Mechanisms to regulate microtubule growth via plus-end binding

Abstract

Microtubules are long filaments that control cellular structure and influence intracellular transport. Motor proteins move along microtubules to transport cargo important for many cellular processes. Microtubules themselves also grow and shrink in length according to cell needs, which helps support changes in cell size, shape, and function. Growth at the plus-ends of microtubules is affected by many factors, two of which are described in the later chapters of this thesis. In Chapter 2, completed work involving the microtubule motor Kinesin-14 is described. Kinesin-14 molecular motors represent an essential class of proteins that bind microtubules and walk towards their minus-ends. Previous studies have described important roles for Kinesin-14 motors at microtubule minus-ends, but their role in regulating plus-end dynamics remains controversial. Kinesin-14 motors have been shown to bind the EB family of microtubule plus-end binding proteins, suggesting that these minus-end directed motors could interact with growing microtubule plus-ends. In this work, we explored the role of minus-end directed Kinesin-14 motor forces in controlling plus-end microtubule dynamics. We found that Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate microtubule lengths in cells. This work shows a new potential pathway by which microtubule growth could be regulated by microtubule associated motor activity. In Chapter 3, completed work involving the direct effects of the female hormone estrogen on microtubule dynamics is described. Estrogen has been implicated in alterations to the microtubule network for a variety of cell types. However, the effects of estrogen on individual microtubules are unknown. In this work we systematically investigated a mechanism by which the naturally occurring 17β-estradiol could alter the length dynamics of individual microtubules. Estradiol pauses microtubule growth without inducing an increased incidence of catastrophe events, similar to the widely used microtubule poison colchicine. This work showed that estrogen’s ability to limit excessive microtubule proliferation could have important implications for sex differences in disease states, and potentially for therapeutic approaches in heart disease and breast cancer. Taken together, the work described below provides two new examples of how plus-ends of microtubules can be perturbed in subtle ways that could lead to important regulation of microtubule growth in multiple cellular contexts. Given that microtubules and their dynamic growth behavior is critical for a myriad of cellular functions, these findings could have broad implications in cell biology research in the future and these studies could inspire many future avenues of inquiry into further plus-end tip structure changes, growth consequences, and the nature and effects of additional protein and molecule interactions with microtubule plus-ends.