DOES THE BINDING OF CLUSTERS OF BASIC RESIDUES TO ACIDIC LIPIDS INDUCE DOMAIN FORMATION IN MEMBRANES

Several proteins that are important components of the calcium/phosphol ipid second messenger system (e.g. phospholipase C, protein kinase C, myristoylated alanine-rich C kinase substrate (MARCKS) and pp60(src)) contain clusters of basic residues that can interact with acidic lipid s on the cytoplasmic surface of plasma membranes. We have studied the membrane binding of MARCKS and pp60(src), peptides that mimic the basi c regions of these proteins, and simple model peptides. Specifically, we determined how the binding of these model peptides depends on (1) t he number of basic residues in the peptide (2) the fraction of acidic lipids in the membrane (3) the ionic strength of the solution (4) the chemical nature of the basic residues (Arg versus Lys) and the acidic phospholipids [phosphatidylglycerol (PG) versus phosphatidylserine (PS )] (5) the pressure and (6) the temperature. The results are consisten t with a simple theoretical model: each basic residue in a peptide bin ds independently to an acidic lipid with an intrinsic microscopic asso ciation constant of 1-10 M(-1) (binding energy similar or equal to 1 k cal/mol). The binding is described with a mass action formalism and th e non-specific electrostatic accumulation of the peptides in the aqueo us diffuse double layer is described with the Gouy-Chapman theory. Thi s Gouy-Chapman/mass action model accounts surprisingly well for the si gmoidal dependence of binding on the percentage of acidic lipids in th e membrane (apparent co-operativity or Hill coefficient >1); the model assumes that the multivalent basic peptides bind >1 acidic lipids and thus induce or stabilize domain formation.