Functional analysis of the M2D helix of the TRK1 potassium transporter of Saccharomyces cerevisiae

Eukaryotic KcsA-related K + transporters mediate physiologically relevant K + and Na + fluxes in fungi and plants. ScTRK1 is a characteristic member of the group, and here we report a mutational analysis of the unique M2 D helix of this transporter. Our results support the theoretical models placing this helix in a relevant position in the pore and interacting with P segments. Most single mutations eliminating positively charged or introducing negatively charged residues reduced the V max of Rb + influx to a half, several together showed an additive effect, and four practically suppressed transport. In contrast, the introduction of only one positively charged residue practically abolished the function of the transporter. Almost all mutations in the M2 D helix affected the two Rb + binding sites of the transporter, mimicking mutations in the selectivity filter.