The surface accessibility of the glycine receptor M2-M3 loop is increased in the channel open state

Mutations in the extracellular M2-M3 loop of the glycine receptor (GlyR) al pha1 subunit have been shown previously to affect channel gating. In this s tudy, the substituted cysteine accessibility method was used to investigate whether a structural rearrangement of the M2-M3 loop accompanies GlyR acti vation. All residues from R271C to V277C were covalently modified by both p ositively charged methanethiosulfonate ethyltrimethylammonium (MTSET) and n egatively charged methanethiosulfonate ethylsulfonate (MTSES), implying tha t these residues form an irregular surface loop. The MTSET modification rat e of all residues from R271C to K276C was faster in the glycine-bound state than in the unliganded state. MTSES modification of A272C, L274C, and V277 C was also faster in the glycine-bound state. These results demonstrate tha t the surface accessibility of the M2-M3 loop is increased as the channel t ransitions from the closed to the open state, implying that either the loop itself or an overlying domain moves during channel activation.