One Scaffold, Two Conformations: The Ring-Flip of the Messenger InsP 8 Occurs under Cytosolic Conditions.

Inositol poly- and pyrophosphates (InsPs and PP-InsPs) are central eukaryotic messengers. These very highly phosphorylated molecules can exist in two distinct conformations, a canonical one with five phosphoryl groups in equatorial positions, and a "flipped" conformation with five axial substituents. Using 13 C-labeled InsPs/PP-InsPs, the behavior of these molecules was investigated by 2D-NMR under solution conditions reminiscent of a cytosolic environment. Remarkably, the most highly phosphorylated messenger 1,5(PP) 2 -InsP 4 (also termed InsP 8 ) readily adopts both conformations at physiological conditions. Environmental factors-such as pH, metal cation composition, and temperature-strongly influence the conformational equilibrium. Thermodynamic data revealed that the transition of InsP 8 from the equatorial to the axial conformation is, in fact, an exothermic process. The speciation of InsPs and PP-InsPs also affects their interaction with protein binding partners; addition of Mg 2+ decreased the binding constant K d of InsP 8 to an SPX protein domain. The results illustrate that PP-InsP speciation reacts very sensitively to solution conditions, suggesting it might act as an environment-responsive molecular switch.