The Minimum M3-M4 Loop Length of Neurotransmitter-activated Pentameric Receptors Is Critical for the Structural Integrity of Cytoplasmic Portals

Abstract

Background: Residue 436 within 5-HT3A cytoplasmic portals determines single channel conductance (␥) and rectification. Results: M3-M4 loop truncation to 75 residues spared inward rectification; truncation to 70 abolished rectification and increased ␥. Human Cys-loop neurotransmitter receptor M3-M4 loops all exceed 70 residues. Conclusion: M3-M4 lengths of Ͼ70 are critical. Significance: Cytoplasmic portals may be essential for normal function of Cys-loop neurotransmitter receptors.

Length of the TM3-4 loop of the glycine receptor modulates receptor desensitization

“…We hypothesize that positively charged amino acids close to TM3 and at the beginning of TM4 of the ICD are essential determinants to retain the overall ability of the receptor for correct folding. This is in line with the proposed function of these residues for cytoplasmic portal formation enabling a funnel of the incoming ions [16].…”

“…Even recently solved structures of Glu-Cl and Lily (GLIC-GlyR chimera) lacked the large ICD [14,15]. Several reports described ␣-helical organization close to TM3 and TM4 [16,17] with basic residues being important determinants for channel properties. Basic motifs at both ends of the GlyR ICD have been reported to determine trafficking and G␤␥ protein binding [6,8].…”

Functional Chimeras of GLIC Obtained by Adding the Intracellular Domain of Anion- and Cation-Conducting Cys-Loop Receptors

“…43,44 It is a target for post-translational modifications like palmitoylation, phosphorylation, and ubiquitination, which can modulate receptor function. 45–47 Additionally, the ICD directly contributes to the modulation of ion flux. For 5-HT 3A R, it has been shown previously that arginines in the MA helix limit the single-channel conductance to subpicosiemen levels.…”

Seeking Structural Specificity: Direct Modulation of Pentameric Ligand-Gated Ion Channels by Alcohols and General Anesthetics

“…To simplify comparison of homologous subunits, the M2 helix is often described using prime notation, such that position 29 occupies the N-terminal intracellular channel vestibule and position 209 is at the C-terminal extracellular end of the pore (Thompson et al, 2010). The loop between a-helices M3 and M4 is highly variable in both sequence and length, in some cases extending over 250 amino acids residues, and in eukaryotic subtypes forms an independent intracellular domain (Jansen et al, 2008) implicated in channel kinetics, conductance, and cytoplasmic interactions (Baptista-Hon et al, 2013). Although lowresolution cryoelectron microscopy data indicate at least one a-helix in the intracellular loop of nicotinic acetylcholine receptors (Unwin, 2005), the absence of this domain in lower organisms (Tasneem et al, 2005) and its variable, likely flexible, nature in eukaryotes make it an ongoing challenge to structural studies.…”