BINDING OF ACYLATED PEPTIDES AND FATTY-ACIDS TO PHOSPHOLIPID-VESICLES- PERTINENCE TO MYRISTOYLATED PROTEINS

We studied the binding of fatty acids and acylated peptides to phospho lipid vesicles by making electrophoretic mobility and equilibrium dial ysis measurements. The binding energies of the anionic form of the fat ty acids and the corresponding acylated glycines were identical; the e nergies increased by 0.8 kcal/mol per number of carbons in the acyl ch ain (N(carbon) = 10, 12, 14, 16), a value identical to that for the cl assical entropy-driven hydrophobic effect discussed by Tanford [The Hy drophobic Effect (1980) Wiley, New York]. The unitary Gibbs free bindi ng energy, DELTAG(u)o, of myristoylated glycine, 8 kcal/mol, is indepe ndent of the nature of the electrically neutral lipids used to form th e vesicles. Similar binding energies were obtained with other myristoy lated peptides (e.g., Gly-Ala, Gly-Ala-Ala). The 8 kcal/mol, which cor responds to an effective dissociation constant of 10(-4) M for myristo ylated peptides with lipids, provides barely enough energy to attach a myristoylated protein in the cytoplasm to the plasma membrane. Thus, other factors that reduce (e.g., hydrophobic interaction of myristate with the covalently attached protein) or enhance (e.g., electrostatic interactions of basic residues with acidic lipids; protein-protein int eractions with intrinsic receptor proteins) the interaction of myristo ylated proteins with membranes are likely to be important and may caus e reversible translocation of these proteins to the membrane. Finally, our results suggest that the mass-dependent entropy price paid by a m olecule when it binds to a membrane and loses one translational and tw o rotational degrees of freedom is small: the membrane binding energy we measure for the neutral form of myristic acid, 12 kcal/mol, agrees with the value predicted from Tanford's measurements of the partitioni ng of the neutral form of fatty acids between water and a bulk organic phase (14 x 0.825 = 12 kcal/mol).