Alpha-helical, but not beta-sheet, propensity of proline is determined by peptide environment.
Proline is established as a potent breaker of both alpha-helical and beta-sheet structures in soluble (globular) proteins. Thus, the frequent occurrence of the Pro residue in the putative transmembrane helices of integral membrane proteins, particularly transport proteins, presents a structural dilemma. We propose that this phenomenon results from the fact that the structural propensity of a given amino acid may be altered to conform to changes imposed by molecular environment. To test this hypothesis on proline, we synthesized model peptides of generic sequence H2N-(Ser-LyS)2-Ala- Leu-Z-Ala-Leu-Z-Trp-Ala-Leu-Z-(Lys-Ser)3-OH (Z = Ala and/or Pro). Peptide conformations were analyzed by circular dichroism spectroscopy in aqueous buffer, SDS, lysophosphatidylglycerol micelles, and organic solvents (methanol, trifluoroethanol, and 2-propanol). The helical propensity of Pro was found to be greatly enhanced in the membrane-mimetic environments of both lipid micelles and organic solvents. Proline was found to stabilize the alpha-helical conformation relative to Ala at elevated temperatures in 2-propanol, an observation that argues against the doctrine that Pro is the most potent alpha-helix breaker as established in aqueous media. Parallel studies in deoxycholate micelles of the temperature-induced conformational transitions of the single-spanning membrane bacteriophage IKe major coat protein, in which the Pro-containing wild type was compared with Pro30 --> Ala mutant, Pro was found to protect the helix, but disrupt the beta-sheet structure as effectively as it does to model peptides in water. The intrinsic capacity of Pro to disrupt beta-sheets was further reflected in a survey of porins where Pro was found to be selectively excluded from the core of membrane-spanning beta-sheet barrels. The overall data provide a rationale for predicting and understanding the structural consequences when Pro occurs in the context of a membrane.
Li
S C
SC
Division of Biochemistry Research, Research Institute, Hospital for Sick Children, Toronto, Ontario, Canada.
Goto
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NK
Williams
K A
KA
Deber
C M
CM
D000595:Amino Acid Sequence; D001483:Base Sequence; D002213:Capsid / Q000235:genetics; D002942:Circular Dichroism; D006358:Hot Temperature; D008567:Membranes, Artificial; D008823:Micelles; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009842:Oligopeptides / Q000737:chemistry*; D018272:Porins / Q000737:chemistry; D011392:Proline / Q000737:chemistry*; D011487:Protein Conformation; D012996:Solutions; D012997:Solvents; D014867:Water
D008567:Membranes, Artificial; D008823:Micelles; D009842:Oligopeptides; D018272:Porins; D012996:Solutions; D012997:Solvents; D014867:Water; D011392:Proline