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Control of eukaryotic phosphate homeostasis by inositol polyphosphate sensor domains

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Science  20 May 2016:
Vol. 352, Issue 6288, pp. 986-990
DOI: 10.1126/science.aad9858

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Peptide domain links phosphate need to uptake

Cellular phosphate (Pi) levels are tightly controlled, but it is not clear how eukaryotic cells actually “measure” the concentration of Pi. Wild et al. now show that inositol polyphosphate (InsP) signaling molecules regulate Pi homeostasis in fungi, plants, and humans by interacting with SPX-domain-containing proteins. SPX domains are found in many eukaryotic Pi transporters, Pi-regulated enzymes, and signaling proteins. InsP binding allowed SPX domains to interact with different target proteins. In plants, one such target protein is a transcription factor. During normal growth, high levels of InsP promoted formation of a SPX protein-transcription factor complex. Under Pi starvation, InsP levels dropped, releasing the transcription factor to promote Pi starvation-response gene transcription.

Science, this issue p. 986

Abstract

Phosphorus is a macronutrient taken up by cells as inorganic phosphate (Pi). How cells sense cellular Pi levels is poorly characterized. Here, we report that SPX domains—which are found in eukaryotic phosphate transporters, signaling proteins, and inorganic polyphosphate polymerases—provide a basic binding surface for inositol polyphosphate signaling molecules (InsPs), the concentrations of which change in response to Pi availability. Substitutions of critical binding surface residues impair InsP binding in vitro, inorganic polyphosphate synthesis in yeast, and Pi transport in Arabidopsis. In plants, InsPs trigger the association of SPX proteins with transcription factors to regulate Pi starvation responses. We propose that InsPs communicate cytosolic Pi levels to SPX domains and enable them to interact with a multitude of proteins to regulate Pi uptake, transport, and storage in fungi, plants, and animals.

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