Abstract
The inositol polyphosphate family of small, cytosolic molecules has a prominent place in the field of cell signalling, and inositol __pyro__phosphates are the most recent addition to this large family. First identified in 1993, they have since been found in all eukaryotic organisms studied. The defining feature of inositol __pyro__phosphates is the presence of the characteristic ‘high energy’ __pyro__phosphate group, which immediately attracted interest in them as possible signalling molecules. In addition to their unique ‘high energy’ __pyro__phosphate bond, their concentration in the cell is tightly regulated with an extremely rapid turnover. This, together with the history of other inositol polyphosphates, makes it likely that they have an important role in intracellular signalling involving some basic cellular processes. This hypothesis is supported by the surprisingly wide range of cellular functions where inositol __pyro__phosphates seem to be involved. A seminal finding was that inositol __pyro__phosphates are able to directly phosphorylate pre‐phosphorylated proteins, thereby identifying an entirely new post‐translational protein modification, namely serine‐__pyro__phosphorylation. Rapid progress has been made in characterising the metabolism of these molecules in the 15 years since their first identification. However, their detailed signalling role in specific cellular processes and in the context of relevant physiological cues has developed more slowly, particularly in mammalian system. We will discuss inositol __pyro__phosphates from the cell signalling perspective, analysing how their intracellular concentration is modulated, what their possible molecular mechanisms of action are, together with the physiological consequences of this novel form of signalling. J. Cell. Physiol. 220: 8–15, 2009. © 2009 Wiley‐Liss, Inc.