The Pros of nACh and 5-HT_3 Receptors

Abstract

Pentameric ligand gated ion channels (pLGIC) mediate fast synaptic transmission in animals, where they are known as Cys-loop receptors. Prokaryotic homologues of these proteins have been found in bacteria, although these do not possess the eponymous Cys-loop. Each receptor is constituted of five subunits which surround a central pore. A detailed analysis of sequences of all these subunits reveals that only the Pro in the Cys-loop is absolutely conserved; this has led to the proposal (which has not been widely adopted) that these receptors should be renamed 'Pro-loop' receptors. In addition to this Pro, there are 3 other Pros that are largely conserved and have been shown to play important but different roles. Here we combine old and new data to compare the characteristics of these Pro residues that allow them to have these distinct roles. Pro has a unique chemistry in proteins because it lacks a backbone hydrogen bond, and has a stronger cis-bias, and greater steric bulk at the amine than other amino acids. Determining which of these properties is essential at which location in Cys-loop receptors can only be accomplished using proline analogues, i.e non-conventional amino acids. These were inserted using nonsense suppression into nACh and 5-HT_3 receptors and receptor characteristics were determined using two electrode voltage clamp in Xenopus oocytes. The data reveal some similarities but also some differences in the roles of Pro at equivalent positions, and identify differences in the specific Pro properties that allow these two related Cys-loop receptors to function.

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