeletal filaments that provide structural support for numerous cellular processes. Despite their high rigidity, microtubules can be dramatically bent in cells and it is unknown how much force a microtubule can withstand before breaking. We find that liquid-liquid phase separation of the kinesin-3 motor KIF1C results in multi-kinesin clusters that entangle neighboring microtubules and impose a high level of mechanical stress that results in microtubule breakage and disassembly. Combining computational simulations and experiments, we show that microtubule fragmentation is enhanced by having a highly processive kinesin motor domain, a stiff clustering mechanism, and sufficient drag force on the microtubules. We estimate a rupture force for microtubules in cells of 70-120 pN, which is lower than previous estimates based on in vitro studies with taxol-stabilized microtubules. These results indicate that the presence of multiple kinesins on a cargo has the potential to cause microtubule breakage. We propose that mechanisms exist to protect microtubule integrity by releasing either the motor-cargo or motor-microtubule interaction, thereby preventing the accumulation of mechanical stress upon the engagement of multi-motor clusters with microtubules.

Mechanical fatigue in microtubules

Mechanical failure of biological nanostructures due to sustained force application has been studied in great detail. In contrast, fatigue failure arising from repeated application of subcritical stresses has received little attention despite its prominent role in engineering and potentially biology. Here, paclitaxel-stabilized microtubules are up to 256 times bent into sinusoidal shapes of varying wavelength and the frequency of breaking events are observed. These experiments allow the calculation of fatigue life parameters for microtubules. Repeated buckling due to 12.5% compression-equal to the compression level experienced by microtubules in contracting cardiomyocytes - results in failure after in average 5 million cycles, whereas at 20.0% compression failure occurs after in average one thousand cycles. The fatigue strength (Basquin) exponent B is estimated as - 0.054±0.009.

Biosensing using antibody-modulated motility of actin filaments on myosin-coated surfaces

Motor proteins, such as myosin and kinesin, are biological molecular motors involved in force generation and intracellular transport within living cells. The characteristics of molecular motors, i.e., their motility over long distances, their capacity of transporting cargoes, and their very efficient energy consumption, recommend them as potential operational elements of a new class of dynamic nano-devices, with potential applications in biosensing, analyte concentrators, and biocomputation. A possible design of a biosensor based on protein molecular motor comprises a surface with immobilized motors propelling cytoskeletal filaments, which are decorated with antibodies, presented as side-branches. Upon biomolecular recognition of these branches by secondary antibodies, the 'extensions' on the cytoskeletal filaments can achieve considerable lengths (longer than several diameters of the cytoskeletal filament carrier), thus geometrically impairing or halting motility. Because the filaments are several micrometers long, this sensing mechanism converts an event in the nanometer range, i.e., antibody-antigen sizes, into an event in the micrometer range: the visualization of the halting of motility of microns-long cytoskeletal filaments. Here we demonstrate the proof of concept of a sensing system comprising heavy-mero-myosin immobilized on surfaces propelling actin filaments decorated with actin antibodies, whose movement is halted upon the recognition with secondary anti-actin antibodies. Because antibodies to the actin-myosin system are involved in several rare diseases, the first possible application for such a device may be their prognosis and diagnosis. The results also provide insights into guidelines for designing highly sensitive and very fast biosensors powered by motor proteins.