Arabidopsis PFA-DSP-Type Phosphohydrolases Target Specific Inositol Pyrophosphate Messengers

Activities and genetic interactions of fission yeast Aps1, a Nudix-type inositol pyrophosphatase and inorganic polyphosphatase

Inositol pyrophosphate 1,5-IP8 regulates expression of a fission yeast phosphate homeostasis regulon, comprising phosphate acquisition genes pho1, pho84, and tgp1, via its action as an agonist of precocious termination of transcription of the upstream lncRNAs that repress PHO mRNA synthesis. 1,5-IP8 levels are dictated by a balance between the Asp1 N-terminal kinase domain that converts 5-IP7 to 1,5-IP8 and three inositol pyrophosphatases-the Asp1 C-terminal domain (a histidine acid phosphatase), Siw14 (a cysteinyl-phosphatase), and Aps1 (a Nudix enzyme). In this study, we report the biochemical and genetic characterization of Aps1 and an analysis of the effects of Asp1, Siw14, and Aps1 mutations on cellular inositol pyrophosphate levels. We find that Aps1's substrate repertoire embraces inorganic polyphosphates, 5-IP7, 1-IP7, and 1,5-IP8. Aps1 displays a ~twofold preference for hydrolysis of 1-IP7 versus 5-IP7 and aps1∆ cells have twofold higher levels of 1-IP7 vis-à-vis wild-type cells. While neither Aps1 nor Siw14 is essential for growth, an aps1∆ siw14∆ double mutation is lethal on YES medium. This lethality is a manifestation of IP8 toxicosis, whereby excessive 1,5-IP8 drives derepression of tgp1, leading to Tgp1-mediated uptake of glycerophosphocholine. We were able to recover an aps1∆ siw14∆ mutant on ePMGT medium lacking glycerophosphocholine and to suppress the severe growth defect of aps1∆ siw14∆ on YES by deleting tgp1. However, the severe growth defect of an aps1∆ asp1-H397A strain could not be alleviated by deleting tgp1, suggesting that 1,5-IP8 levels in this double-pyrophosphatase mutant exceed a threshold beyond which overzealous termination affects other genes, which results in cytotoxicity.

IMPORTANCE

Repression of the fission yeast PHO genes tgp1, pho1, and pho84 by lncRNA-mediated interference is sensitive to changes in the metabolism of 1,5-IP8, a signaling molecule that acts as an agonist of precocious lncRNA termination. 1,5-IP8 is formed by phosphorylation of 5-IP7 and catabolized by inositol pyrophosphatases from three distinct enzyme families: Asp1 (a histidine acid phosphatase), Siw14 (a cysteinyl phosphatase), and Aps1 (a Nudix hydrolase). This study entails a biochemical characterization of Aps1 and an analysis of how Asp1, Siw14, and Aps1 mutations impact growth and inositol pyrophosphate pools in vivo. Aps1 catalyzes hydrolysis of inorganic polyphosphates, 5-IP7, 1-IP7, and 1,5-IP8 in vitro, with a ~twofold preference for 1-IP7 over 5-IP7. aps1∆ cells have twofold higher levels of 1-IP7 than wild-type cells. An aps1∆ siw14∆ double mutation is lethal because excessive 1,5-IP8 triggers derepression of tgp1, leading to toxic uptake of glycerophosphocholine.

Protein pyrophosphorylation by inositol phosphates: a novel post-translational modification in plants?

Inositol pyrophosphates (PP-InsPs) are energy-rich molecules harboring one or more diphosphate moieties. PP-InsPs are found in all eukaryotes evaluated and their functional versatility is reflected in the various cellular events in which they take part. These include, among others, insulin signaling and intracellular trafficking in mammals, as well as innate immunity and hormone and phosphate signaling in plants. The molecular mechanisms by which PP-InsPs exert such functions are proposed to rely on the allosteric regulation via direct binding to proteins, by competing with other ligands, or by protein pyrophosphorylation. The latter is the focus of this review, where we outline a historical perspective surrounding the first findings, almost 20 years ago, that certain proteins can be phosphorylated by PP-InsPs in vitro. Strikingly, in vitro phosphorylation occurs by an apparent enzyme-independent but Mg2+-dependent transfer of the β-phosphoryl group of an inositol pyrophosphate to an already phosphorylated serine residue at Glu/Asp-rich protein regions. Ribosome biogenesis, vesicle trafficking and transcription are among the cellular events suggested to be modulated by protein pyrophosphorylation in yeast and mammals. Here we discuss the latest efforts in identifying targets of protein pyrophosphorylation, pointing out the methodological challenges that have hindered the full understanding of this unique post-translational modification, and focusing on the latest advances in mass spectrometry that finally provided convincing evidence that PP-InsP-mediated pyrophosphorylation also occurs in vivo. We also speculate about the relevance of this post-translational modification in plants in a discussion centered around the protein kinase CK2, whose activity is critical for pyrophosphorylation of animal and yeast proteins. This enzyme is widely present in plant species and several of its functions overlap with those of PP-InsPs. Until now, there is virtually no data on pyrophosphorylation of plant proteins, which is an exciting field that remains to be explored.

Crystal Structure and Enzymology of Solanum tuberosum Inositol Tris/Tetrakisphosphate Kinase 1 (StITPK1)

Inositol phosphates and their pyrophosphorylated derivatives are responsive to the phosphate supply and are agents of phosphate homeostasis and other aspects of physiology. It seems likely that the enzymes that interconvert these signals work against the prevailing milieu of mixed populations of competing substrates and products. The synthesis of inositol pyrophosphates is mediated in plants by two classes of ATP-grasp fold kinase: PPIP5 kinases, known as VIH, and members of the inositol tris/tetrakisphosphate kinase (ITPK) family, specifically ITPK1/2. A molecular explanation of the contribution of ITPK1/2 to inositol pyrophosphate synthesis and turnover in plants is incomplete: the absence of nucleotide in published crystal structures limits the explanation of phosphotransfer reactions, and little is known of the affinity of potential substrates and competitors for ITPK1. Herein, we describe a complex of ADP and StITPK1 at 2.26 Å resolution and use a simple fluorescence polarization approach to compare the affinity of binding of diverse inositol phosphates, inositol pyrophosphates, and analogues. By simple HPLC, we reveal the novel catalytic capability of ITPK1 for different inositol pyrophosphates and show Ins(3,4,5,6)P4 to be a potent inhibitor of the inositol pyrophosphate-synthesizing activity of ITPK1. We further describe the exquisite specificity of ITPK1 for the myo-isomer among naturally occurring inositol hexakisphosphates.

Activities, substrate specificity, and genetic interactions of fission yeast Siw14, a cysteinyl-phosphatase-type inositol pyrophosphatase

IMPORTANCE

The inositol pyrophosphate signaling molecule 1,5-IP8 modulates fission yeast phosphate homeostasis via its action as an agonist of RNA 3'-processing and transcription termination. Cellular 1,5-IP8 levels are determined by a balance between the activities of the inositol polyphosphate kinase Asp1 and several inositol pyrophosphatase enzymes. Here, we characterize Schizosaccharomyces pombe Siw14 (SpSiw14) as a cysteinyl-phosphatase-family pyrophosphatase enzyme capable of hydrolyzing the phosphoanhydride substrates inorganic pyrophosphate, inorganic polyphosphate, and inositol pyrophosphates 5-IP7, 1-IP7, and 1,5-IP8. Genetic analyses implicate SpSiw14 in 1,5-IP8 catabolism in vivo, insofar as: loss of SpSiw14 activity is lethal in the absence of the Nudix-type inositol pyrophosphatase enzyme Aps1; and siw14∆ aps1∆ lethality depends on synthesis of 1,5-IP8 by the Asp1 kinase. Suppression of siw14∆ aps1∆ lethality by loss-of-function mutations of 3'-processing/termination factors points to precocious transcription termination as the cause of 1,5-IP8 toxicosis.

Intracellular phosphate recycling systems for survival during phosphate starvation in plants

Phosphorus (P) is an essential nutrient for plant growth and plants use inorganic phosphate (Pi) as their P source, but its bioavailable form, orthophosphate, is often limited in soils. Hence, plants have several mechanisms for adaptation to Pi starvation. One of the most common response strategies is "Pi recycling" in which catabolic enzymes degrade intracellular constituents, such as phosphoesters, nucleic acids and glycerophospholipids to salvage Pi. Recently, several other intracellular degradation systems have been discovered that salvage Pi from organelles. Also, one of sphingolipids has recently been identified as a degradation target for Pi recycling. So, in this mini-review we summarize the current state of knowledge, including research findings, about the targets and degradation processes for Pi recycling under Pi starvation, in order to further our knowledge of the whole mechanism of Pi recycling.