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

Covering: 1982 to 2009 4-Hydroxy-2-pyridone alkaloids exert diverse biological effects, ranging from antifungal, antibacterial, insecticidal and cytotoxic activity to the induction of neurite outgrowth in different cell assays. For this reason, this class of compounds has attracted much attention in the scientific community, resulting in the elucidation of the biosynthesis of some members which are formed by fungal polyketide synthases via tetramic acids by fungal polyketide synthases as well as different total synthesis approaches. This article will give a summary of the so far known structures, overview the different activities of the compounds and disclose the synthetic attempts since 1982.

Diphosphorylated inositol polyphosphates, also referred to as inositol pyrophosphates, are important signaling molecules that regulate critical cellular activities in many eukaryotic organisms, such as membrane trafficking, telomere maintenance, ribosome biogenesis, and apoptosis. In mammals and fungi, two distinct classes of inositol phosphate kinases mediate biosynthesis of inositol pyrophosphates: Kcs1/IP6K- and Vip1/PPIP5K-like proteins. Here, we report that PPIP5K homologs are widely distributed in plants and that Arabidopsis thaliana VIH1 and VIH2 are functional PPIP5K enzymes. We show a specific induction of inositol pyrophosphate InsP₈ by jasmonate and demonstrate that steady state and jasmonate-induced pools of InsP₈ in Arabidopsis seedlings depend on VIH2. We identify a role of VIH2 in regulating jasmonate perception and plant defenses against herbivorous insects and necrotrophic fungi. In silico docking experiments and radioligand binding-based reconstitution assays show high-affinity binding of inositol pyrophosphates to the F-box protein COI1-JAZ jasmonate coreceptor complex and suggest that coincidence detection of jasmonate and InsP₈ by COI1-JAZ is a critical component in jasmonate-regulated defenses.

The analysis of myo-inositol phosphates (InsPs) and myo-inositol pyrophosphates (PP-InsPs) is a daunting challenge due to the large number of possible isomers, the absence of a chromophore, the high charge density, the low abundance, and the instability of the esters and anhydrides. Given their importance in biology, an analytical approach to follow and understand this complex signaling hub is desirable. Here, capillary electrophoresis (CE) coupled to electrospray ionization mass spectrometry (ESI-MS) is implemented to analyze complex mixtures of InsPs and PP-InsPs with high sensitivity. Stable isotope labeled (SIL) internal standards allow for matrix-independent quantitative assignment. The method is validated in wild-type and knockout mammalian cell lines and in model organisms. SIL-CE-ESI-MS enables the accurate monitoring of InsPs and PP-InsPs arising from compartmentalized cellular synthesis pathways, by feeding cells with either [¹³C₆]-myo-inositol or [¹³C₆]-D-glucose. In doing so, we provide evidence for the existence of unknown inositol synthesis pathways in mammals, highlighting the potential of this method to dissect inositol phosphate metabolism and signalling.

Significance: Biological roles of inorganic phosphate require careful regulation of its transport across cell membranes, but mechanisms are poorly understood. We demonstrate that a major cellular phosphate export protein, XPR1, is regulated by the most “energetic” of cell signaling molecules—1,5-bisdiphosphoinositol 1,2,3,4-tetrakisphosphate (InsP ₈ ). We derive this conclusion by showing reduced XPR1-mediated phosphate efflux when InsP ₈ synthesis was attenuated by genetic or pharmacological techniques; phosphate efflux was rescued by using liposomes to deliver into cells metabolically resistant InsP ₈ analogues. Genetic elimination of InsP ₈ from an osteosarcoma cell line perturbed phosphate homeostasis and accelerated differentiation into a biomineralization phenotype. We propose that mutations in gene products that regulate InsP ₈ synthesis might compromise XPR1 function, with pathological consequences for bone maintenance and ectopic calcification.

In plants, phosphate (P_i) homeostasis is regulated by the interaction of PHR transcription factors with stand-alone SPX proteins, which act as sensors for inositol pyrophosphates. In this study, we combined different methods to obtain a comprehensive picture of how inositol (pyro)phosphate metabolism is regulated by P_i and dependent on the inositol phosphate kinase ITPK1. We found that inositol pyrophosphates are more responsive to P_i than lower inositol phosphates, a response conserved across kingdoms. Using the capillary electrophoresis electrospray ionization mass spectrometry (CE-ESI-MS) we could separate different InsP₇ isomers in Arabidopsis and rice, and identify 4/6-InsP₇ and a PP-InsP₄ isomer hitherto not reported in plants. We found that the inositol pyrophosphates 1/3-InsP₇, 5-InsP₇, and InsP₈ increase several fold in shoots after P_i resupply and that tissue-specific accumulation of inositol pyrophosphates relies on ITPK1 activities and MRP5-dependent InsP₆ compartmentalization. Notably, ITPK1 is critical for P_i-dependent 5-InsP₇ and InsP₈ synthesis in planta and its activity regulates P_i starvation responses in a PHR-dependent manner. Furthermore, we demonstrated that ITPK1-mediated conversion of InsP₆ to 5-InsP₇ requires high ATP concentrations and that Arabidopsis ITPK1 has an ADP phosphotransferase activity to dephosphorylate specifically 5-InsP₇ under low ATP. Collectively, our study provides new insights into P_i-dependent changes in nutritional and energetic states with the synthesis of regulatory inositol pyrophosphates.