Abstract
Inositol phosphates (InsPs) are intricately tied to lipid signaling, as at least one portion of the inositol phosphate signaling pool is derived from hydrolysis of the lipid precursor, phosphatidyl inositol (4,5) bisphosphate. The focus of this review is on the inositol pyrophosphates, which are a novel group of InsP signaling molecules containing diphosphate or triphosphate chains (i.e., PPx) attached to the inositol ring. These PPx-InsPs are emerging as critical players in the integration of cellular metabolism and stress signaling in non-plant eukaryotes. Most eukaryotes synthesize the precursor molecule, myo-inositol (1,2,3,4,5,6)-hexakisphosphate (InsP6), which can serve as a signaling molecule or as storage compound of inositol, phosphorus, and minerals (referred to as phytic acid). Even though plants produce huge amounts of precursor InsP6 in seeds, almost no attention has been paid to whether PPx-InsPs exist in plants, and if so, what roles these molecules play. Recent work has delineated that Arabidopsis has two genes capable of PP-InsP5 synthesis, and PPx-InsPs have been detected across the plant kingdom. This review will detail the known roles of PPx-InsPs in yeast and animal systems, and provide a description of recent data on the synthesis and accumulation of these novel molecules in plants, and potential roles in signaling.
Highlights
Myo-inositol signaling is much like a language in that each molecular species used in the pathway, whether lipid or soluble in nature, can convey specific information to the cell, like a word
Comprehensive analyses of both inositol and inositol phospholipids in signaling have been previously reviewed (Van Leeuwen et al, 2004; Gillaspy, 2011; Heilmann and Heilmann, 2014), so this review focuses on new inositol signaling molecules, the di-phospho (PP) and tri-phospho (PPP) inositol phosphates (PPx-Inositol phosphates (InsPs)), known as inositol pyrophosphates
The main questions regarding PPx-InsPs are: can these molecules be detected in plants, how are they synthesized and what type of information do they convey? Recent work addressing these questions will be discussed in the broader context of understanding how PPx-InsPs function in eukaryotes
Summary
Myo-inositol (inositol) signaling is much like a language in that each molecular species used in the pathway, whether lipid or soluble in nature, can convey specific information to the cell, like a word. Each combination of different numbers and positions of phosphates on the inositol ring and the presence of diacylglycerol linked via the C1 of inositol, convey unique information (see Figure 1) Comprehensive analyses of both inositol and inositol phospholipids in signaling have been previously reviewed (Van Leeuwen et al, 2004; Gillaspy, 2011; Heilmann and Heilmann, 2014), so this review focuses on new inositol signaling molecules, the di-phospho (PP) and tri-phospho (PPP) inositol phosphates (PPx-InsPs), known as inositol pyrophosphates. HISTORY AND STRUCTURE OF PPx-INsPs PPx-InsPs were first identified in Dictyostelium in 1993 (Glennon and Shears, 1993; Hawkins et al, 1993; Menniti et al, 1993; Stephens et al, 1993) They are similar to ATP and polyphosphates in that they contain a linear chain of two (PP) or three (PPP) phosphates separated by pyrophosphate bonds, linked to an InsP molecule (see Figure 2).
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