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Research Projects
The goal of the lab is to understand the molecular basis of microbial differentiation. We focus on the Cdc42-depndent MAPK pathway that regulates filamentous growth in yeast. Our lab has ongoing project in four areas: 1) Identify proteins that regulate MAPK pathways. We have identified a scaffold-type adaptor (Bem4) that interacts with Cdc42 and bears resemblance to members of the evolutionarily conserved family of Smg GDS GTPase regulators. We have recently uncovered a role for polar landmarks (Rsr1) in regulating the pathway. Identifying new proteins and figuring out how they regulate MAP kinase pathways is a central goal of the lab. 2) Define how signaling mucins regulate MAPK pathways. We have identified a mucin-type glycoprotein (Msb2) that operates at the head of the MAP kinase pathway and regulates filamentous growth. Yeast is an ideal model genetic system to figure out what mucins might be sensing and how this type of protein is activated. 3) Identify triggers of filamentous growth. In many fungal species, including pathogens, filamentous growth remains mysterious from the perspective of the signals that lead to virulence. We have shown that glucose depletion triggers filamentous growth in yeast. Understanding how nutrient signals regulate filamentous growth and identifying new triggers of the response are important to understand this morphogenetic and pathogenic behavior. 4) Uncover regulatory mechanisms that impact fungal pathogenicity. Using genomic approaches including ones developed in the lab (e.g. secretion profiling), we have identified genetic pathways and networks that regulate filamentous growth. We are currently applying these approaches to fungal pathogens. The major focus is on signaling mucin regulation in the fungal pathogen Candida albicans.
The problem: signaling pathways share components
One area of interest is to understand how the Rho GTPase Cdc42 is specified to one of the many pathways in which it operates. Cdc42 is a master regulator of cell polarity and signal transduction. The particular focus is to understand how Cdc42 regulates mitogen activated protein kinase (MAPK) pathways in yeast. Cdc42 is 81% identical between yeast and humans, and the mechanisms that underlie cell polarity and MAPK signaling are evolutionarily conserved. In yeast, as in other systems, Cdc42 has multiple functions. One main function of Cdc42 is to establish polarity at sites marked by bud-site-selection proteins. In fact, Cdc42 is an essential protein because of its role in bud emergence. Cdc42 also regulates MAPK pathways, which are signal transduction pathways that control diverse responses from the response to stress to cell differentiation.
One MAPK pathway that Cdc42 regulates is called the Filamentous Growth (or invasive/pseudohyphal) pathway. In response to limiting carbon or nitrogen sources, the Filamentous Growth pathway and other pathways control differentiation to the filamentous cell type. Such cells grow as branched interconnected filaments. Cdc42 also regulates MAPK pathways that control mating, in which cells detect peptide pheromones and form ‘shmoos’ towards complementary partners, and the response to osmotic stress [HOG], which does not cause does not a morphogenetic response. The Filamentous Growth, Mating, and HOG pathway are induced by different stimuli and regulate non-overlapping transcriptional and morphological responses through the same GTPase.
In addition to Cdc42, other proteins are shared among MAPK pathways in yeast. Three MAPK pathways require Cdc42, its activator, the GEF Cdc24, the p21 activated kinase (PAK) Ste20, and MAPKKK Ste11. A subset of proteins are required for activation of any two pathways. The complicated picture that has emerged by studying these pathways provides a simplified version of the networks that regulate metazoan development. Figuring out signaling specificity in yeast provides a template for signal specification in general.
Pathway-specific regulators function at the level of Cdc42 in the fMAPK pathway. One is the signaling mucin Msb2. Signaling mucins are evolutionarily conserved MAPK regulatory proteins that interact with RAS and Rho-type GTPases to regulate signal transduction pathways. Msb2 interacts with Cdc42 and other general factors. The second regulator is a scaffold, Bem4. Bem4 is unique among scaffolds in that it binds to Cdc42 and bears homology to members of the Small GTPase Guanine Dissociation Stimulator (Smg GDS) family of GTPase regulators. In mammals, Smg GDS proteins regulate small GTPases and play diverse roles in polarity and signaling. The third connection is polar landmarks culminating in the bud-site GTPase Rsr1. Collectively, these proteins lead to the activation of Cdc42 in a specific context. Understanding how these proteins lead to the specific activation of a signaling GTPase is currently under investigation.
Studying Mucin Glycoprotein Signaling in the Pathogen Candida albicans
Filamentous growth is a microbial differentiation response that is common to many fungal species. In fungal pathogens like the human opportunistic fungal pathogen Candida albicans, filamentous growth is required for virulence. Identifying the signaling molecules that regulate filamentous growth in yeast provides a template for the regulation of filamentous growth in fungal pathogens. The discovery of Msb2 as a regulator of the filamentous growth pathway has had application in plant and animal fungal systems.
In a collaborative project with Dr. Mira Edgerton in the Department of Oral Biology at UB, we are defining the roles of CaMsb2 in regulating the Cek MAP kinase pathway. One recent discovery was a role for CaMsb2 in relaying changes in temperature to the Cek pathway.
In the figure below, cells lacking CaMsb2 show a growth defect at 42˚C (left). This corresponds to a defect in Cek1 phospohrylation (middle panel) together with the cell wall protein and heat shock regulator Ssa1. At right is a model for how CaMsb2 might control temperature sensing in this organism.
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