Abstract

K2P TASK2 K+ channels have been shown to underlie IK, vol, the cell volume regulated K+ conductance involved in regulatory volume decrease, RVD, in Ehrlich cells and proximal tubule cells (Niemeyer et al., 2001; Barrière et al., 2003). RVD is the process in which cells swollen in hypotonicity recover their volume by activation of K+ and Cl- channels followed by KCl efflux and osmotically-obliged water. Using Ehrlich cells we have also shown that IK, vol is tonically inhibited by an active G protein and that this inhibition is relieved by cell swelling and G protein inactivation (Niemeyer et al., 2002). We now show results that suggest that TASK2 channels are modulated by Gβγ subunits of heterotrimeric G protein. In whole cell patch-clamp studies of TASK-2 expressed in HEK-293 cells with 100 μM GTP and 1 mM MgATP, current became stable at 60 ± 4 % (SEM, n=9) of initial value at >5 min after the onset of the recording. When GTP-γ-S was used as a replacement for GTP there was a strong inhibition of TASK2 currents (15 ± 2 %, n=12), but if GDP-β-S was used instead no inhibition was seen (79 ± 5 %, n=7). Intracellular addition of purified Gβγ inhibited TASK-2 (27 ± 4 %, n=7) current, independently of whether GTP or GDP-β-S was present. The effects of GTP-γ-S and Gβγ were abolished by neutralisation of TASK-2 C-terminus double lysine residues K257-K258 or K296-K297. Tagged proteins and immunoprecipitation assays were used to see if there is a physical interaction between Gβ1-4 subunits and TASK-2. TASK-2 was seen to associate with Gβ1 or Gβ2 only (n=6), in agreement with the highest expression of the subunits in proximal tubule cells. Coimmunoprecipitation was impeded by mutating C-terminus K257-K258 (but not K296-K297) to alanines (n=4 for each mutant). Finally we investigated whether the G protein effect might be involved in the extra and intracellular pH-gating of TASK-2. Gating by extra- or intracellular pH was unaltered in GTP-γ-S-insensitive TASK-2-K257A-K258A mutant. Shrinking TASK-2-expressing HEK-293 cells hypertonic, 400 mOsm solution decreased the current to 0.36 ± 0.03 (n=7) of its initial value. The same manoeuvre had a significantly diminished effect on TASK-2-K257A-K258A-expressing cells, with a decrease in current to only 0.71 ± 0.03 (n=10) of the initial value. Our data are compatible with the concept that TASK2 channels are modulated by Gβγ subunits of heterotrimeric G protein. We propose that this modulation is a novel way in which TASK-2 can be tuned to its physiological functions.