Biochemical Characterization of the Arabidopsis K+ Channels KAT1 and AKT1 Expressed or Co-expressed in Insect Cells

KAT1 and AKT1 belong to the multigenic family of the inwardly rectifying Shaker-like plant K+ channels. They were biochemically characterized after expression in insect cells using recombinant baculoviruses. The channels were solubilized from microsomal fractions prepared from infected cells (among eight different detergents only one, L-alpha-lysophosphatidylcholine, was efficient for solubilization), and purified to homogeneity using immunoaffinity (KAT1) or ion-exchange and size exclusion (AKT1) techniques. The following results were obtained with the purified polypeptides: (i) neither KAT1 nor AKT1 was found to be glycosylated; (ii) both polypeptides were mainly present as homotetrameric structures, supporting the hypothesis of a tetrameric structure for the functional channels; (iii) no heteromeric KAT1/AKT1 assembly was detected when the two polypeptides were co-expressed in insect cells. The use of the two-hybrid system in yeast also failed to detect any interaction between KAT1 and AKT1 polypeptides. Because of these negative results, the hypothesis that plant K+-channel subunits are able to co-assemble without any discrimination, previously put forward based on co-expression in Xenopus oocytes of various K+-channel subunits (including KAT1 and AKT1), has still to be supported by independent approaches. Co-localization of channel subunits within the same plant tissue/cell does not allow us to conclude that the subunits form heteromultimeric channels.