ELECTROSTATICS AND THE MEMBRANE ASSOCIATION OF SRC - THEORY AND EXPERIMENT

The binding of Src to phospholipid membranes requires both hydrophobic insertion of its myristate into the hydrocarbon interior of the membr ane and nonspecific electrostatic interaction of its N-terminal cluste r of basic residues with acidic phospholipids. We provide a theoretica l description of the electrostatic partitioning of Src onto phospholip id membranes. Specifically, we use molecular models to represent a non myristoylated peptide corresponding to residues 2-19 of Src [nonmyr-Sr c(2-19); GSSKSKPKDPSQRRSLE-NH2] and a phospholipid bilayer, calculate the electrostatic interaction by solving the nonlinear Poisson-Boltzma nn equation, and predict the molar partition coefficient using statist ical thermodynamics. The theoretical predictions agree with experiment al data obtained by measuring the partitioning of nonmyr-Src(2-19) ont o phospholipid vesicles: membrane binding increases as the mole percen t of acidic lipid in the vesicles is increased, the ionic strength of the solution is decreased, or the net positive ch;uge of the peptide i s increased. The theoretical model also correctly predicts the measure d partitioning of the myristoylated peptide, myr-Src(2-19); for exampl e, adding 33% acidic lipid to electrically neutral vesicles increases the partitioning of myr-Src(2-19) 100-fold. Phosphorylating either ser ine 12 (by protein kinase C) or serine 17 (by cAMP-dependent protein k inase) decreases the partitioning of myr-Src(2-19) onto vesicles conta ining acidic lipid 10-fold. We investigated the effect of phosphorylat ion on the localization of Src to biological membranes by expressing f usion constructs of Src's N terminus with a soluble carrier protein in COS-1 cells; phosphorylation produces a small shift in the distributi on of the Src chimeras from the plasma membrane to the cytosol.