Ion Channels

Many outward-rectifying K⁺ channels in plants show an unusual voltage sensitivity: gating of the channel interacts with [K⁺] outside so that the net K⁺ flux through the channel is always directed outward. This dependence on extracellular K⁺ for gating makes good "physiological sense" in an environment where the concentration of the cation often varies by more than 100-fold. It also raises fundamental biological questions about the mechanism of K⁺ sensing. Kinetic analysis has shown that 2 K⁺ ions interact with the channel to gate its activity in Vicia guard cells, and at a site (or sites) remarkably similar in its physico-chemical properties to the channel pore [Blatt and Gradmann (1997) J. Membr. Biol. 158,241].

This K⁺-sensitivity is intrinsic to Kv-like K⁺ channels such as SKOR and GORK of Arabidopsis as it is to the TOK1/YKC1 channel of yeast. The structures of the yeast and plant channels are very different, however, indicating that the K⁺-sensitivities of these two channel structures come about through convergent evolution [see Vergani, et al. (1998) EMBO J. 17,7190; Johansson and Blatt (2006) Biochem J. 393,645].

Our work with the SKOR K⁺ channel - part of a collaboration with Dr I. Dreyer (University of Potsdam) - has yielded a very different picture of the mechanics of K⁺ sensitivity in the plant K⁺ channel. In this case, K⁺-dependent gating arises from interactions of the K⁺ ion within the channel pore which, in turn, affect the conformation of helices lining the pore and the gate. The mechanism is analogous to that proposed by Clay Armstrong in his the 'foot-in-the-door' model [see Yellen (2002) Nature 419,35]: K⁺ ions within the pore effectively hold the channel open by preventing the protein structure from relaxing to close the gate. However, gating in the plant K⁺ channels flies in the face of conventional thinking. It seems that K+ occupation of the pore actually promotes closure of the gate [Johansson, et al. (2006) Plant J, 46,269-81]! How this works remains an open question, but it is clear that K⁺-dependent gating in SKOR is the very first example for which we can demonstrate the molecular mechanism for nutrient sensing in a plant cell.

The SKOR channel is also subject to regulation by reactive oxygen species (ROS), consistent with nutrient stress-associated increases in ROS and the role for SKOR in vascular charge balance and K⁺ delivery to the shoot [Schachtmann (2007) Ann.Rev.Plant Biol. 58,47]. Again it appears that this ROS sensitivity is ‘hard-wired’ within the structure of the SKOR channel protein, in this case through reversible chemical modification of the protein. Our recent work uncovered a single cysteine residue essential for ROS sensitivity – and for sensitivity to known Cys-modifying reagents – and located within the voltage sensor domain of this channel [Garcia-Mata, et al. (2010) J. Biol. Chem. 285,29286]. We could show that ROS access to this site is voltage-dependent, as expected for conformational changes of the voltage sensor domain, and molecular dynamic modelling supported this conclusion, indicating a water-access to the site only when the channel is in the depolarised conformation. This behaviour complements the ROS-sensitivity of SKOR-mediated K⁺ delivery to the shoot, and it opens the door to manipulating mineral nutrient relocation between root and shoot.