Membrane-binding amphipathic alpha-helical peptide derived from CTP:phosphocholine cytidylyltransferase.

Abstract

A peptide corresponding to a portion of the amphipathic alpha-helical region of CTP:phosphocholine cytidylyltransferase was synthesized. This region of the enzyme was proposed to be the membrane-binding domain [Kalmar, G.B., Kay, R.J., Lachance, A., Aebersold, R., & Cornell, R.B. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 6029]. We have shown that the peptide is physically associated with PG vesicles. CD of the peptide in buffer suggested a primarily random structure, while, in the presence of trifluoroethanol, the peptide was alpha-helical. Anionic lipid vesicles promoted an alpha-helical conformation, whereas neutral or cationic lipid vesicles did not alter the random structure of the peptide, suggesting a selective stabilization of the alpha-helix by anionic membranes. The fluorescence of the single tryptophan residue, which lies on the hydrophobic face of the amphipathic alpha-helix, was studied. Anionic lipid vesicles specifically induced a shift in the fluorescence to a lower wavelength. Fluorescence quenching by the aqueous-phase quencher, I-, and the lipid-phase quencher 9,10-dibromo-PC was used to determine the accessibility of the tryptophan to each of these environments. The presence of anionic lipid vesicles, but not nonanionic lipid vesicles, decreased the quenching by I- suggesting that, in the presence of anionic lipids, the tryptophan residue is poorly accessible to the aqueous I-. Dibromo-PC significantly quenched the fluorescence only when present in anionic vesicles, confirming the membrane location of the tryptophan residue and the lipid specificity of this interaction.(ABSTRACT TRUNCATED AT 250 WORDS)