How Actin-Myosin Interactions Differ With Different Isoforms of Myosin

Correspondence to Saul Winegrad, MD, Department of Physiology, School of Medicine, University of Pennsylvania, 37th and Hamilton Walk, Philadelphia, PA 19104-6085.

Key Words: myosin • myosin heavy chain • in vitro motility • unitary force • unitary displacement • actin-myosin interaction

The development of in vitro techniques for assaying the mechanical properties of individual actin-myosin interactions has provided investigators with a powerful tool to address questions about fundamental properties of the force-generating reactions that produce movement of cells or organelles within cells. These techniques are capable of measuring the force and/or displacement produced by the interaction of a single force generator with a single actin filament. The load on the force generator can be varied in a controlled manner to allow the sampling of force and velocity under a variety of conditions. Progress is being made toward measuring the amount of ATP split during these reactions. There are, however, limitations to the existing in vitro motility assays, such as the inability to control the orientation of the molecules rigorously and the quantitative effects of brownian motion on the force and displacement transients that are produced by the interaction between myosin and actin. One consequence of these problems and the different approaches of the several laboratories using in vitro assays is the existence of a range of values for the force and displacement produced by the unitary force–generating set of reactions. Most results indicate forces between 2 and 10 pN and displacements from 5 to 15 nm with means of about 5 pN and 5 nm, respectively. The latter is about the periodicity of actin in the thin filament. In spite of these problems, which are slowly responding to the ingenuity of investigators using the techniques, much useful information has been produced. . . . [Full Text of this Article]