Functional aspects of inorganic phosphate transport

Abstract: Inorganic phosphate is an essential nutrient for all organisms. It is required for many cellular components as nucleic acids and phospholipids, and as energy-carrying compounds such as ATP. Thus, a regulated uptake of this pivotal nutrient is of outermost importance. Depending of the availability of phosphate in the surroundings the yeast Saccharomyces cerevisiae make use of two different systems for transporting phosphate into the interior of the cell: a low-affinity system that is active during surplus phosphate conditions and a high-affinity system that is active when the availability becomes limited. This thesis focuses on the high-affinity system, which is comprised of the Pho84 and Pho89 transporters. Of the two transporters, Pho84 is the predominant one, responsible for almost all phosphate uptake during low phosphate conditions, and the contribution of Pho89 is of minor importance. Hence Pho84 is by far the most well characterized phosphate transporter. Even though much is known about phosphate transporters in yeast little in known about how phosphate is transported. The work in this thesis aims to broaden the knowledge about the transport mechanism by the means of site-directed mutagenesis and functional characterization. Also the similarity of Pho84 to glucose sensors and the potential role of conserved residues in phosphate signaling are investigated. By the use of a high-affinity system deletion strain (∆Pho84 ∆Pho89), we also managed to investigate the functional importance of well conserved residues in Pho89. In summary: the work presented in this thesis has contributed to increase the knowledge about transport mechanisms in phosphate transporters.