The Role of the Cytoplasimic Domain in pH-Dependent Gating by the KCSA Channel

Abstract

Ion channels are membrane-spanning proteins that regulate ion flow across the membrane by responding to environmental changes. These changes are sensed by a sensor region, which then conveys the information to the pore region, which induces channel opening or closing. However, the mechanism for this conveyance remains unknown.The KcsA channel is a representative potassium channel activated by protons (changes in pH). It is a tetramer with four homologous subunits, each of which has two transmembrane segments which form a transmembrane pore, an α-helix at the N terminus, and a cytoplasmic domain made of 35 amino acids at the C terminus. Studies have argued the region that senses protons and regulates gating is entirely within the transmembrane segments. However, we have found that the cytoplasmic domain may also be involved in these two functions.Here, we show that the charged amino acids in the cytoplasmic domain, which makes up about half of this domain's amino acids, play an important role in pH-dependent gating. To investigate their effects, we made these amino acids neutral and measured gate activity. The mutant channel could be activated at what is normally inactive pH. This suggests that charges on the cytoplasmic domain generate electrostatic repulsion or attraction within the tetramer and influence KcsA channel activation in response to pH changes.