Yeast studies reveal moonlighting functions of the ancient actin cytoskeleton

Abstract

Classic functions of the actin cytoskeleton include control of cell size and shape and the internal organization of cells. These functions are manifest in cellular processes of fundamental importance throughout biology such as the generation of cell polarity, cell migration, cell adhesion, and cell division. However, studies in the unicellular model eukaryote Saccharomyces cerevisiae (Baker's yeast) are giving insights into other functions in which the actin cytoskeleton plays a critical role. These include endocytosis, control of protein translation, and determination of protein 3-dimensional shape (especially conversion of normal cellular proteins into prions). Here, we present a concise overview of these new "moonlighting" roles for the actin cytoskeleton and how some of these roles might lie at the heart of important molecular switches. This is an exciting time for researchers interested in the actin cytoskeleton. We show here how studies of actin are leading us into many new and exciting realms at the interface of genetics, biochemistry, and cell biology. While many of the pioneering studies have been conducted using yeast, the conservation of the actin cytoskeleton and its component proteins throughout eukaryotes suggests that these new roles for the actin cytoskeleton may not be restricted to yeast cells but rather may reflect new roles for the actin cytoskeleton of all eukaryotes.

Keywords: Arp2/3; Lsb2p; Sup35p; Yih1p; amyloid; eEF1A; formin.

Figure 1. The yeast actin cytoskeleton and its importance for endocytosis

A. Visualisation of actin cytoskeleton via fluorescence microscopy of S. cerevisiae cells, fixed and stained with fluorophore-conjugated phalloidin (F-actin specific reagent). Scale bar, 5 µm. B. Internalisation and vacuolar accumulation of the fluorescent endocytic dye Lucifer Yellow by endocytosis in S. cerevisiae cells. Scale bar, 5 µm.

Figure 1. The yeast actin cytoskeleton and its importance for endocytosis

A. Visualisation of actin cytoskeleton via fluorescence microscopy of S. cerevisiae cells, fixed and stained with fluorophore-conjugated phalloidin (F-actin specific reagent). Scale bar, 5 µm. B. Internalisation and vacuolar accumulation of the fluorescent endocytic dye Lucifer Yellow by endocytosis in S. cerevisiae cells. Scale bar, 5 µm.

Figure 2. Actin-Translation connections found in yeasts

A. eEF1A bound to F-actin and Yih1p bound to G-actin do not participate in translation and in controlling Gcn2p-mediated translational regulation, respectively. B. eEF1A can be released from F-actin by either Bni1p or the α subunit of the guanine nucleotide exchange factor eEF1B, and may then participate in protein synthesis. Studies support the idea that Yih1p released from G-actin sequesters Gcn1p thereby inhibiting Gcn2p function. For more detailed explanation please see text.