| Regional Biophysics Meeting 2005, March 16-20, Zreče, Slovenia | [MembBiophys] |
Ion channels are transport proteins present in the cell membranes of all living organisms. KDCl (K+ Daucus carota 1) is a potassium channel cloned from carrot roots. It does not express functional channels when injected alone in Xenopus laevis oocytes. When co-injected with KAT1 (K+ Arabidopsis thaliana 1), KDC1 participates in the formation of heteromeric KDC1:KAT1 channels which show peculiar properties in response to heavy metals: the current mediated by the KDC1:KAT1 channel is not inhibited (as usual for potassium channels) but, on the contrary, potentiated by metal ions like Zn2+ and Ni2+. Compared to all plant inward rectifying potassium channels, the carrot channel KDC1 has two histidine pairs (H161,H162) in the S3-S4 and (H224,H225) in the S5-S6 linkers. The molecular mechanisms responsible for the KDC1 modulation by zinc were investigated by voltage-clamp technique. For this purpose, the aminoacids of the two putative binding sites were mutated into alanines. In mutated channels, the single histidine mutation in position 224 and the double histidine mutation in position 224-225 as well as in position 161-162 suppressed zinc potentiation. Results obtained by the addition of other metals, like by Pb2+ or Cd2+, suggest that Zn2+ potentiation cannot be due to simple electrostatic interactions between zinc and channel residues. We are currently investigating transport properties of another plant potassium channels cloned from Daucus carota : DKT1. DKT1 is expressed together with KDC1 in roots, shoot apical meristem and embryo cotyledons, suggesting a possible “ in vivo” interaction between the two channels. We verified that also DKT1 is unable to form functional homomeric channels when injected into Xenopus oocytes. Instead, DKT1 coinjected with KDC1 generated robust and stable inward currents. We may speculate that plants may have adopted a modulatory-strategy based on heteromeric channel formation to respond to different environmental conditions.
Email: naso@ge.ibf.cnr.it
Address: Istituto di Biofisica, CNR, Genova., Via De Marini 6, 16149 Genova, Italy