Differential roles for the low-affinity phosphate transporters Pho87 and Pho90 in Saccharomyces cerevisiae

摘要

pWhen starved of Psubi/sub, yeast cells activate the PHO signalling pathway, wherein the Pho4 transcription factor mediates expression of genes involved in Psubi/sub acquisition, such as iPHO84/i, encoding the high-affinity Hsup+/sup/Psubi/sub symporter. In contrast, transcription of iPHO87/i and iPHO90/i, encoding the low-affinity Hsup+/sup/Psubi/sub transport system, is independent of phosphate status. In the present work, we reveal that, upon Psubi/sub starvation, these low-affinity Psubi/sub transporters are endocytosed and targeted to the vacuole. For Pho87, this process strictly depends on iSPL2/i, another Pho4-dependent gene that encodes a protein known to interact with the N-terminal SPX domain of the transporter. In contrast, the vacuolar targeting of Pho90 upon Psubi/sub starvation is independent of both Pho4 and Spl2, although it still requires its SPX domain. Furthermore, both Pho87 and Pho90 are also targeted to the vacuole upon carbon-source starvation or upon treatment with rapamycin, which mimics nitrogen starvation, but although these responses are independent of PHO pathway signalling, they again require the N-terminal SPX domain of the transporters. These observations suggest that other SPX-interacting proteins must be involved. In addition, we show that Pho90 is the most important Psubi/sub transporter under high Psubi/sub conditions in the absence of a high-affinity Psubi/sub-transport system. Taken together, our results illustrate that Pho87 and Pho90 represent non-redundant Psubi/sub transporters, which are tuned by the integration of multiple nutrient signalling mechanisms in order to adjust Psubi/sub-transport capacity to the general nutritional status of the environment./p