Logo for University of Iowa Health Care This logo represents the University of Iowa Health Care

Soraya Johnson

Mentor: W. Scott Moye-Rowley, Ph.D.
Lab Room: 6-513 BSB
Lab Phone: 319-335-7875

Investigating Links Between Sphingolipid Biosynthesis and the PDR Pathway in Yeast

Multi-drug resistance, which is a common phenomenon in several types of cancer as well in fungal infections of the immunocompromised, is often the result of over-expression of ATP binding cassette (ABC) transporter proteins. Our lab studies multi-drug resistance in the model yeast, Saccharomyces cerevisiae, in which multi-drug resistance is referred to as pleiotropic drug resistance (PDR). Specifically, I am interested in understanding how the (PDR) pathway influences sphingolipid homeostasis. Earlier work from our laboratory and others has shown that the expression of several genes involved in sphingolipid biosynthesis is transcriptionally regulated by the Pdr1p and Pdr3p transcription factors. Complex sphingolipids are important components of the eukaryotic plasma membrane, while sphingolipid intermediates such as phytosphingosine (PHS) play important roles in intracellular signaling.

It is known that deletion of both the Pdr5 and Yor1 plasma membrane localized ABC transporter proteins confers resistance to the sphingolipid intermediate phytosphingosine (PHS), which is thought to be mediated by the plasma membrane protein Rsb1. Increased levels of PHS are toxic to the cell due to mislocalization of nutrient transporters. In our hands, Rsb1 plays a role in PHS tolerance of this strain, but it does not appear to be the sole cause for resistance. I am currently examining possible changes in the levels of the tryptophan permease, Tat2, as a possible alternative explanation for the PHS resistance of the pdr5yor1 deletion strain.

Interestingly, I discovered that strains lacking both Pdr5 and Yor1 are also resistant to a sphingolipid biosynthesis inhibitor, Aureobasidin A (AbA), which is independent of Rsb1. AbA inhibits the sphingolipid pathway downstream of PHS by targeting inositol phosphorylceramide synthase (AUR1). Several assays have determined that AUR1 itself does not appear to be altered in the pdr5yor1 double deletion strain, therefore I conducted an EMS mutagenesis screen to determine the protein(s) involved in tolerance to AbA. There appear to be two classes of mutant genes, one involved in transcriptional control and the other involved in sphingolipid honeostasis. I am investigating the roles of these genes in the increased resistance to AbA seen in the absence of Pdr5 and Yor1. These data are uncovering new links between sphingolipid synthesis and the pleiotropic drug resistance regulatory pathway.

Johnson SS, Hanson PK, Manoharlal R, Brice SE, Cowart LA, Moye-Rowley WS. Regulation of yeast nutrient permease endocytosis by ATP-binding cassette transporters and a seven-transmembrane protein, RSB1. J Biol Chem. 2010 Nov 12;285(46):35792-802. Epub 2010 Sep 8. PubMed PMID: 20826817; PubMed Central PMCID: PMC2975203.

Westin ER, Chavez E, Lee KM, Gourronc FA, Riley S, Lansdorp PM, Goldman FD, Klingelhutz AJ. Telomere restoration and extension of proliferative lifespan in dyskeratosis congenita fibroblasts. Aging Cell. 2007 Jun;6(3):383-94. Epub 2007 Mar 23. PubMed PMID: 17381549; PubMed Central PMCID: PMC2225626