Physiological regulation of the high-affinity phosphate uptake system in the yeast Saccharomyces cerevisiae

Sammanfattning: Yeast cells can be found in a wide variety of environments, demonstrating that these cells are capable of adapting to changes in the external milieu. Inorganic phosphate (Pi) is an essential nutrient for all organisms and Saccharomyces cerevisiae cells take up phosphate from the external environment via two different systems, i.e., a high-affinity system consisting of the Pho84 and Pho89 permeases and a low-affinity system composed of the Pho87, Pho90 and Pho91 permeases. Expression of the Pho84 and Pho89 permeases is regulated in response to the extracellular concentration of Pi, whereas the low-affinity system is thought to be expressed constitutively. The Pho84 permease couples phosphate uptake with protons and has an acidic pH optimum. In contrast, Pho89 is a cation-coupled phosphate transporter with a preference for sodium ions and an alkaline pH optimum.In the present thesis the regulation of the two high-affinity phosphate transporters in response to alterations in the external concentration of Pi has been examined. The Pho84 permease is the major supplier of phosphate under Pi-limiting conditions, whereas the Pho89 permease plays only a minor role under such conditions. Instead, Pho89 is proposed to be involved in supplying phosphate to the cells in different stressful situations. In a strain from which all of the phosphate transporters except Pho89 have been deleted, this protein is up-regulated sufficiently to sustain cell growth under both high- and low-phosphate conditions even in the presence of an acidic environment.In addition to being regulated on the transcriptional level, Pho84 is also regulated post-translationally and is in response to Pi addition or depletion internalised and sorted to the vacuole for degradation. This post-translational regulation is independent of the PHO pathway that mediates transcriptional regulation.