Conformations of proline residues in membrane environments

✍ Scribed by Charles M. Deber; Mira Glibowicka; G. Andrew Woolley

• Publisher: Wiley (John Wiley & Sons)
• Year: 1990
• Tongue: English
• Weight: 799 KB
• Volume: 29
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360290120

✦ Synopsis

Although noted as hydrophilic residues with helix-breaking potential, proline residues are observed in putatively a-helical transmembrane (TM) segments of many channel-forming integral membrane proteins. In addition to the recognized property of X-Pro peptide bonds (where X = any amino acid) to occur in cis as well as trans isomeric states, the tertiary amide character of the X-Pro bond confers increased propensity for involvement of its carbonyl group in specific H-bonded structures (e.g., pand y-turns) and/or liganding interactions with positively charged species. To examine this latter situation in further detail, we identified Leu-Pro-Phe as a consensus sequence triad based on actual occurrences of intramembranous Pro residues in transport protein T M segments.

Accordingly, we have undertaken the synthesis of hydrophobic peptides with potential membrane affinity, of which t-butyloxycarbonyi-L-Ala-L-Ala-L-Ala-L-Leu-L-Pro-L-Phe-OH (t-Boc-AAALPF-OH) is an initial compound. Partitioning of this peptide into model membrane environments composed of lipid micelles induces specific conformation(s) for the membrane-bound hexapeptide, as monitored by 75-MHz I3C-nmr spectral behavior of C-enriched Leu and Pro carbonyl carbons, and by 300-MHz 'H-nmr spectra of peptide a, p, and aromatic protons. Data are interpreted in terms of an intramolecularly H-bonded inverse y-turn conformation in the membrane environment involving the Leu-Pro-Phe triad. The inherent structural instability of a Pro-containing segment in a T M helix due to the multiplicity of possible local conformations is discussed as a functional aspect of membrane-buried prolines in transport proteins.