Abstract #48

Signal transduction in yeast via PAQR receptors. Brian Kupchak¹, Lisa Regalla¹, Nancy Villa¹, Christina Appin¹, Anna Vagstad¹, Charlene Wolford², Ashley Cowart², Yusuf Hannun², Thomas Lyons¹. 1) Chemistry, University of Florida, Gainesville, FL; 2) Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston.
   The yeast genome encodes four proteins of the PAQR receptor family. One such protein, Izh2p, has been shown to function as a receptor for the plant defensin, osmotin. PAQR proteins belong to a large and ubiquitous family of receptors that are made up of 7 transmembrane domains. Due to the superficial similarity of these proteins to G-protein coupled receptors and some ambiguous preliminary data, these proteins have been postulated to comprise a new class of GPCR. We have developed a reporter assay to monitor activity of these receptors using the FET3 promoter fused to the lacZ gene. This reporter has allowed us to probe the mechanism of signal transduction via these receptors. Our data suggest that PAQR proteins signal via the Pkh1p and Pkh2p sphingosine dependent kinases and initiate a cascade that requires PKA, Rim15p, AMPK and the Nrg1p and Nrg2p repressors. We have also discovered that these receptors increase endogenous sphingosine and share significant structural similarity with alkaline ceramidases suggesting that they may act as ligand-activated enzymes producing sphingoid base second messengers. Based on this data we propose that the PAQR proteins directly activate Pkh1p/Pkh2p which in turn activates PKA, causes the relocalization of Rim15p and Snf1p resulting in binding of Nrg1p/Nrg2p to the FET3 promoter. Our data show that the PAQR receptors negatively regulate the expression of high-affinity iron- and zinc-uptake transporters. Finally, we have demonstrated that 7 human PAQR receptors can be functionally reconstituted in yeast and that they activate the same signal transduction pathway. This shows that there is conservation of the PAQR signaling module in higher eukaryotes.

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