Nicotinic Acetylcholine Receptor - Structure

Structure

Nicotinic receptors, with a molecular mass of 290 kDa, are made up of five subunits, arranged symmetrically around a central pore. Each subunit comprises four transmembrane domains with both the N- and C-terminus located extracellularly. They possess similarities with GABA_A receptors, glycine receptors, and the type 3 serotonin receptors (which are all ionotropic receptors), or the signature Cys-loop proteins.

In vertebrates, nicotinic receptors are broadly classified into two subtypes based on their primary sites of expression: muscle-type nicotinic receptors and neuronal-type nicotinic receptors. In the muscle-type receptors, found at the neuromuscular junction, receptors are either the embryonic form, composed of α1, β1, δ, and γ subunits in a 2:1:1:1 ratio, or the adult form composed of α1, β1, δ, and ε subunits in a 2:1:1:1 ratio. The neuronal subtypes are various homomeric or heteromeric combinations of twelve different nicotinic receptor subunits: α2 through α10 and β2 through β4. Examples of the neuronal subtypes include: (α4)₃(β2)₂, (α4)₂(β2)₃, and (α7)₅. In both muscle-type and neuronal-type receptors, the subunits are somewhat similar to one another, especially in the hydrophobic regions.