Figure 3. KEA3 regulates the composition of the p.m.f. by mediating potassium/proton antiport.

(a) The CPA2 domain of KEA3 is homologous to that of known K⁺/H⁺ antiporters. A percent identity matrix is shown for Arabidopsis KEA2 and KEA3, E. coli KefC and NhaA and T. thermophilus NapA. (b) The locations of two key substrate-binding amino acid residues in TM5 of the CPA2 domain19 are shown on a model of KEA3. (c) Low levels of Nigericin complement the kea3 NPQ phenotype. NPQ induction on transition from dark to 70 μmol m⁻² s⁻¹ light was measured in WT (Ws) and kea3-2 leaves incubated in water (mock) or 0.03 μM nigericin. (d,e) p.m.f. kinetics in kea3 mutants are largely unaffected. NPQ and ECS_t (which reports the magnitude of the p.m.f.) were measured near-simultaneously in WT (Ws) and kea3-2 by Chl fluorescence and dark-induced relaxation kinetics, respectively, in single leaves during a transition from dark to 90 μmol m⁻² s⁻¹. (f) After 100 s of low light, kea3 mutants show increased ΔpH and decreased Δψ. Full ECS decay kinetics were recorded after 100 s of low light (90 μmol m⁻² s⁻¹) to measure ΔpH and Δψ in WT (Col-0) and kea3-1. The average of six independent measurements per genotype was plotted as a moving average with interval 5 (see also Supplementary Fig. 9). (g) The experiment in f was repeated at different light intensities, and the fraction of the p.m.f. contributed by ΔpH was plotted. Asterisks indicate where WT and kea3 differ significantly (*P<0.04, Student’s t-test). (c–e,g) Error bars represent s.e.m. (n=6).