Characterization of a tobacco TPK-type K⁺ channel as a novel tonoplast K⁺ channel using yeast tonoplasts

The tonoplast K⁺ membrane transport system plays a crucial role in maintaining K⁺ homeostasis in plant cells. Here, we isolated cDNAs encoding a two-pore K⁺ channel (NtTPK1) from Nicotiana tabacum cv. SR1 and cultured BY-2 tobacco cells. Two of the four variants of NtTPK1 contained VHG and GHG instead of the GYG signature sequence in the second pore region. All four products were functional when expressed in the Escherichia coli cell membrane, and NtTPK1 was targeted to the tonoplast in tobacco cells. Two of the three promoter sequences isolated from N. tabacum cv. SR1 were active, and expression from these was increased ∼2-fold by salt stress or high osmotic shock. To determine the properties of NtTPK1, we enlarged mutant yeast cells with inactivated endogenous tonoplast channels and prepared tonoplasts suitable for patch clamp recording allowing the NtTPK1-related channel conductance to be distinguished from the small endogenous currents. NtTPK1 exhibited strong selectivity for K⁺ over Na⁺. NtTPK1 activity was sensitive to spermidine and spermine, which were shown to be present in tobacco cells. NtTPK1 was active in the absence of Ca²⁺, but a cytosolic concentration of 45 μM Ca²⁺ resulted in a 2-fold increase in the amplitude of the K⁺ current. Acidification of the cytosol to pH5.5 also markedly increased NtTPK1-mediated K⁺ currents. These results show that NtTPK1 is a novel tonoplast K⁺ channel belonging to a different group from the previously characterized vacuolar channels SV, FV, and VK.