Organized living: formation mechanisms and functions of plasma membrane domains in yeast Natasza E. Zio ´ lkowska 1* , Romain Christiano 2* and Tobias C. Walther 2 1 Organelle Architecture and Dynamics, Max Planck Institute of Biochemistry, 82158 Martinsried, Germany 2 Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06520, USA Plasma membrane proteins and lipids organize into lateral domains of specific composition. Domain forma- tion is achieved by a combination of lipid–lipid and lipid– protein interactions, membrane-binding protein scaf- folds and protein fences. The resulting domains function in membrane protein turnover and homeostasis, as well as in cell signaling. We review the mechanisms generat- ing plasma membrane domains and the functional con- sequences of this organization, focusing on recent findings from research on the yeast model system. The plasma membrane of yeast is highly organized The plasma membrane delineates the cell and separates it from its environment. The plasma membrane is now rec- ognized as a complex organelle organized into lateral domains. This compartmentalization is thought to allow precise spatiotemporal control of plasma membrane pro- cesses, such as establishing and maintaining cell polarity, transducing signals and membrane protein and lipid turn- over. Because of the availability of powerful genetic, bio- chemical and cell biological methods, Saccharomyces cerevisiae offers unique opportunities as a model system for studying plasma membrane organization. Here, we build on other reviews [1–4] and describe the basic prin- ciples and functions of membrane organization, focusing particularly on research in S. cerevisiae. Plasma membrane composition and structure The plasma membrane consists of several hundreds of highly diverse lipid and protein species. Lipids differ in their headgroups and their fatty acids of varying length and saturation [5,6]. The abundance of a particular lipid species is not the same in both leaflets of the membrane bilayer. Negatively charged lipids, such as phosphatidyl- serine and phosphatidylinositides, localize mostly to the inner membrane leaflet, whereas phosphatidylcholine and sphingolipids are more abundant in the outer leaflet [7]. This bilayer asymmetry is important for lipid–lipid and lipid–protein interactions on either side of the membrane, including specific interactions of lipids with receptor, cy- toskeletal and signaling molecules. The second major class of plasma membrane compo- nents consists of proteins, which are very diverse in amino acid sequence and properties. The genome of the yeast S. cerevisiae encodes approximately 300 different plasma membrane proteins (as derived from gene ontology anno- tation; see www.yeastgenome.org/). The human genome is predicted to contain a much larger number of genes encod- ing plasma membrane proteins (3000–3200 from gene ontology annotation). Proteins and lipids organize into lateral membrane domains at different scales. Large domains extend many micrometers, sometimes occupying a large fraction of the cell surface, segregating it into a few distinct compart- ments. Examples of such large-scale domains include the apical membrane of epithelial cells or the bud of yeast cells. On a smaller scale, proteins and specific lipids demix into neighboring, often dynamic, compartments in the scale of nanometers to a few micrometers (Figure 1). This type of domain includes, for example, yeast eisosomes/membrane compartment containing Can1 (MCC). In this review, we focus mostly on these small-scale domains. Plasma membrane organization in yeast: domains and networks Rapid research progress during the past few years has revealed that the yeast plasma membrane features a promi- nent membrane domain pattern (Figure 2). Currently, three such domains are known: the MCC, MCT [membrane com- partment containing target of rapamycin kinase complex 2 (TORC2)] and MCP (membrane compartment containing Pma1). These domains appear microscopically either as mutually exclusive punctae (MCC and MCT) or as a network percolating between the other domains (MCP) [2,8–11]. The MCC is enriched in transporters, such as Can1, and is thought to contain elevated sterol levels [9]. Each MCC appears ultrastructurally as a 50-nm-wide and 300-nm- long furrow [12,13] that is coated by eisosomes, which are large, peripheral membrane protein complexes [14]. Dele- tion of the gene encoding the main eisosome component Pil1 leads to loss of the normal punctate pattern of eiso- somes/MCC proteins and their clustering into one or a few eisosome remnants per cell [9,14]. Furthermore, sterols require Pil1 for their normal distribution, because the normal punctate staining of the fluorescent sterol-binding dye filipin collapses in pil1D cells [9]. Review Corresponding author: Walther, T.C. (tobias.walther@yale.edu) Keywords: lipid rafts; microdomains; eisosomes; protein scaffolds; BAR domains; protein fences. * These authors contributed equally to this work. 0962-8924/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2011.12.002 Trends in Cell Biology, March 2012, Vol. 22, No. 3 151