Subunit-selective role of the M3 transmembrane domain of the nicotinic acetylcholine receptor in channel gating

The nicotinic acetylcholine receptor (AChR) can be either hetero-pentameric, composed of α and non-α subunits, or homo-pentameric, composed of α7 subunits. To explore the subunit-selective contributions of transmembrane domains to channel gating we analyzed single-channel activity of chimeric muscle AChRs. We exchanged M3 between α1 and ɛ or α7 subunits. The replacement of M3 in α1 by ɛM3 significantly alters activation properties. Channel activity appears as bursts of openings whose durations are 20-fold longer than those of wild-type AChRs. In contrast, 7-fold briefer openings are observed in AChRs containing the reverse ɛ chimeric subunit. The duration of the open state decreases with the increase in the number of α1M3 segments, indicating additive contributions of M3 of all subunits to channel closing. Each α1M3 segment decreases the energy barrier of the closing process by ∼ 0.8 kcal/mol. Partial chimeric subunits show that small stretches of the M3 segment contribute additively to the open duration. The replacement of α1 sequence by α7 in M3 leads to 3-fold briefer openings whereas in M1 it leads to 10-fold prolonged openings, revealing that the subunit-selective role is unique to each transmembrane segment.