Membrane topologies and main features of the Kv and Kir potassium channel subtypes. (a, b) Schematic representation of the membrane topology of (a) Kv and (b) Kir channels. Note that one subunit of the tetrameric structure is shown (see Figures 3b, 4). Transmembrane helices are numbered S1-S6 in Kv channels and M1 and M2 in Kir channels; P, pore helix; S, signature sequence; N, amino terminus; C, carboxyl terminus; T1, conserved T1 domain (see Figure 4). The extracellular side is towards the top. Adapted with permission from .