Pronounced differences between the native K+ channels and KAT1 and KST1 α-subunit homomers of guard cells

Stomatal opening is the result of K+-salt accumulation in guard cells. Potassium uptake in these motor cells is mediated by voltage-dependent, K+-selective ion channels. Here we compare the in-vitro properties of two guard-cell K+-channel alpha-subunits from Arabidopsis thaliana (L.) Heynh. (KAT1) and Solanum tuberosum L. (KST1) after heterologous expression with the respective K+-transport characteristics' in their mother cell. The KAT1 and KST1 subunits when expressed in Xenopus oocytes shared the basic features of the K+-uptake channels in the corresponding guard cells, including voltage dependence and single-channel conductance. Besides these similarities, the electrophysiological comparison of K+ channels in the homologous and the heterologous expression systems revealed pronounced differences with respect to modulation and block by extracellular cations. In the presence of 1 mM Cs+, 50% of the guard-cell K+-uptake channels (GCKCl(in)) in A. thaliana and S. tuberosum, were inhibited upon hyperpolarization to -90 mV. For a similar effect on KAT1 and KST1 in oocytes, voltages as negative as -155 mV were required. In contrast, compared to the K+ channels in vivo the functional alpha-subunit homomers almost lacked a voltage-dependent block by extracellular Ca2+. Similar to the block by Cs+ and Ca2+, the acid activation of the alpha-homomers was less pronounced in oocytes. Upon acidification the voltage-dependence shifted by 82 and 90 mV for GCKCL(in), in A. thaliana and S. tuberosum, respectively, but only by 25 mV for KAT1 and KST1. From the differences in K+-channel modulation in vivo and after heterologous expression we conclude that the properties of functional guard-cell K+-uptake channels result either from the heterometric assembly of different alpha-subunits or evolve from cell-type-specific posttranslational modification.