Fig. 2. The human 2P domain K+channel subunits. (A ) Membrane topology of a background K+channel with four transmembrane segments and two P domains. Both amino and carboxy termini are intracellular. (B ) Phylogenetic tree of the human 2P domain K+channels. (C ) I-V curves of TASK-1, an open rectifier, constructed with voltage ramps of 600 ms in duration from −120 to 100 mV in a physiological (150 mm K+intracellular and 5 mm K+extracellular) and in a symmetrical K+gradient (150 mm K+intracellular and 150 mm K+extracellular). (D ) I-V curves of TREK-1, an outward rectifier. (E ) I-V curves of TWIK-2, a mild inward rectifier.