Geyer, E. A. | bioRxiv Science

bioRxiv2022

Stimulating microtubule growth is not the essential function of the microtubule polymerase Stu2

ch-TOG family proteins, including the budding yeast Stu2, are essential for spindle formation and chromosome segregation. Such functions depend on an array of activities ranging from microtubule nucleation, polymerization and depolymerization, to conferring tension sensitivity to kinetochores. This functional diversity makes it challenging to dissect these various functions and understand their relative importance. Here, we developed separation-of-function mutants and used artificial tethering tools to elucidate several important mechanistic insights into Stu2s essential role. We show that Stu2s microtubule polymerization activity depends on its basic linker region but is surprisingly dispensable for viability; that in fact, Stu2 carries out an essential kinetochore-associated function; and finally, that Stu2s precise location within the kinetochore is critical for its function, suggesting a spatial separation mode of action may underlie its ability to confer tension sensitivity. Our findings highlight the significance of Stu2s kinetochore role and provide insights into the molecular mechanisms by which it performs its various functions.

cell biology↗

bioRxiv2018

Direct observation of individual tubulin dimers binding to growing microtubules

The biochemical basis of microtubule growth has remained elusive for over thirty years despite being fundamental for both cell division and associated chemotherapy strategies. Here, we combine interferometric scattering microscopy with recombinant tubulin to monitor individual tubulins binding to and dissociating from growing microtubule tips. We make the first direct, single-molecule measurements of tubulin on- and off-rates. We detect two populations of transient dwell times, and determine via binding-interface mutants that they are separated by the formation of inter-protofilament bonds. Applying a computational model, we find that slow association kinetics with strong interactions along protofilaments best recapitulate our data, and furthermore predict plus-end tapering. Overall, we provide the most direct and complete quantification of how microtubules grow to date.\n\nSIGNIFICANCEMicrotubule polymerization dynamics are fundamental to cell migration and cell division, where they are targets for chemotherapy drugs. Despite significant progress, the precise structural and biochemical events occurring at growing microtubule tips are not well defined, and better understanding is necessary for discriminating mechanisms of microtubule dynamics regulation in cells. Here, we visualize individual tubulin subunits reversibly and irreversibly interacting with dynamic microtubule tips, and thereby directly measure tubulin on- and off-rates. By analyzing plus-tip residence times of wild-type and mutant tubulin, we characterize the relative contributions of longitudinal (along protofilaments) and lateral (between protofilaments) bond energies to microtubule growth. This work provides insights into microtubule tip structure and potential modes of microtubule dynamics regulation.

biophysics↗