Position and ionization state of Asp in the core of membrane-inserted alpha helices control both the equilibrium between transmembrane and nontransmembrane helix topography and transmembrane helix positioning.
The behavior of model-membrane-inserted polyLeu-rich peptides containing Asp residues located at various positions in their hydrophobic core was investigated. The topography of the bilayer-inserted alpha helices formed by these peptides was evaluated by measuring the emission lambda(max) and quenching the fluorescence of a Trp at the center of the peptide sequence. When Asp residues were protonated (at low pH), peptides that were incorporated into vesicles composed of dioleoylphosphatidylcholine (DOPC) adopted a topography in which the polyLeu sequence predominantly formed a normal transmembrane (TM) helix. When Asp residues were ionized (at neutral or high pH), topography was altered in a manner that would allow the charged Asp residues to reside near the bilayer surface. In DOPC vesicles, most peptides repositioned so that the longest segment of consecutive hydrophobic residues (12 residue minimum) formed a truncated/shifted TM structure. However, peptides with one or two charged Asp residues close to the center of the hydrophobic sequence and thus lacking even a 12-residue continuous hydrophobic segment, formed a helical non-TM state locating near the bilayer surface. At low pH, incorporation of the peptides into thicker bilayers composed of dierucoylphosphatidylcholine (DEuPC) resulted in the formation of a mixture of the normal TM state and the non-TM helical state located near the bilayer surface. In DEuPC vesicles at high pH, the non-TM state tended to predominate. How Asp-ionization-dependent shifts in helix topography may regulate the function of membrane proteins exposed to environments with differing pH in vivo (e.g., endosomes) is discussed.
Caputo
Gregory A
GA
Department of Biochemistry and Cell Biology, Stony Brook University, State University of New York, Stony Brook, New York 11794-5215, USA.
London
Erwin
E
D001224:Aspartic Acid / Q000737:chemistry*; D002942:Circular Dichroism; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D007477:Ions / Q000737:chemistry; D008081:Liposomes / Q000737:chemistry* / Q000378:metabolism*; D008563:Membrane Lipids / Q000737:chemistry / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D010455:Peptides / Q000737:chemistry / Q000378:metabolism; D010713:Phosphatidylcholines / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000737:chemistry
D007477:Ions; D008081:Liposomes; D008563:Membrane Lipids; D008565:Membrane Proteins; D010455:Peptides; D010713:Phosphatidylcholines; C027819:polyleucine; D001224:Aspartic Acid; D014364:Tryptophan; C017251:1,2-oleoylphosphatidylcholine