Microtubules (MTs) constitute the largest components of the eukaryotic cytoskeleton and facilitate a plethora of
diverse functions including intracellular transport, cellular motility, and, cell division. During mitosis, MTs
aggregate to form the mitotic spindle, making them a potent drug target for many successful chemotherapeutic
agents, including paclitaxel and vinblastine, known as spindle poisons. MT-targeting drugs operate by interfering
with dynamic instability (DI): the ability of MTs to rapidly switch from polymerizing to depolymerizing (referred to
as catastrophe) and vice-versa. Paclitaxel operates by decreasing catastrophe rate while vinblastine encourages
catastrophe and inhibits polymerization. A full understanding of MT catastrophe will greatly aid in the design of
spindle poisons with fewer off-target effects, as well as greatly advance general understanding of DI.
Each MT is composed of aß-tubulin heterodimers, stacked head-to-tail in protofilaments (PFs) which are aligned
laterally to form a hollow tube. Both a- and ß-tubulin bind guanosine triphosphate (GTP) and hydrolysis of GTP
to GDP (guanosine diphosphate) at the ß-tubulin binding site is hypothesized to induce stress on the MT lattice.
This stress gradually builds until the subunits at the MT end undergo GTP hydrolysis, at which point PFs begin
to peel apart and catastrophe has occurred. Lag between GTP hydrolysis and polymerization creates a construct
referred to as the GTP cap: a group of subunits at the MT end that have yet to hydrolyze GTP, release the
product inorganic phosphate (Pi), or undergo a structural transition. Recent studies have caused doubt in the
field on the nature of this transition and an atomistic understanding of the underlying mechanisms will lead to a
full understanding of catastrophe. I propose to computationally resolve three key aspects of catastrophe: the
mechanism of GTP hydrolysis, the release of Pi, and the structural coupling between PFs leading to catastrophe.
First, I will use enhanced sampling methodology to uncover the enzymatic mechanism of GTP hydrolysis, with
emphasis placed on potential catalytic residues belonging to a-tubulin, which sits atop ß-tubulin upon
polymerization to form the active site. Subsequently, I will develop novel computational techniques to determine
the pathway of Pi release post-hydrolysis and examine the potential for structural change upon release. Lastly, I
will develop a coarse-grained (CG) model of a full MT, using rates determined from the previous studies, able to
undergo catastrophe to examine how hydrolysis and Pi release in neighboring subunits affects the potential for
these reactions to occur in a particular subunit. This will give an unprecedentedly detailed view of the loss of the
GTP cap and the steps leading to catastrophe. Additionally, I will collaborate with two leading experimentalists
in the MT community to develop mutants that specifically test my hypotheses and to obtain lattice parameters of
MTs doped with spindle poisons. This will allow me to integrate the effects of drugs into the CG model and
examine how their effects propagate along an MT. These results and the developed models will greatly advance
the understanding of DI and hopefully lead to the development of gentler MT-targeting therapies in the future.