A novel K+ channel expressed in carrot roots with a low susceptibility toward metal ions

Kdc1 is a novel K+-channel gene cloned from carrot roots, and which is also present in cultured carrot cells. We investigated the characteristics of t he ionic current elicited in Xenopus oocytes coinjected with KDC1 (K+-Daucu s carota 1) and KAT1 (from Arabidopsis thaliana) RNA. Expressed hereromeric channels displayed inward-rectifying potassium currents whose kinetics, vo ltage characteristics, and inhibition by metal ions depended on KDC1:KAT1 r atios. At low KDC1:KAT1 ratios, Zn2+ inhibition of heteromeric K+ current w as less pronounced compared to homomeric KAT1 channels, while at higher KDC 1:KAT1 ratios, the addition of Zn2+ even produced an increase in current. U nder the same conditions, the Ni2+ inhibition of the current was also reduc ed, but no current increase was observed. These effects might be explained by the unusual amino acid composition of the KDC1 protein in terms of histi dine residues that are absent in the pore region, but abundant (four per su bunit) in the proximity of the pore entrance. Channels like KDC1 could be a t least partially responsible for the higher resistance of carrot cells in the presence of metals.