The preference of tryptophan for membrane interfaces: insights from N-methylation of tryptophans in gramicidin channels.
To better understand the structural and functional roles of tryptophan at the membrane/water interface in membrane proteins, we examined the structural and functional consequences of Trp --> 1-methyl-tryptophan substitutions in membrane-spanning gramicidin A channels. Gramicidin A channels are miniproteins that are anchored to the interface by four Trps near the C terminus of each subunit in a membrane-spanning dimer. We masked the hydrogen bonding ability of individual or multiple Trps by 1-methylation of the indole ring and examined the structural and functional changes using circular dichroism spectroscopy, size exclusion chromatography, solid state (2)H NMR spectroscopy, and single channel analysis. N-Methylation causes distinct changes in the subunit conformational preference, channel-forming propensity, single channel conductance and lifetime, and average indole ring orientations within the membrane-spanning channels. The extent of the local ring dynamic wobble does not increase, and may decrease slightly, when the indole NH is replaced by the non-hydrogen-bonding and more bulky and hydrophobic N-CH(3) group. The changes in conformational preference, which are associated with a shift in the distribution of the aromatic residues across the bilayer, are similar to those observed previously with Trp --> Phe substitutions. We conclude that indole N-H hydrogen bonding is of major importance for the folding of gramicidin channels. The changes in ion permeability, however, are quite different for Trp --> Phe and Trp --> 1-methyl-tryptophan substitutions, indicating that the indole dipole moment and perhaps also ring size and are important for ion permeation through these channels.
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, USA.
Haiyan
H
Sun
Denise V
DV
Greathouse
Olaf S
OS
Andersen
Roger E
RE
Koeppe
D002850:Chromatography, Gel; D002942:Circular Dichroism; D003903:Deuterium / Q000378:metabolism; D006096:Gramicidin / Q000737:chemistry / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D008745:Methylation; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D014364:Tryptophan / Q000031:analogs & derivatives / Q000737:chemistry* / Q000378:metabolism*
D007473:Ion Channels; D008565:Membrane Proteins; D006096:Gramicidin; C028129:tryptophan methyl ester; D014364:Tryptophan; D003903:Deuterium