Micromanipulation of Macromolecules: How to Measure the Stiffness of Single Microtubules

Abstract

Optical tweezers are a combination of an intense light source and a standard light microscope, making it possible to grab and manipulate optically refracting particles in solution with the momentum of light. Ashkin and Dziedzic first used optical tweezers to manipulate biological objects such as viruses and bacteria in 1987. Since then, single-beam laser traps have frequently been employed to hold, move, and deform cells and subcellular particles. Examples include, but are not restricted to, the holding of yeast cells within a trap for more than one cell cycle (Ashkin et al. 1987), the manipulation of nuclei and organelles in plant cells (Ashkin and Dziedzic 1989; Leitz et al. 1994) and protozoa (Aufderheide et al. 1992), the displacement of chromosomes or chromosomes fragments in cultured cells (Berns et al. 1989; Seeger et al. 1991), the blockage of axonal transport (MArtenson et al. 1993), and cell sorting (Buican 1991; for reviews, see Block 1990; Kuo and Sheetz 1992; Weber and Greulich 1992). These studies have demonstrated convingcingly that intracellular organelles up to the size of nuclei as well as whole cells can be displaced or deformed without damaging effects. Apart from these in vivo experiments, optical tweezers-based techniques are widely used to measure physical parameters of single molecular motors and their step sizes (Finer et al. 1994; Kuo and Sheets 1993; Svoboda et al. 1993), or the elastic parameters of DNA (Perkins et al. 1995) or microtubules (Kurachi et al. 1995).