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Interactions in the Membrane

The inositol-containing phospholipids, or phosphoinositides, are membrane phospholipids with both structural and regulatory functions. Phosphatidylinositol-4,5-bisphosphate (PIP₂) has been implicated in the regulation of multiple ion channels at the cytoplasmic face of the membrane. However, direct evidence for a phosphoinositide-channel interaction is critically lacking.

In this study, Decha Enkvetchakul and colleagues use an ⁸⁶Rb⁺ uptake assay to provide the first evidence for PIP₂ regulation of K⁺ channels reconstituted into liposomes. The unique contribution of this study is the demonstration of a direct and concentration-dependent interaction in an entirely defined system. It is interesting to note that, in contrast to mammalian Kir channels, which are activated by PIP₂, the prokaryotic KirBac1.1 is inhibited by the phosphoinositide. However, the efficacy of modulation in both cases is proportional to the number of phosphates on the negatively charged head group of PIP₂, suggesting that a common binding mechanism may trigger allosteric events with opposite outcomes. This research lends new importance to previous studies that documented the modulatory effects of phosphoinositide-channel interactions but were unable to exclude intermediate players or cascades as the mechanism of action.

FOOTNOTES

See referenced article, J. Biol. Chem. 2005, 280, 35785-35788

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