The crystal structure of a protein channel provides clues about the mechanisms that control the closure of pores found in the epidermis of plant leaves. Excitingly, the protein channel folds in a way never seen before. See Article p.1074 SLAC1 is a recently identified anion channel found in the leaves of plants, where it controls turgor pressure and stomatal opening in response to environmental factors including carbon dioxide, ozone and drought. The X-ray crystal structure of a bacterial homologue of SLAC1 — the tellurite resistance protein TehA from Haemophilus influenzae — has now been determined. Structure-inspired mutagenesis was used to analyse the conductance properties of the channel. Electrostatic features of the pore suggest that selectivity among different anions is largely a function of the energetic cost of ion dehydration. This work, together with further studies of the function of the bacterial protein, suggests that SLAC1 and TehA represent a large family of selective anion channels controlled by environmental stimuli.