Cytoplasmic contractile proteins

In reviewing work on cytoplasmic contractile proteins and the contributions made to this field by The Journal of Cell Biology, some perspective is gained by first pointing out that most cell biologists were not aware of the existence of these important cellular constituents more than 12 years ago. Times have changed, and today cytoplasmic contractile protein research is one of the busiest areas in cell biology. My purpose here is to highlight some of the important events in the growth of this field and to forecast some future trends . More exhaustive coverage of the field is found in recent books (1, 2) and review articles (3-7). Closely related historical reviews on cellular motility by R. D. Allen and muscle by Franzini-Armstrong and Peachy are included in this volume . Without question, the most important landmark in this field was the independent purification of actin and myosin from the slime mold Physarum by Hatano and co-workers (8, 9) in Japan and by Adelman and Taylor (10) in the United States in the late 1960s (Fig . 1) . To be sure, there were earlier reports by Loewy (11) and others (12) describing "actomyosin-like" proteins in nonmuscle cells, but all of these preparations were too crude to be characterized convincingly. However, once highly purified contractile proteins were available, it was straightforward to establish that they shared many important features with their muscle counterparts and to make a strong argument that they participate in cellular motile mechanisms . A second major event was the publication in The Journal of Cell Biology in 1969 of a paper by Ishikawa et al . (13) that described a morphological technique for identifying actin filaments in cells by electron microscopy. Their technique was simply to treat glycerated cells with muscle heavy meromyosin that decorated cytoplasmic thin filaments with arrowheadshaped complexes (Fig . 2), identical with those originally observed along heavy meromyosin-decorated pure actin filaments (Fig. 3) by Huxley (14) . Armed with this technique, morphologists found actin virtually everywhere in nature (reviewed in reference 3) . More recently a second generation of morphologists has used fluorescent antibodies to localize actin (15), myosin (16), and additional accessory proteins (17) in many cell types. This work has led to a large number ofstudies characterizing the cytoplasmic contractile protein molecules and their distributions in cells. Other lines of investigation in this area have included efforts to demonstrate the involvement of the con-