K+ channel structures. Secondary structures of pore forming α-subunits of the: A) 2TMD-1P Kir channels, B) 6TMD-1P voltage-gated K+ channels and C) 4TMD-2P twin pore domain K+ channels. Functional channels are a tetramer of α-subunits (or dimer in the case of the 4TMD-2P), arranged so the extracellular P loops form the pore region of the channel. The charged TMD S4 in the 6TMD-1P structure is thought to be involved in the voltage-gating of these channels. Channel gating may also be modified by the presence of auxiliary β-subunits.