YEASTBOOK CELL SIGNALING & DEVELOPMENT Architecture and Biosynthesis of the Saccharomyces cerevisiae Cell Wall Peter Orlean1 Department of Microbiology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ABSTRACT The wall gives a Saccharomyces cerevisiae cell its osmotic integrity; defines cell shape during budding growth, mating, sporulation, and pseudohypha formation; and presents adhesive glycoproteins to other yeast cells. The wall consists of b1,3- and b1,6- glucans, a small amount of chitin, and many different proteins that may bear N- and O-linked glycans and a glycolipid anchor. These components become cross-linked in various ways to form higher-order complexes. Wall composition and degree of cross-linking vary during growth and development and change in response to cell wall stress. This article reviews wall biogenesis in vegetative cells, covering the structure of wall components and how they are cross-linked; the biosynthesis of N- and O-linked glycans, glycosylphos- phatidylinositol membrane anchors, b1,3- and b1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins. TABLE OF CONTENTS Abstract 775 Introduction 777 Wall Composition and Architecture 777 Polysaccharides 778 Chitin: 778 b-Glucans: 778 Cross-links between polysaccharides: 779 Cell wall mannoproteins 779 GPI proteins: 780 Mild alkali-releasable proteins: 780 Disulfide-linked proteins: 780 Strategies to identify CWP 780 Cell wall phenotypes 781 Precursors and Carrier Lipids 781 Sugar nucleotides 781 Dolichol and dolichol phosphate sugars 781 Dolichol phosphate synthesis: 781 Continued Copyright © 2012 by the Genetics Society of America doi: 10.1534/genetics.112.144485 Manuscript received May 17, 2012; accepted for publication August 6, 2012 Supporting information is available online at http://www.genetics.org/lookup/suppl/doi:10.1534/genetics.112.144485/-/DC1. 1Address for correspondence: Department of Microbiology, University of Illinois at Urbana-Champaign, B-213 Chemical and Life Sciences Laboratory, 601 South Goodwin Ave., Urbana, IL 61801. E-mail: [email protected] Genetics, Vol. 192, 775–818 November 2012 775 CONTENTS, continued Dol-P-Man and Dol-P-Glc synthesis: 781 Biosynthesis of Wall Components Along the Secretory Pathway 781 N-Glycosylation 782 Assembly and transfer of the Dol-PP-linked precursor oligosaccharide: 782 Steps on the cytoplasmic face of the ER membrane: 782 Transmembrane translocation of Dol-PP-oligosaccharides: 782 Lumenal steps in Dol-PP-oligosaccharide assembly: 783 Oligosaccharide transfer to protein: 783 N-glycan processing in the ER and glycoprotein quality control: 783 Mannan elaboration in the Golgi: 784 Formation of core-type N-glycan and mannan outer chains: 784 Mannan side branching and mannose phosphate addition: 784 O-Mannosylation 785 Protein O-mannosyltransferases in the ER: 785 Extension and phosphorylation of O-linked manno-oligosaccharide chains: 785 Importance and functions of O-mannosyl glycans: 785 GPI anchoring 785 GPI structure and proteins that receive GPIs: 785 GPI structure: 785 Identification of GPI proteins: 786 Assembly of the GPI precursor and its attachment to protein in the ER: 786 Steps on the cytoplasmic face of ER membrane: 786 Lumenal steps in GPI assembly: 787 GPI transfer to protein: 788 Remodeling of protein-bound GPIs: 788 Sugar nucleotide transport 789 GDP-Man transport: 789 Other sugar nucleotide transport activities: 789 Biosynthesis of Wall Components at the Plasma Membrane 789 Chitin 789 Septum formation: 789 Chitin synthase biochemistry: 790 S. cerevisiae’s chitin synthases and auxiliary proteins: 791 Chitin synthase I: 791 Chitin synthase II and proteins impacting its localization and activity: 791 Chitin synthase III and proteins impacting its localization and activity: 792 Chitin synthesis in response to cell wall stress: 793 Chitin synthase III in mating and ascospore wall formation: 794 b1,3-Glucan 794 Fks family of b1,3-glucan synthases: 794 Roles of the Fks proteins in b1,3-glucan synthesis: 794 Rho1 GTPase, a regulatory subunit of b1,3-glucan synthase: 795 b1,6-Glucan 795 In vitro synthesis of b1,6-glucan 795 Proteins involved in b1,6-glucan assembly 796 ER proteins: 796 Homologs of the UGGT/calnexin protein quality control machinery: 796 Fungus-specific ER chaperones required for b1,6-glucan synthesis: 796 More widely distributed secretory pathway proteins: 797 Kre6 and Skn1: 797 Kre9 and Knh1: 797 Plasma membrane protein Kre1: 797 How might b1,6-glucan be made?: 797 Continued 776 P. Orlean CONTENTS, continued Remodeling and Cross-Linking Activities at the Cell Surface 797 Order of incorporation of components into the cell wall 797 Incorporation of GPI proteins into the wall 798 Incorporation of PIR proteins into the wall 798 Cross-linkage of chitin to b1,6- and b1,3-glucan 799 Cell Wall-Active and Nonenzymatic Surface Proteins and Their Functions 799 Known and predicted enzymes 799 Chitinases: 799 b1,3-glucanases: 799 Exg1, Exg2, and Ssg/Spr1 exo-b1,3-glucanases: 799 Bgl2, Scw4, Scw10, and Scw11 endo-b1,3-glucanases: 800 Eng1/Dse4 and Eng2/Acf2 endo-b1,3-glucanases: 800 Gas1 family b1,3-glucanosyltransferases: 800 Yapsin aspartyl proteases: 800 Nonenzymatic CWPs 801 Structural GPI proteins: 801 Sps2 family: 801 Tip1 family: 801 Sed1 and Spi1: 801 Ccw12: 801 Other nonenzymatic GPI proteins: 802 Flocculins and agglutinins: 802 Non-GPI-CWP: 802 PIR proteins: 802 Scw3 (Sun4): 803 Srl1: 803 What Is Next? 803 HE wall gives Saccharomyces cerevisiae its morphologies direct, and the number of genes that encode enzymes directly Tduring budding growth, pseudohypha formation, mat- involved in biosynthesis or remodeling of the wall, or non- ing, and sporulation; it preserves the cell’s osmotic integrity; enzymatic wall proteins, is now 180 (see Supporting Infor- and it provides a scaffold to present agglutinins and floccu- mation, Table S1). This review covers these proteins, with lins to other yeast cells. The wall consists of mannoproteins, emphasis on the wall of vegetative cells during the budding b-glucans, and a small amount of chitin, which become cycle and in response to stress. Wall synthetic activities will be cross-linked in various ways. Wall composition and organi- covered in the context of their cellular localization, starting zation vary during growth and development. During the with precursors in the cytoplasm, proceeding along the secre- budding cycle, deposition of chitin is tightly controlled, tory pathway from the endoplasmic reticulum (ER) to the and expression of certain hydrolases involved in cell separa- plasma membrane, and culminating with the events outside tion is daughter cell-specific. The wall can be weakened, and the plasma membrane that generate covalent cross-links be- the cell consequently stressed, by treatment with polysaccha- tween wall components. Additional information about indi- ride binding agents such as Calcofluor White (CFW), Congo vidual proteins and the phenotypes of strains lacking them is Red, sodium dodecyl sulfate (SDS), aminoglycoside antibio- presented in File S1, File S2, File S3, File S4, File S5, File S6, tics, and b-glucanase preparations or by mutational loss of File S7, File S8,andFile S9. Earlier work on the yeast cell capacity to make a wall component. Such stresses commonly wall has been reviewed by Ballou (1982), Fleet (1991), activate the cell wall integrity (CWI) pathway (Levin 2011) Orlean (1997), Kapteyn et al. (1999a), Cabib et al. (2001), and result in compensatory synthesis of wall material. Klis et al. (2002, 2006), and Lesage and Bussey (2006). Up to a quarter of the genes in S. cerevisiae have some role in maintenance of a normal wall. From the results of a survey Wall Composition and Architecture of deletion strains for cell wall phenotypes, De Groot et al. (2001) estimated that 1200 genes, not counting essential The wall accounts for 15–30% of the dry weight of a vege- ones, impact the wall. Most of the effects, however, are in- tative S. cerevisiae cell (Aguilar-Uscanga and François 2003; S. cerevisiae Cell Wall 777 Yin et al. 2007). It is 110–200 nm wide, as estimated from Polysaccharides transmission electron micrographs and by using an atomic Wall polysaccharides are typically separated into three force microscope to detect surface accessibility of “molecular fractions defined on the basis of their solubility in alkali rulers” consisting of versions of the plasma membrane sensor and acid (Fleet 1991). These fractions contain differing rel- Wsc1 with different lengths (Dupres et al. 2010; Yamaguchi ative amounts of b1,3- and b1,6-linked glucans and mannan et al. 2011). The wall’s major components are b1,3- and (Magnelli et al. 2002) and also differ in whether and to what b1,6-linked glucans, mannoproteins, and chitin, which can extent the glucans are cross-linked to chitin, which deter- be covalently joined to form higher-order complexes. The mines their solubility in alkali. Determination of Man-to-Glc b1,6-glucan, although quantitatively a minor component ratios in total acid hydrolysates of walls has been useful in of the wall, has a central role in cross-linking wall compo- assessing the impact of mutations on wall composition (Ram nents (Kollar et al. 1997). Some mannoproteins have or are et al. 1994; Dallies et al. 1998). Digestion of isolated walls predicted to have enzymatic activity as hydrolases or cross- and wall fractions with linkage-specific glycosidases has linkers; others may have structural roles or mediate “social been used to quantify wall components and determine the activity” by serving as mating agglutinins or flocculins. fine structure of b1,6-glucan (Boone et al. 1990; Magnelli Among the latter, Flo1 and Flo11 promote formation of ex- et al. 2002; Aimanianda et al. 2009), as well as to generate tensive mats of cells, or biofilms (Reynolds and Fink 2001; oligosaccharides for structural analysis and characterization Beauvais et al. 2009; Bojsen et al. 2012). of linkages between polymers (Kollar et al.