Contribution of actin to the structure of the cytoplasmic matrix

The realization that actin is a constituent of nonmuscle cells (1) and the identification of actin filaments in the periphery ofsuch cells (2) opened up a new world ofmolecular biology. For the first time it became possible to explain in detail a body of phenomena described over two centuries concerning important aspects of cell shape, movement, and consistency. Although actin comprises only one of several intracellular fiber systems of cells, it is an extremely important one . This essay briefly reviews some present concepts of the contribution of actin to the "cytoplasmic matrix." The appreciation of actin's existence in nonmuscle cells immediately provided some intuitively attractive ideas concerning the behavior of such cells . First, actin is a globular monomer that assembles reversibly to form long fibers, and this assembly increases the apparent viscosity of an actin solution (3) . Therefore, changes in the state of assembly in different parts of a cell could account for differences in cytoplasmic consistency . Second, long actin fibers, if sufficiently stiff and especially if organized as bundles, could maintain the cell or parts of cells in a particular configuration . Third, actin, working together with myosin, could generate the contractility observed in these cells . Fourth, actin fibers could act as cables to tie parts ofthe cell, including its investing membrane, together. These four basic ideas, which concern "mechanical" properties of actin fibers, continue in their broadest sense to be valid . However, it has become apparent that the functions ofactin, influenced by a seemingly endless number of "actin-binding proteins," are extraordinarily complex . Although this complexity seems formidable and confusing at first glance, it actually is a testimony to the marvellous versatility of this highly conserved protein (4) and to nature's ingenuity in the engineering ofcytoplasm . Research on actin in cytoplasmic structure and function has been advancing on two fronts . On one, investigators study the details of actin assembly and rheology in vitro and the influence of purified actin-associated proteins on these properties . The rheological behavior ofcrude cytoplasmic extracts has often provided the direction for such research . On the other, cell biologists examine the morphology ofactin in cells in the light and the electron microscope and also determine the location of the actin-associated proteins in the cell .