Structural Studies of the Gating Mechanism in a Pentameric Ligand-Gated Ion Channel Containing Two Additional N-Terminal Periplasmic Domains

Pentameric Ligand-gated ion channels (pLGICs) are allosteric membrane proteins mediating rapid chemo-electrical signal transduction triggered by the binding of neurotransmitters. They can be regulated by a variety of allosteric modulators, including some with important pharmaceutical compounds. pLGICs also exist in prokaryotes and it is not unusual to find them fused to other modules, thus displaying a different diversity in domain architectures compared to their eukaryotic counterparts. We present here two high resolution crystal structures of DeCLIC, a pLGIC from proteobacteria Desulfofustis, with large N-terminal domains (NTDs) directly fused to the conventional ligand binding domain (LBDs), in two different conformations. The one obtained at high Ca 2+ concentration shows a closed pore. In contrast, the other one (with no added Ca 2+) displays a widely open pore. Consistent with the observed structures, functional recordings indicated DeCLIC channels were active under calcium-free high-pH conditions, but ceased conducting upon addition of calcium. The observed positional arrangement of the NTDs suggest they can play a role as structural regulatory modules because they establish tight interactions with the orthosteric site in the LBDs only in the open pore conformation. Moreover, the global compaction and twisting motions of the NTDs and LBDs in the open form builds up a constriction gate in the vestibule of the LBD, also seen in eukaryotic receptors, which likely finely tunes the ion conductance. This new pair of structures illustrates the possible role of additional periplasmic domains, and provides new insights into the functional and structural diversity of the pLGIC family