All living organisms, from bacteria and yeast to mammalian cells, must respond to a fluctuating extracellular environment. Information contained in such fluctuations is signaled via the cell surface to the interior of the cell, resulting in responses such as altered gene expression and protein activity. Membrane transport proteins play a central role in such signal transduction. These proteins are responsible for the uptake of essential nutrients, ions and metabolites as well.
Transport of inorganic phosphate across the plasma membrane is the first step in phosphate utilization by the cell. The intracellular level of phosphate in the yeast Saccharomyces cerevisiae is regulated by the PHO network of scattered genes coding for both structural and regulatory units.
In S. cerevisiae, phosphate uptake across the plasma membrane is mediated by specific proteins proposed to be organized into at least three different transport systems. The PHO84 and PHO89 genes encode two high-affinity phosphate co-transporters. Hydropathy analysis suggests that these proteins are arranged into 12 transmembrane domains. While the derepressible Pho84 protein catalyzes proton-coupled phosphate transport at acidic pH, the Pho89 protein, which is also derepressible, catalyzes sodium-dependent phosphate uptake at alkaline pH.
Three different model systems have been developed for the characterization of the Pho84 protein, i.e., inside-out plasma membrane vesicles and reconstitution into liposomes alone or with cytochrome c oxidase. The results obtained suggest that the phosphate transport is bidirectional and that it is the direction of the driving-force rather than the orientation of the protein, which determines the direction of transport of inorganic phosphate catalyzed by Pho84p. Each component of the Dp which drives phosphate uptake has been investigated in the co-reconstituted model system and the findings suggest that this transport can be driven by either one of the components. Divalent cations such as Mn2+ and Co2+ stimulate phosphate uptake by proteoliposomes, suggesting that a metal-phosphate complex is most likely the entity being transported.
Stockholm: Stockholm University , 1999. , 39 p.