Characterization of a calmodulin-binding transporter from the plasma membrane of barley aleurone

We have used Arabidopsis calmodulin (CaM) covalently coupled to horseradish peroxidase to screen a barley aleurone cDNA expression library for CaM binding proteins. The deduced amino acid sequence of one cDNA obtained by this screen was shown to be a unique protein of 702 amino acids with CaM and cyclic nucleotide binding domains at the carboxyl terminus and high similarity to olfactory and K+ channels. This cDNA was designated HvCBT1 (Hordeum vulgare CaM binding transporter). Hydropathy plots of HvCBT1 showed the presence of six putative transmembrane domains, but sequence alignment indicated a pore domain that was unlike the consensus domains in K+ and olfactory channels. Expression of a subclone of amino acids 482-702 in Escherichia coli generated a peptide that bound CaM. When a fusion protein of HvCBT1 and green fluorescent protein was expressed in barley aleurone protoplasts, fluorescence accumulated in the plasma membrane. Expression of HvCBT1 in the K+ transport deficient Saccharomyces cerevisiae mutant CY162 showed no rescue of the mutant phenotype. However, growth of CY162 expressing HvCBT1 with its pore mutated to GYGD, the consensus sequence of K+ channels, was compromised. We interpret these data as indicating that HvCBT1 acts to interfere with ion transport.