Analysis of Arabidopsis TPK2 and KCO3 reveals structural properties required for K⁺ channel function

Arabidopsis thaliana contains five tandem-pore domain potassium channels, TPK1-TPK5 and the related one-pore domain potassium channel, KCO3. Although KCO3 is unlikely to be an active channel, it still has a physiological role in plant cells. TPK2 is most similar to KCO3 and both are localized to the tonoplast. However, their function remains poorly understood. Here, taking advantage of the similarities between TPK2 and KCO3, we evaluated Ca²⁺ binding to the EF hands in TPK2, and the elements of KCO3 required for K⁺ channel activity. Presence of both EF-hand motifs in TPK2 resulted in Ca²⁺ binding, but EF1 or EF2 alone failed to interact with Ca²⁺. The EF hands were not required for K⁺ transport activity. EF1 contains two cysteines separated by two amino acids. Replacement of both cysteines with serines in TPK2 increased Ca²⁺ binding. We generated a two-pore domain chimeric K⁺ channel by replacing the missing pore region in KCO3 with a pore domain of TPK2. Alternatively, we generated two versions of simple one-pore domain K⁺ channels by removal of an extra region from KCO3. The chimera and one of the simple one-pore variants were functional channels. This strongly suggests that KCO3 is not a pseudogene and KCO3 retains components required for the formation of a functional K⁺ channel and oligomerization. Our results contribute to our understanding of the structural properties required for K⁺ channel activity.