Optical Tweezers-Based Measurements of Forces and Dynamics at Microtubule Ends.

Microtubules are dynamic cytoskeletal polymers that polymerize and depolymerize while interacting with different proteins and structures within the cell. The highly regulated dynamic properties as well as the pushing and pulling forces generated by dynamic microtubule ends play important roles in processes such as in cell division. For instance, microtubule end-binding proteins are known to affect dramatically the dynamic properties of microtubules, and cortical dyneins are known to mediate pulling forces on microtubule ends. We discuss in this chapter our efforts to reconstitute these systems in vitro and mimic their interactions with structures within the cell using micro-fabricated barriers. Using an optical tweezers setup, we investigate the dynamics and forces of microtubules growing against functionalized barriers in the absence and presence of end-binding proteins and barrier-attached motor proteins. This setup allows high-speed as well as nanometer and piconewton resolution measurements on dynamic microtubules.

Force generation by dynamic microtubules in vitro.

Biopolymers are essential for cellular organization. They bridge the cell interior, forming a framework that is used as a reference for different cellular organelles. This framework, called the cytoskeleton, is not static but constantly reorganizes. The dynamics of the cytoskeleton allows the cell to rearrange its interior for various processes, such as cell division. This dynamic reorganization relies at least partly on forces that arise from the assembly and disassembly of cytoskeletal biopolymers. In many cases, these forces are generated when biopolymers interact with the cell boundary. This chapter focuses on force generation by and regulation of microtubules (MTs) that interact with growth-opposing barriers. We describe three in vitro assays that can be used to mimic MT interactions with the cell boundary. The essential components in each of our minimal systems are (functionalized) microfabricated barriers against which we grow MTs under different conditions. We describe in detail the different methods and assays necessary to realize these in vitro experiments.

Facilitated diffusion with DNA coiling.

When DNA-binding proteins search for their specific binding site on a DNA molecule they alternate between linear 1-dimensional diffusion along the DNA molecule, mediated by nonspecific binding, and 3-dimensional volume excursion events between successive dissociation from and rebinding to DNA. If the DNA molecule is kept in a straight configuration, for instance, by optical tweezers, these 3-dimensional excursions may be divided into long volume excursions and short hops along the DNA. These short hops correspond to immediate rebindings after dissociation such that a rebinding event to the DNA occurs at a site that is close to the site of the preceding dissociation. When the DNA molecule is allowed to coil up, immediate rebinding may also lead to so-called intersegmental jumps, i.e., immediate rebindings to a DNA segment that is far away from the unbinding site when measured in the chemical distance along the DNA, but close by in the embedding 3-dimensional space. This effect is made possible by DNA looping. The significance of intersegmental jumps was recently demonstrated in a single DNA optical tweezers setup. Here we present a theoretical approach in which we explicitly take the effect of DNA coiling into account. By including the spatial correlations of the short hops we demonstrate how the facilitated diffusion model can be extended to account for intersegmental jumping at varying DNA densities. It is also shown that our approach provides a quantitative interpretation of the experimentally measured enhancement of the target location by DNA-binding proteins.