Actin and actin-binding proteins in yeast

Recently it has been shown that the amoeboid form of Dictyosteliurn discoideurn can chemotax in the absence of either myosin, a-actinin or severin without immediately obvious defects [reviewed by Bray and Vasiliev, 19891. These observations lead one to wonder what contractile proteins do in nonmuscle cells. A number of investigators have begun in the past few years to study the actin cytoskeleton in the yeast Saccharornyces cerevisiae with the hope of associating the biochemical activities of contractile proteins observed in vitro with in vivo functions. Although yeast do not exhibit the full range of biological behaviors of higher organisms such as Dictyosteliurn, the simplicity of yeast can be an advantage in elucidating functional interactions. Moreover, S. cerevisiae offers the most sophisticated genetics available in a eukaryote (see the accompanying review by Stearns). A number of components of the yeast actin cytoskeleton have now been identified, and, although this work is still in its infancy, genetic studies on several of these proteins are already shedding light on their in vivo functions. Since yeast are nonmotile and have not been shown to exhibit actin-based organelle transport or cytoplasmic streaming, possible roles for yeast actin are not immediately obvious. Clues about what actin might be doing in yeast were first provided by studies on the organization of the actin cytoskeleton [Kilmartin and Adams, 1984; Adams and Pringle, 19841. S. cerevisiae cells replicate by growing in a highly polarized manner to form buds, which ultimately become daughter cells. Budding occurs when a spatially restricted region of the cell surface enlarges by the incorporation of new membrane and cell wall material. Fluorescence microscopy has shown that the yeast actin cytoskeleton is disposed in a manner that reflects the asymmetry of growth [Kilmartin and Adams, 1984; Adams and Pringle, 19841. The figure shows schematically how actin (encoded by the ACT1 gene) is organized in a budding yeast cell. Cables are aligned along the growth axis and cortical patches of actin are concentrated at the growing cell surface in the bud. These patches first appear clustered in a ring at the cell cortex early in the cell cycle (not shown); this ring predicts the precise site of bud emergence. Late in the cell cycle, the patches cluster in the region of the neck separating the daughter from the mother cell. Localized cell wall growth in this region results in the separation of the mother and daughter cells. The patches often seem to be located at the ends of actin filaments. Staining of the patches with rhodamine-phalloidin suggests that they contain filamentous actin [Adams and Pringle, 19841. The tight association of the actin patches with regions of surface growth suggests that they might be parts of the machinery responsible for localized membrane and cell wall insertion. The cables are well positioned to mediate directed transport of organelles synthesized in the mother cell to the bud. Understanding the function of actin in yeast will require an understanding of its organization on an ultrastructural level. The actin cables seen by fluorescence microscopy are likely to be filament bundles. Bundles of filaments have been observed in electron micrographs of yeast [Adams and Pringle, 19841; immunogold-labeling of yeast with antiactin antisera is needed to determine whether these filaments are composed of actin. Labeling actin filaments with myosin heads will determine if the actin filaments in a yeast cell have a uniform polarity. A uniform filament polarity with the barbed ends attached to the bud surface would make it possible for myosin-like motors to move organelles toward the bud surface. No structures corresponding to the cortical actin patches have been observed in the electron microscope. The patches might be transmembrane linkages between the