How do muscles produce work? Using optical tweezers to study molecular machines
Submitted by sis on 09 March 2007
There is currently much enthusiasm for nanotechnology, the field that explores and seeks to control the microscopic world. In my lab at King’s College, London, we use an apparatus called ‘optical tweezers’ to manipulate and study these motors individually. Microscopic man-made machines may still be the stuff of science fiction, but through experiments of this kind, nature may well teach us our first lesson.
By understanding how these molecules work in detail, we can gain insight into the ways that nature, through evolution, has engineered many different forms of life from remarkably few templates. Many kinds of single-molecule ‘machines’ exist in nature, and the tasks they perform are as varied as they are vital. They enable living cells to function, move and reproduce. For example, some of them transport nutrients across the cell by ‘walking’ along an intracellular railroad network. Molecular-sized motors probably play a crucial role in cell division by separating chromosomes after they have replicated. Other types of motors rotate the flagellum that propels sperm cells and some bacteria. Many of these motors are structurally very similar, and yet they perform distinct tasks.
Muscle contraction is achieved by sliding two kinds of filaments past one another. One of these filaments is made of myosin (the motor), the other of a substance called actin (see figure above). The two kinds of filaments overlap in the muscle cells to maximise their interactions.
During each cycle, the two filaments slide past each other by a short distance – this is the basis of muscle contraction. The forces and movements produced by a single cycle are minute, but the combination of millions of myosin molecules acting simultaneously amplifies the effect by many orders of magnitude.
Biologists have been studying muscle and myosin for many decades, but for all our present knowledge, there still remains a vast region of mystery: how strongly do the individual myosin molecules pull and over what distance? How is the duration of one cycle affected by the chemical environment? We know that the total work done by the myosin molecule is strongly affected by the external force against which the motor has to pull, but the relationship between them is still poorly understood. Because of their small size, motor molecules like myosin react to their environment very differently than would larger molecules. For example, in contrast to a car engine, the effects of viscosity in the environment and of the continual bombardment of water molecules (the phenomenon known as Brownian motion) are considerable. At the molecular scale, moving through water actually feels more like what we would experience when swimming in turbulent honey!
Scientists are insatiably curious. Theorists devise models to suggest how muscles work, but at the end of the day, only experiments will really push our understanding further. However, a detailed understanding of myosin cannot be gained by looking at whole muscles because what we see is only the overall effect of many myosin motors acting independently. What is needed instead is a way to control and examine the myosin molecules in isolation. But this is a monstrous challenge in itself. These are obviously not objects you can simply hold with a steady hand!
Our experiment at King’s College is designed to recreate, under the microscope, the basic unit of a contractible muscle. The aim is to investigate how a single myosin molecule responds to an external force applied to an actin filament via the optical tweezers. First we produce two traps by shining two laser beams into our microscope objective (see figure). Each trap is then made to catch one micron-sized plastic bead that is specially coated to bind actin filaments. A single filament is caught and stretched between the beads, producing a kind of dumbbell. We now steer this dumbbell towards another bead that is stuck to the surface of our microscope cover slip and coated with myosin molecules. When the conditions are just right, we manage to make just one myosin molecule, located near the top of the central bead, interact with the actin filament in the dumbbell, just as it would do in a whole muscle fibre.
It goes without saying that these experiments require a good dose of patience. They also require the collaboration of scientists with very different domains of expertise. Biologists are crucial to connect the results of the experiments to the known physiological behaviour of muscles, and to ensure that the project remains on the right track. Physicists are equally important in properly setting up the optical and electronic components of the apparatus and in analysing the results quantitatively. As in any area of research, the frontiers of our knowledge are only extended by trying out new things and by being endlessly curious and inquisitive. So the next time you think there is nothing simpler than throwing a basketball, think again.
Nanotechnology is a fashionable subject in science fiction as well as in cutting-edge research and, in this interesting article, Alexandre Lewalle tells us about nanomotors in muscle cells (the tiny actin and myosin filaments) and about the ways in which they are investigated by means of nanotools (the optical tweezers).
The article, written in an enjoyable style with clear examples and vivid metaphors, is suitable for biology teachers willing to update their knowledge and for upper secondary school students interested in the forefront of research.