Differential roles for the low-affinity phosphate transporters Pho87 and Pho90 in Saccharomyces cerevisiae
Résumé
When starved for phosphate (Pi), yeast cells activate the PHO-signaling pathway, wherein the Pho4 transcription factor mediates expression of genes involved in Pi-acquisition, such as PHO84, encoding the high-affinity H+/Pi-symporter. In contrast, transcription of PHO87 and PHO90, encoding the low-affinity H+/Pi-transport system, is independent of phosphate status. Here, we reveal that upon Pi-starvation, these low-affinity phosphate transporters are endocytosed and targeted to the vacuole. For Pho87, this process strictly depends on SPL2, 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 Pi-starvation is independent of both Pho4 and Spl2, though it still requires its SPX-domain. Furthermore, both Pho87 and Pho90 are also targeted to the vacuole upon glucose starvation or upon treatment with rapamycin, which mimics nitrogen-starvation, but, while these responses are independent of PHO-pathway signaling, 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 Pi-transporter under high Pi conditions in the absence of a high-affinity Pi-transport system. Taken together, our results illustrate that Pho87 and Pho90 represent non-redundant Pi-transporters, which are tuned by the integration of multiple nutrient signaling mechanisms in order to adjust Pi-transport capacity to the general nutritional status of the environment.
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