1. A model to explain K + -sensing of plant K out channels Supplemental Material to Johansson, Wulfetange, Porée, Michard, Lacombe, Sentenac, Thibaud, Mueller-Roeber, Blatt, Dreyer “ K + dependence of the Arabidopsis K + channel SKOR indicates an unique gating mechanism coupled to ion permeation ” dreyer@rz.uni-potsdam.de

2. Supplemental Material, Johansson et al. In the following only those details of a plant K out channel are displayed which are important to explain the K + -sensing mechanism: The P-domain and the S6 segment of 2 (out of 4) subunits.

3. Supplemental Material, Johansson et al. An open K out channel mediates K + efflux.

4. open Supplemental Material, Johansson et al. Spontaneous (voltage-dependent) closing occurs via rearrangements of C-terminal parts of the S6 segment. closed

5. openclosed Supplemental Material, Johansson et al. The ionic compositions of the pore and the cavity equilibrate with the external solution. When the external K + concentration is low also the occupancy of the pore by K + is low. low K + ext. high K + ext. When the external K + concentration is high also the occupancy of the pore by K + is high.

6. openclosed low K + ext. high K + ext. Supplemental Material, Johansson et al. When the external K + concentration is high also the occupancy of the pore by K + is high.  The pore is not flexible but rather rigid. Interactive forces between the pore (SKOR-M286) and S6 (SKOR-D312) induce then further conformational changes in S6 which stabilise the closed conformation.

7. openclosed low K + ext. high K + ext. Supplemental Material, Johansson et al. These rearrangements reduce the interactive forces between the pore and S6.  No further conformational changes in S6 are induced. When K + ext. is low  the pore is not crowded with K + ions  the pore is flexible and undergoes structural rearrangements (low-K + structure, Zhou et al., 2001, Nature 414, 43-48 ).

8. openclosed low K + ext. high K + ext. Supplemental Material, Johansson et al. Spontaneous rearrangements of the C-terminal part of S6 “re-open” the channel. However, the “low-K + structure” of the pore does not allow K + flux.

9. openclosed low K + ext. high K + ext. Supplemental Material, Johansson et al. The cavity is flushed by internal K +.

10. openclosed low K + ext. high K + ext. Supplemental Material, Johansson et al. When the internal K + concentration is large enough, K + ions can alter the conformation of the pore (high-K + structure, Zhou et al., 2001, Nature 414, 43-48 ).

11. openclosed low K + ext. high K + ext. Supplemental Material, Johansson et al. The K out channel mediates K + efflux again. open

12. Summary

13. Extracellular K + stabilises a closed conformation of plant K out channels. Supplemental Material, Johansson et al.

14. The model explains mutant phenotypes: SKOR-M286L

15. closed low K + ext. Supplemental Material, Johansson et al. open high K + ext. : C 2high low K + ext. : C 2low high K + ext. Prediction for the mutant SKOR-M286L The SKOR-mutation M286  L eliminates the interactive pore-S6 forces. The stabilised closed state C 2high at high K + ext. does not exist anymore. At high K + ext. the channel remains in state C 1. In contrast, C 2low at low K + ext. (low-K + pore structure) still exists. As a consequence, the channel SKOR-M286L appears to be K + ext. -activated.

16. Supplemental Material, Johansson et al.SKOR-M286L: elimination of the K + -induced stabilisation of the closed state C 2high  Number of open channels increases with increasing K + ext.

17. The model explains mutant phenotypes: SKOR-D312N

18. closed low K + ext. Supplemental Material, Johansson et al. Prediction for the mutant SKOR-D312N open high K + ext. high K + ext. : C 2high low K + ext. : C 2low The SKOR-mutation D312  N eliminates the interactive pore-S6 forces. The stabilised closed state C 2high at high K + ext. does not exist anymore. At high K + ext. the channel remains in state C 1. In contrast, C 2low at low K + ext. (low-K + pore structure) still exists. As a consequence, the channel SKOR-D312N appears to be K + ext. -activated.

19. Supplemental Material, Johansson et al.SKOR-D312N: elimination of the K + -induced stabilisation of the closed state C 2high  Number of open channels increases with increasing K + ext.

20. Mathematical Appendix An Analytical Consideration of the Model

21. The transitions O  O* and C 2low  C 1low are strongly energetically disfavoured. The only [K + ext. ]-dependent transition is C 1low  C 1. d and a are voltage-dependent rate-constants. h 1, h 2, k d, k a, L, and r can be voltage-dependent or voltage-independent. Supplemental Material, Johansson et al.

23. Experimentally accessible is p O : Some further simplifications of the equation: 

24. Supplemental Material, Johansson et al. Wild-type SKOR : Increasing [K + ] (=increasing x)  decreasing p O Mutants SKOR-M286L and SKOR-D312N :  p O increases with increasing [K + ] (=increasing x) Predictions for the K + -dependence at a fixed voltage