Transduction of Osmostress Signals by the Yeast Sln1 Protein

William Marsh Rice University, Houston TX

Investigators

Abstract

One universal stress for cells is a change in the availability of water, called osmotic stress. Previous studies by Dr. Gustin's group and others have identified an osmotic stress signaling pathway in Baker's yeast, a pathway that is controlled by several surface osmosensors including the protein Sln1. Although much is known about how various surface sensors detect specific chemicals, very little is known about the mechanism of osmosensing by proteins like Sln1. The purpose of this project is to determine exactly how the cell surface Sln1 protein works to convert the external signal of osmotic stress into activation of signaling pathways inside the cell. Sln1 senses two types of osmotic stress. When the salt concentration outside the cell is increased (hypertonic stress), Sln1 is turned off. When cells are placed into dilute solutions (hypotonic stress), water enters the cell to balance the higher salt inside. In cells with a stiff exterior wall like fungi or plants, this water entry increases the pressure inside (called turgor). Sln1 is stimulated by the increase in turgor. To better understand how Sln1 is stimulated, hyperactive mutants of Sln1 that behaved as if the cell had higher turgor pressure were isolated. Sln1 is normally present as a pair (dimer) of identical proteins (monomers). The important insight from these genetic results is that mutations that activate Sln1 are predicted to break bonds between its monomers, allowing sliding of one monomer by another in the region where Sln1 is embedded in the membrane. These data suggest the hypothesis that Sln1 signaling is mediated by turgor-induced breakage in bonds between the membrane-embedded portions of the protein. To test this hypothesis, this project will generate and test additional mutants to determine whether weakening or strengthening bonds between monomers have the predicted effect on Sln1 activity. In addition, this research will map the membrane-embedded regions of Sln1 and the contact points between the monomers in the region of the membrane. Comparison of these two properties between normal and hyperactive mutants of Sln1 will generate a structural model of how Sln1 changes shape during its activation. Findings from this study will provide an important paradigm for understanding the activation mechanism for the many Sln1-related sensor proteins found in plants and fungi. Broader Impacts: Students from various levels will be involved in various aspects of this project. Dr. Gustin has been mentoring undergraduate and graduates students including those from underrepresented groups, many of whom have gone on to careers in scientific research, education and administration. Every year Dr. Gustin hosts an average of 10 undergraduate students who carry out sub projects either during the whole year or as summer interns.