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Cell wall plasticity

The yeast cell wall confers shape and has a protective function, providing resistance to mechanical and osmotic stresses. Apart from providing the location for receptors and targets for flocculation, the porosity of the outer protein layer limits entry of larger molecules, such as enzymes that might damage the more fragile plasma membrane (Klis et al., 2002; Free, 2013). The wall must be capable of responding to the requirements of cellular growth and proliferation, and conversely adopt a more rigid protective form under non-growth-permitting conditions. It follows that systems must be available for sensing cell wall structure and making appropriate adjustments in response to signals from the central metabolic pathways. Based on whole-cell transcription studies, some 1200 genes have been implicated in cell wall-related functions, demonstrating how regulation in the structure of this organelle is integral to physiological responses of the cell to the environment (De Groot et al., 2001). Comparisons of stationary phase and exponentially growing walls show many differences. The former are less permeable, show different protein profiles, and are more resistant to cell wall-degrading enzymes. Approximately a 6-fold increase in disulfide bridges in stationary phase cells has been observed (de Nobel et al., 1990). In addition to these changes, anaerobiosis is accompanied by altered levels of transcription of several genes involved in the production of cell wall proteins (Klis et al., 2006).

Remodelling of cell wall structure is accomplished through the action of the cell wall integrity signalling pathway (Levin, 2011). A response to environmental stresses is sensed via a set of cell- wall-located receptors via a G-protein termed Rho1 (Ras-homologous family of GTPases). The same protein is also involved in the regulation of cell wall synthetic reactions associated with progression through the cell cycle. It follows that this process requires sensitivity to the internal cues driving budding. Rho1 coordinates a multitude of functions related to cell wall structure and biogenesis. These include coordination of synthesis of ^-glucan at selected sites on the cell wall, the associated endocytosis of the necessary building materials, and organization of the actin cytoskeleton. The cell response to the passage into quiescence, triggered by nutrient starvation and/or application of external stresses, is also mediated by the Rhol-directed signalling pathway (Levin, 2011). More recently, Rhol has also been implicated in another signalling pathway in yeast involved in regulation of homeostasis of plasma membrane fluidity (Lockshon et al., 2012).

Rhol is one of a family of six related GTPases that are found in S. cerevisiae, located at the surface of the plasma membrane. Two of these are essential, Rho as discussed and Cdc42, the latter being vital for establishment ofcell polarity and bud formation. Proper function of the Rho system is dependent on correct location and orientation in the plasma membrane and on the activity of guanosine nucleotide exchange factors and GTPase-activating proteins. The first of these transmit signals from cell-surface receptor proteins of the initiating stimulus (Radico and Heinisch, 2010). Somewhat fascinatingly, some of these sensors have been shown to possess rigid polypeptides that project into the periplasm and possibly the wall, where they may serve as linear nanosprings probing mechanical status (Dupres et al., 2009). Phosphoinositides have been shown to have importance in membrane functionality and it appears that part of this activity involves activation of the Rho sensors (Odorizzi et al., 2000).

Rho1 exerts its effects to the relevant parts of yeast metabolism via a MAP kinase cascade. An important component is the protein kinase, Pkc1, which is itself activated by GTP-bound Rho1 (Kamada et al., 1996). It constitutes one of five MAPK signalling pathways that regulate a multitude of critical cellular functions, which apart from the cell wall integrity sensing pathway include mating response, sporulation, and the morphological changes associated with transitions to pseudohyphal habits associated with those strains capable of this response (Gustin et al., 1998).

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