Architecture and biosynthesis of the Saccharomyces cerevisiae cell wall

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 β1,3- and β1,6-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, glycosylphosphatidylinositol membrane anchors, β1,3- and β1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins.

Figure 1

Wall components and cross-links between them. (A) Reducing end of chitin linked to a side-branching β1,3-Glc on β1,6-glucan. (B) Reducing end of chitin linked to a nonreducing end of β1,3-glucan. (C) Reducing end of β1,3-glucan chain linked to a side-branching β1,6-Glc on β1,3-glucan. (D) Reducing end of GPI glycan (possibly the α1,4-Man) to internal Glc in β1,6-glucan (linkage to nonreducing end of β1,6-glucan is also possible). (E) Ester linkages between β1,3-Glc and γ-carboxyl groups of glutamates in PIR protein internal repeats. (F) Disulfide link between CWP. Chemical treatments used to release CWP are indicated.

Figure 2

Assembly of the Dol-PP-linked precursor oligosaccharide in N-glycosylation, its transfer to protein, and subsequent glycan processing. Residues added at the cytoplasmic face of the ER membrane originate from sugar nucleotides, whereas Dol-P sugars generated at the cytoplasmic face of the membrane are the donors in lumenal transfers. Symbols are adaptations of those used by the Consortium of Glycobiology Editors in Essentials in Glycobiology (Varki and Sharon 2009).

Figure 3

Formation of mannan outer chains and core-type N-glycans in the Golgi. Protein-bound Man₈-GlcNAc₂ structures are first acted on by the Och1 α1,6-Man-T in the cis-Golgi. The initiating α1,6-Man is then elongated with ∼10 α1,6-linked Man by mannan polymerase (M-Pol)-I, and this chain is then extended with up to ∼50 α1,6-linked Man by M-Pol-II. Kre2/Mnt1, Ktr1, Ktr2, Ktr3, and Yur1 collectively add α1,2-linked mannoses. Core-type glycans are formed when an α1,2-linked Man is added to the Och1-derived α1,6-Man. Symbols are as used in Figure 2.

Figure 4

Biosynthesis of O-linked glycans. (A) Addition of α-Man by protein O-mannosyltransferases in the ER lumen. Pmt4 homodimers act on membrane proteins or GPI proteins. Representative Pmt heterodimers are shown. (B) Extension of O-linked manno-oligosaccharides in the Golgi. Ktr1 family members have a collective role in adding α1,2-linked mannoses, and Mnt1 family members add α1,3-linked mannoses. The dominant Man-T active at each step are shown in boldface type. Man-P may be added to saccharides with two α1,2-linked Man.

Figure 5

Biosynthesis of the GPI precursor and its transfer to protein in the ER membrane. GlcNAc addition to PI and de-N-acetylation of GlcNAc-PI to GlcN-PI occur at the cytoplasmic face of the ER membrane, and further additions to the GPI occur on the lumenal side of the ER membrane. Gpi18 and Mcd4 need not act in a defined order. Man₃- and Man₄-GPIs either bearing Etn-P on Man-2 but not Man-1 or without any Etn-Ps (not shown) have also been detected in radiolabeling experiments with certain late-stage GPI assembly mutants.

Figure 6

Remodeling of protein-bound GPIs. The inositol palmitoyl group and the sn-2 acyl chain are removed by Bst1 and Per1, respectively, and Gup1 transfers a C_26:0 acyl chain to the sn-2 position. Cwh43 can replace diphosphatidic acid with ceramide phosphate (shown here) or diacylglycerol with ceramide. Etn-P on Man-1 and Man-2 may be removed by Ted1 and Cdc1. Steps through Etn-P removal occur in the ER. An α1,2- or an α1,3-linked Man is added to Man-4 in the Golgi by as yet unknown Man-T. At the plasma membrane, the GPI can be cleaved, possibly between GlcN and Man, and the reducing end of the GPI remnant transferred to β1,6-glucan. Symbols are as used in Figure 1 and Figure 5.

Figure 7

Roles of chitin synthases II and III in chitin deposition during budding growth. (A) Chitin synthase III synthesizes a chitin ring (blue) around the base of the emerging bud. (B) The plasma membrane invaginates and chitin synthase II synthesizes the primary septum (red). No chitin is made in the lateral walls of the bud. (C) Secondary septa (green) are laid down on the mother- and daughter-cell sides of the primary septum, and chitin synthase III starts synthesizing lateral wall chitin in the bud (blue). (D) After cell separation, the bud scar (which is formed from the chitin ring made by Chs3), most of the primary septum made by Chs2, as well as secondary septal material deposited on the mother cell side, remain on the mother cell. The birth scar on the daughter cell contains residual chitin from the primary septum as well as secondary septal material. (E and F) Chitinase digestion of the primary septum from the daughter-cell side facilitates cell separation, and lateral wall chitin synthesis continues as the daughter cell grows. Figure is adapted from Cabib and Duran (2005).

Figure 8

Trafficking and regulation of Chs2. Cell cycle-regulated expression of CHS2 peaks at the G₂-M phase transition, and Chs2 is synthesized at the ER. Phosphorylation of Chs2 by Cdk1 retains Chs2 in the ER. Upon chromosomal separation, Cdc14-dependent dephosphorylation of Chs2 allows release of the protein from the ER and its transit to the mother cell–bud junction. Inn1 and Cyk3, localized at the division site, are involved in Chs2 activation. After primary septum formation is complete, Chs2 is endocytosed and degraded. Localization, function, and subsequent removal of Chs2 when the primary septum is complete depend on phosphorylation by Dbf2. Figure is adapted from Lesage and Bussey (2006).

Figure 9

Overview of Chs3 trafficking. Chs3, synthesized in the ER, requires palmitoylation by Pfa4 and association with Chs7 to exit the ER. In the trans-Golgi, Chs3 association with exomer components Chs5 and Chs6 facilitates incorporation of Chs3 into secretory vesicles for delivery to the plasma membrane at the site of chitin ring formation. Localization and activation of Chs3 depends on association with Chs4, whose association with the septin ring is mediated in turn via an interaction with Bni4. In cells with medium-sized buds, Chs3 is retrieved from the plasma membrane and sequestered in chitosomes in an endocytic process depending on End4 and later recruited back to the neck region in a Chs6-dependent manner. During the cell wall stress response, Rho1 and Pkc1 trigger mobilization of Chs3 from chitosomes to the plasma membrane for synthesis of extra chitin in the lateral wall. Figure is adapted from Lesage and Bussey (2006).