Preparation of dual-color polarity-marked fluorescent microtubule seeds

In Vitro Reconstitution of Microtubule Dynamics and Severing Imaged by Label-Free Interference-Reflection Microscopy

The dynamic architecture of the microtubule cytoskeleton is crucial for cell division, motility and morphogenesis. The dynamic properties of microtubules-growth, shrinkage, nucleation, and severing-are regulated by an arsenal of microtubule-associated proteins (MAPs). The activities of many of these MAPs have been reconstituted in vitro using microscope assays. As an alternative to fluorescence microscopy, interference-reflection microscopy (IRM) has been introduced as an easy-to-use, wide-field imaging technique that allows label-free visualization of microtubules with high contrast and speed. IRM circumvents several problems associated with fluorescence microscopy including the high concentrations of tubulin required for fluorescent labeling, the potential perturbation of function caused by the fluorophores, and the risks of photodamage. IRM can be implemented on a standard epifluorescence microscope at low cost and can be combined with fluorescence techniques like total-internal-reflection-fluorescence (TIRF) microscopy. Here we describe the experimental procedure to image microtubule dynamics and severing using IRM , providing practical tips and guidelines to resolve possible experimental hurdles.

Ectopic A-lattice seams destabilize microtubules

Natural microtubules typically include one A-lattice seam within an otherwise helically symmetric B-lattice tube. It is currently unclear how A-lattice seams influence microtubule dynamic instability. Here we find that including extra A-lattice seams in GMPCPP microtubules, structural analogues of the GTP caps of dynamic microtubules, destabilizes them, enhancing their median shrinkage rate by >20-fold. Dynamic microtubules nucleated by seeds containing extra A-lattice seams have growth rates similar to microtubules nucleated by B-lattice seeds, yet have increased catastrophe frequencies at both ends. Furthermore, binding B-lattice GDP microtubules to a rigor kinesin surface stabilizes them against shrinkage, whereas microtubules with extra A-lattice seams are stabilized only slightly. Our data suggest that introducing extra A-lattice seams into dynamic microtubules destabilizes them by destabilizing their GTP caps. On this basis, we propose that the single A-lattice seam of natural B-lattice MTs may act as a trigger point, and potentially a regulation point, for catastrophe.

Reconstituting the motility of isolated intracellular cargoes

Kinesin, dynein, and myosin transport intracellular cargoes including organelles, membrane-bound vesicles, and mRNA along the cytoskeleton. These motor proteins work collectively in teams to transport cargoes over long distances and navigate around obstacles in the cell. In addition, several types of motors often interact on the same cargo to allow bidirectional transport and switching between the actin and microtubule networks. To examine transport of native cargoes in a simplified in vitro system, techniques have been developed to isolate endogenous cargoes and reconstitute their motility. Isolated cargoes can be tracked and manipulated with high precision using total internal reflection fluorescence microscopy and optical trapping. Through use of native cargoes, we can examine vesicular transport in a minimal system while retaining endogenous motor stoichiometry and the biochemical and mechanical characteristics of both motor and cargo.