When starved of Pi, yeast cells activate the PHO signalling 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. In the present work, we reveal that, upon Pi starvation, these low-affinity Pi 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, 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 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 signalling mechanisms in order to adjust Pi-transport capacity to the general nutritional status of the environment.

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