Bernd Müller-Röber

Inward rectifying potassium (K⁺ _in) channels play an important role in turgor regulation and ion uptake in higher plants. Here, we report a previously unrecognized feature of these proteins: K⁺ _in channel C‐terminal polypeptides mediate channel protein interactions. Using a C‐terminal fragment of potato guard cell K⁺ _in channel KST1 in a yeast two‐hybrid screen two novel putative K⁺ _in channel proteins (SKT2 and SKT3) were identified by interaction of their C‐termini which contained a conserved domain (K_HA). Interactions were confirmed by Western blot‐related assays utilizing K⁺ _in channel C‐termini fused to green fluorescence protein. Although deletion of the K_HA‐domain abolished these interactions, K⁺ _in currents were still detectable by patch‐clamp measurements of insect cells expressing these KST1 mutants, indicating that formation of a functional channel does not depend on this C‐terminal domain.

Mitochondrial citrate synthase (EC 4.1.3.7) represents the first enzyme of the tricarboxylic acid cycle, catalyzing the condensation of acetyl-CoA and oxaloacetate, finally yielding citrate and CoA. We report here the isolation of cDNA clones encoding citrate synthase from Nicotiana tabacum, Beta vulgaris and Populus. Nucleotide and deduced amino acid sequences were compared with previously published sequences of mitochondrial citrate synthases from Arabidopsis thaliana and potato, as well as with the sequence of glyoxysomal citrate synthase from pumpkin. Homologies between the various plant mitochondrial enzymes were in the range from 77.2% (potato vs. Arabidopsis) to 94.2% (potato vs. tobacco) on the nucleotide level (coding regions only), and in the range from 70.1% to 90.4% (potato vs. Arabidopsis, and potato vs. tobacco, respectively) on the amino acid level. Identities of the mitochondrial isozymes to the pumpkin glyoxysomal enzyme were below 30% on the nucleotide and amino acid level. In Northern blot experiments citrate synthase mRNA was detected in all tissues analyzed. However, levels of expression showed tissue dependency despite the fact that citrate synthase is usually considered a house-keeping enzyme. Whether these different levels of expression reflect tissuespecifc variations with respect to basic metabolism awaits further analysis.