Conformation-dependent hydrophobic photolabeling of the nicotinic receptor: Electrophysiology-coordinated photochemistry and mass spectrometry

摘要

We characterized the differential accessibility of the nicotinic acetylcholine receptor α1 subunit in the open, closed, and desensitized states by using electrophysiology-coordinated photolabeling by several lipophilic probes followed by mass spectrometric analysis. Voltage-clamped oocytes expressing receptors were preincubated with one of the lipophilic probes and were continually exposed to acetylcholine; UV irradiation was applied during 500-ms pulses to + 40 or to -140 mV (which produced closed or ≈50% open receptors, respectively). In the open state, there was specific probe incorporation within the N-terminal domain at residues that align with the β8–β9 loop of the acetylcholine-binding protein. In the closed state, probe incorporation was identified at several sites of the N-terminal domain within the conserved cysteine loop (residues 128–142), the cytoplasmic loop (M3–M4), and M4. The labeling pattern in the M4 region is consistent with previous results, further defining the lipid-exposed face of this transmembrane α-helix. These results show regions within the N-terminal domain that are involved in gating-dependent conformational shifts, confirm that the cysteine loop resides at or near the protein-membrane interface, and show that segments of the M3–M4 loop are near to the lipid bilayer.