TASK-2 (K2P5.1) is a two-pore domain, background type K+ channel belonging to the TALK subgroup of the K2P family of proteins. K2P channels, of which there exist 15 mammalian members, are four transmembrane domain proteins harbouring two pore domains per subunit and forming dimers with pseudo-fourfold symmetry. These channels provide the molecular substratum of the long known background K+ conductance and, although initially believed to behave as mere passive non-regulated permeation pathways, they have revealed themselves exquisitely controlled by a variety of gating mechanisms modulating their opening and closing. TASK-2 is activated by extracellular alkalinization and inhibited by acidification. This property is thought to be responsible for its role in maintaining a hyperpolarised membrane potential during bicarbonate reabsorption in proximal tubule epithelial cells, and its absence from KO mice leads to a condition reminiscent of human proximal renal tubular acidosis syndrome (6). In addition, TASK-2 is expressed in neurons of the retrotrapezoid nucleus that function as central respiratory chemoreceptors and there it appears to function as a CO2 and pH sensor to modulate excitability with an impact in the central control breathing (5). Consistent with a possible function of TASK-2 as a CO2-sensor we have also recently uncovered a pHi-dependent gating mechanism (2) and a direct effect of CO2 (4). In this presentation we will review progress in our laboratory that starts to reveal the molecular mechanism of some of these regulations as well as the emergence of new modulation modes that might attune the activity of TASK-2 to its physiological functions. Activation of TASK-2 by extracellular alkalinization is mediated by neutralization of R224 located towards the extracellular end of transmembrane domain (TM) 4, near the second pore domain [3]. Protonated R224 exerts an electrostatic effect on the selectivity filter that impedes ion movement. Its deprotonation at exceptionally low extracellular pH (pHo) values is attributed to a hydrophobic environment revealed by molecular models of TASK-2 [7]. In addition to its regulation by pHo, TASK-2 is gated open by intracellular alkalinization (2). Intracellular-facing K245 acts as pHi sensor as its mutation to histidine acid-shifts the pHi-dependence curve of TASK-2. Our more recent work has focused into additional modes of regulation of TASK-2. Channel activity is inhibited by Gβγ subunits of heterotrimeric G protein (1) TASK-2 is strongly inhibited when GTP-γ-S, but not GDP-β-S, is used to replace intracellular GTP, or by addition of intracellular purified Gβγ subunits. The action of GTP-γ-S and Gβγ subunits are dependent on TASK-2 C terminus double lysine residues K257-K258 and K296-K297, whose neutralisation abolishes the effect. Immunoprecipitation and membrane yeast two hybrid assays using tagged proteins reveal an interaction between Gβ1 and Gβ2 subunits and TASK-2. Interaction is impeded by mutating K257-K258 (but not K296-K297) to alanines. Our data are compatible with the concept that TASK-2 channels are modulated by Gβγ subunits of heterotrimeric G protein. This could be linked to a hormonal regulation of TASK-2 in the kidney, although actions of Gβγ are also known to occur by receptor-independent processes. Finally, unpublished results reveal that TASK-2 activity promptly runs down in the absence of intracellular ATP either in whole-cell recordings or in isolated inside-out (i-o) patches of membrane from cells expressing the channel. That the run-down is most probably linked to loss of membrane phosphatidylinositol 4,5-bisphosphate (PIP2) is suggested by the fact that it is: mimicked by PIP2 scavenger neomycin, reversed by superfusion of i-o patches with DiC8-PIP2 and reproduced by PIP2 hydrolysis elicited with a co-expressed membrane-bound voltage-sensitive phosphatase. It is suggested that membrane PIP2 interacting with positively charged residues in the channel is an important determinant of channel activity. Mutagenesis of candidate residues is in progress to characterise the nature of such an interaction.