The pentameric ligand-gated ion channels (pLGICs) constitute a family of neurotransmitter receptors that are key players in the control of electric signaling at chemical synapses. The family codes for a conserved scaffold of channel proteins that open in response to the binding of neurotransmitter molecules. We have determined the X-ray structures of two prokaryotic family members from the bacterium Erwinia chrysanthemi (ELIC) at 0.33 nm resolution [1] and from the bacterium Gloeobacter violaceus (GLIC) at 0.31 nm resolution [2]. Both proteins form cation selective channels and bear most of the structural hallmarks of the family including the N-terminal extracellular ligand-binding domain and the four helices of the pore domain. Despite the overall similarity, both structures adopt distinct conformations of the ion conduction pathway. The structure of ELIC shows a non-conductive state with rings of hydrophobic residues at the extracellular side of the pore preventing ion permeation. This hydrophobic barrier has opened in the structure of GLIC to a funnel shaped pore, where a ring of conserved glutamate residues at the intracellular constriction creates an ion-coordination site. GLIC is thus believed to represent a conducting conformation of the channel. In combination, both structures suggest a novel gating mechanism for pentameric ligand-gated ion channels where channel opening proceeds by a change in the tilt of the pore-forming helices [3]. The structures thus provide a first detailed view into how a pLGIC may open and selectively conduct ions.