Structure of the pore-forming transmembrane domain of a ligand-gated ion channel.
The structure of the pore-forming transmembrane domain of the nicotinic acetylcholine receptor from Torpedo has been investigated by infrared spectroscopy. Treatment of affinity-purified receptor with either Pronase or proteinase K digests the extramembranous domains (roughly 75% of the protein mass), leaving hydrophobic membrane-imbedded peptides 3-6 kDa in size that are resistant to peptide (1)H/(2)H exchange. Infrared spectra of the transmembrane domain preparations exhibit relatively sharp and symmetric amide I and amide II band contours centered near 1655 and 1545 cm(-)1, respectively, in both (1)H(2)O and (2)H(2)O. The amide I band is very similar to the amide I bands observed in the spectra of alpha-helical proteins, such as myoglobin and bacteriorhodopsin, that lack beta structure and exhibit much less beta-sheet character than is observed in proteins with as little as 20% beta sheet. Curve-fitting estimates 75-80% alpha-helical character, with the remaining peptides likely adopting extended and/or turn structures at the bilayer surface. Infrared dichroism spectra are consistent with transmembrane alpha-helices oriented perpendicular to the bilayer surface. The evidence strongly suggests that the transmembrane domain of the nicotinic receptor, the most intensively studied ligand-gated ion channel, is composed of five bundles of four transmembrane alpha-helices.
Méthot
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Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa, Ontario, K1H 8M5, Canada.
Ritchie
B D
BD
Blanton
M P
MP
Baenziger
J E
JE
D000818:Animals; D002942:Circular Dichroism; D019286:Endopeptidase K / Q000737:chemistry; D007473:Ion Channels / Q000737:chemistry; D008024:Ligands; D011402:Pronase / Q000737:chemistry; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011978:Receptors, Nicotinic / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D014101:Torpedo
D007473:Ion Channels; D008024:Ligands; D011978:Receptors, Nicotinic; D019286:Endopeptidase K; D011402:Pronase