Distinct Specificities of Inwardly Rectifying K Channels

Activation of several inwardly rectifying K channels (Kir) requires the presence of phosphatidylinositol 4,5bisphosphate (PtdIns(4,5)P2). The constitutively active Kir2.1 (IRK1) channels interact with PtdIns(4,5)P2 strongly, whereas the G-protein activated Kir3.1/3.4 channels (GIRK1/GIRK4), show only weak interactions with PtdIns(4,5)P2. We investigated whether these inwardly rectifying K channels displayed distinct specificities for different phosphoinositides. IRK1, but not GIRK1/GIRK4 channels, showed a marked specificity toward phosphates in the 4,5 head group positions. GIRK1/ GIRK4 channels were activated with a similar efficacy by PtdIns(3,4)P2, PtdIns(3,5)P2, PtdIns(4,5)P2, and PtdIns(3,4,5)P3. In contrast, IRK1 channels were not activated by PtdIns(3,4)P2 and only marginally by high concentrations of PtdIns(3,5)P2. Similarly, high concentrations of PtdIns(3,4,5)P3 were required to activate IRK1 channels. For either channel, PtdIns(4)P was much less effective than PtdIns(4,5)P2, whereas PtdIns was inactive. In contrast to the dependence on the position of phosphates of the phospholipid head group, GIRK1/GIRK4, but not IRK1 channel activation, showed a remarkable dependence on the phospholipid acyl chains. GIRK1/GIRK4 channels were activated most effectively by the natural arachidonyl stearyl PtdIns(4,5)P2 and much less by the synthetic dipalmitoyl analog, whereas IRK1 channels were activated equally by dipalmitoyl and arachidonyl stearyl PtdIns(4,5)P2. Incorporation of PtdInsP2 into the membrane is necessary for activation, as the short chain water soluble diC4 PtdIns(4,5)P2 did not activate either channel, whereas activation by diC8 PtdIns(4, 5)P2 required high concentrations.