The Role of LAM Genes in the Pheromone-Induced Cell Death of S. cerevisiae Yeast

Interrelationship between the Non-Vesicular Transport of Sterols and Their Distribution between the Rafts and the Non-Raft Phase of the Plasma Membrane

Sterols significantly affect the barrier properties of the membrane, which might explains the fact that their concentration is maximal in the plasma membrane (PM). Together with sphingolipids, sterols form rafts, i.e., bilayer regions whose physicochemical properties differ from those of the surrounding PM. The presence of rafts allows membrane proteins to choose the lipid environment optimal for their functioning (in terms of thickness, rigidity, spontaneous curvature, and lateral pressure profile of the bilayer). The ratio between sterols and sphingolipids in the rafts is close to stoichiometric. Theoretically, excess sterol outside the rafts can critically reduce the degree of order of membrane phospholipids. Sterols are synthesized in the endoplasmic reticulum (ER). The active (against the concentration gradient) transport of sterols from the ER to the PM is driven by proteins of the Osh family, while Lam proteins provide passive reverse transport of sterols from the PM to the ER. Inactivation of Osh proteins does not reduce the total level of sterols in the PM but reduces the rate of their movement inside the PM (the mechanisms underlying this effect remains unclear). Therefore, the vesicular transport of sterols from the ER to the PM is probably more active than the non-vesicular transport carried out by Osh proteins. Since sterols are more rigidly anchored and less sterically accessible in the rafts than outside them, we suggested that Lam proteins transport excess sterols from the non-raft phase of the PM to the ER, and Osh proteins return them back to the PM. In this way, the mutual activity of the Osh and lam proteins provides the rotation of sterols between the non-raft fraction of the PM and rafts, with the enrichment of the latter. It is possible that with a decrease in the sterol concentration in the non-raft fraction of the membrane, the rate of the Lam-dependent transport decreases since the degree of order of phospholipids and, consequently, the strength of retention of sterol molecules in the membrane increases, which might represent a mechanisms maintaining the concentration and distribution of sterols in the PM.

Structural Role of Plasma Membrane Sterols in Osmotic Stress Tolerance of Yeast Saccharomyces cerevisiae

Yeast S. cerevisiae has been shown to suppress a sterol biosynthesis as a response to hyperosmotic stress. In the case of sodium stress, the failure to suppress biosynthesis leads to an increase in cytosolic sodium. The major yeast sterol, ergosterol, is known to regulate functioning of plasma membrane proteins. Therefore, it has been suggested that the suppression of its biosynthesis is needed to adjust the activity of the plasma membrane sodium pumps and channels. However, as the sterol concentration is in the range of thirty to forty percent of total plasma membrane lipids, it is believed that its primary biological role is not regulatory but structural. Here we studied how lowering the sterol content affects the response of a lipid bilayer to an osmotic stress. In accordance with previous observations, we found that a decrease of the sterol fraction increases a water permeability of the liposomal membranes. Yet, we also found that sterol-free giant unilamellar vesicles reduced their volume during transient application of the hyperosmotic stress to a greater extent than the sterol-rich ones. Furthermore, our data suggest that lowering the sterol content in yeast cells allows the shrinkage to prevent the osmotic pressure-induced plasma membrane rupture. We also found that mutant yeast cells with the elevated level of sterol accumulated propidium iodide when exposed to mild hyperosmotic conditions followed by hypoosmotic stress. It is likely that the decrease in a plasma membrane sterol content stimulates a drop in cell volume under hyperosmotic stress, which is beneficial in the case of a subsequent hypo-osmotic one.