Ion transport in muscle acetylcholine receptor maintained by conserved salt bridges between the pore and lipid membrane

Pores through ion channels rapidly transport small inorganic ions along their electrochemical gradients. Here, applying single- channel electrophysiology and mutagenesis to the archetypal muscle nicotinic acetylcholine receptor (AChR) channel, we show that a conserved pore- peripheral salt bridge partners with those in the other subunits to regulate ion transport. Disrupting the salt bridges in all five receptor subunits greatly decreases the amplitude of the unitary current and increases its fluctuations. However, disrupting individual salt bridges has unequal effects that depend on the structural status of the other salt bridges. The AChR epsilon- and delta-subunits are structurally unique in harboring a putative palmitoylation site near each salt bridge and bordering the lipid membrane. The effects of disrupting the palmitoylation sites mirror those of disrupting the salt bridges, but the effect of disrupting either of these structures depends on the structural status of the other. Thus, rapid ion transport through the AChR channel is maintained by functionally interdependent salt bridges linking the pore to the lipid membrane.