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

Abstract

When starved of P(i), yeast cells activate the PHO signalling pathway, wherein the Pho4 transcription factor mediates expression of genes involved in P(i) acquisition, such as PHO84, encoding the high-affinity H(+)/P(i) symporter. In contrast, transcription of PHO87 and PHO90, encoding the low-affinity H(+)/P(i) transport system, is independent of phosphate status. In the present work, we reveal that, upon P(i) starvation, these low-affinity P(i) 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 P(i) transporter under high P(i) conditions in the absence of a high-affinity P(i)-transport system. Taken together, our results illustrate that Pho87 and Pho90 represent non-redundant P(i) transporters, which are tuned by the integration of multiple nutrient signalling mechanisms in order to adjust P(i)-transport capacity to the general nutritional status of the environment.

Phosphate homeostasis in the yeast Saccharomyces cerevisiae, the key role of the SPX domain‐containing proteins

“…These apparently contradictory results might arise from the different experimental conditions used in the two studies. It has also been shown that Pho87 and Pho90 can also be targeted to the vacuole under conditions other than Pi deficiency, such as carbon-source starvation or nitrogen deficiency, which are independent from the PHO signaling pathway suggesting that other SPX-interacting proteins, besides Spl2, are involved in this mechanism [38]. However, independently of the conditions tested, the vacuolar targeting of both Pho87 and Pho90 strictly required the presence of their N-terminal SPX domain.…”

“…Indeed, inactivation of cell surface proteins, such as transporters and signaling receptors, in response to changes in nutrient availability by ubiquitin-mediated endocytosis and vacuolar targeting is a common strategy employed by cells [38]. Furthermore, the vacuolar targeting of both Pho87 and Pho90 strictly required the presence of the SPX domain [38]. Although a previous study showed that Spl2 could interact with the SPX domain of Pho90, the vacuolar targeting of Pho90 upon Pi stress does not seem to require interaction with Spl2 [45].…”

SPX1 is a phosphate-dependent inhibitor of PHOSPHATE STARVATION RESPONSE 1 in Arabidopsis

“…Several Pi homeostasisrelated proteins in yeast, including Pi sensors, share an SPX domain (28)(29)(30)(31)(32), so-called because it is present in the suppressor of yeast gpa1 (Syg1), the yeast cyclin-dependent kinase inhibitor (Pho81), and the human xenotropic and polytropic retrovirus receptor 1 (XPR1). Several plant proteins bearing this domain are involved in Pi starvation signaling (33)(34)(35)(36)(37); nonetheless, demonstration of a role for plant SPX proteins as Pi sensors is lacking.…”

OsSPX-MFS3, a vacuolar phosphate efflux transporter, is involved in maintaining Pi homeostasis in rice

“…PHO87 and PHO90 are plasma membrane proteins and mediate Pi uptake into cells under replete Pi condition. Although the expression of these low-affinity Pi transporters is independent of Pi status, PHO87 and PHO90 are targeted to the vacuole and degraded under Pi starvation conditions (Ghillebert et al, 2011). The PHO91 protein is localized on the tonoplast and is responsible for transport of Pi from the vacuole lumen to the cytosol (Hürlimann et al, 2007).…”