text: The Cys-loop ligand-gated ion channel superfamily is composed of nicotinic acetylcholine, GABA, GABA-ρ, glycine, 5-HT, and zinc-activated (ZAC) receptors. These receptors are composed of five protein subunits which form a pentameric arrangement around a central pore. There are usually 2 alpha subunits and 3 other beta, gamma, or delta subunits (some consist of 5 alpha subunits). 
  The name of the family refers to a characteristic loop formed by 13 highly conserved amino acids between two cysteine (Cys) residues, which form a disulfide bond near the N-terminal extracellular domain.
  Cys-loop receptors are known only in eukaryotes, but are part of a larger family of pentameric ligand-gated ion channels. Only the Cys-loop clade includes the pair of bridging cysteine residues. The larger superfamily includes bacterial (e.g. GLIC) as well as non-Cys-loop eukaryotic receptors, and is referred to as "pentameric ligand-gated ion channels", or "Pro-loop receptors".
  All subunits consist of a large conserved extracellular N-terminal domain, three highly conserved transmembrane domains, a cytoplasmic loop of variable size and amino acid sequence, and a fourth transmembrane region with a relatively short and variable extracellular  C-terminal domain.
  Neurotransmitters bind at the interface between subunits in the extracellular domain.
  Each subunit contains four membrane-spanning alpha helices (M1, M2, M3, M4).  The pore is formed primarily by the M2 helices. The M3-M4 linker is the intracellular domain that binds the cytoskeleton.