The Acetylcholine Receptor

The nicotinic acetylcholine receptor found on the skeletal muscle cell is greatly affected by the autoimmune response in myasthenia gravis. The nicotinic acetylcholine receptor is a well studied receptor. The Torpedo electric ray has tissue with high quantities of neuromuscular junctions, and the electric organ that the ray uses to stun its prey has up to half of it's innervated membrane occupied with acetylcholine receptors. This has made it an excellent resource for studing acetylcholine receptors. Through biochemical analysis of the receptors, the amino acid sequence of the receptor has been found (alpha subunit amino acid sequence shown to the right).

The acetylcholine receptor is a pentaramic protein consisting of five subunits (2 alpha units, one beta unit, one gamma unit, and one delta unit); each subunit encoded by a seperate gene. For all five subunits to assemble correctly the gene expression must be precisely coordinated. The five subunits are arranged in a barrel-like configuration around a central ion pore. The initial configuration is alpha-beta-alpha-gamma-delta or alpha-gamma-alpha-beta-delta.

Acetylcholine binds to the alpha subunit, which consists of 457 amino acids. The main binding site for acetylcholine is on the alpha subunit within a pocket of the external part of the peptide chain (shown by a square in the diagram at left-click for larger version of icon with text). The coiled structures within the diagram represent alpha-helix M2 segments lining the channel. M1-4 are hydrophobic intramembranous sequences of 19-27 amino acids that may span the membranes of the receptor. Intracellular ions are collected within the folds of the receptor and attracted to charged residues within the walls of the folds. Residues are located at the ends of the pores to help determine the ionic selectivity of the channel: oppositely charged residues attract, therefore the negative receptors of an acetylcholine receptor attract cations. Acetylcholine reacts with the residues to form weak bonds which cause an alosteric change in the subunit configurations and allows ions to enter the channel. The channel is nonselective between cations, producing an inward flow of positive charges. These positive charges initiate the action potential which causes the muscle to contract.

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