Binding of polylysine to charged bilayer membranes: molecular organization of a lipid.peptide complex

The interaction between a positively charged peptide (poly-L-lysine) and model membranes containing charged lipids has been investigated. Conformational changes of the polypeptide as well as changes in the membrane lipid distribution were observed upon lipid-protein agglutination: 1. The strong binding of polylysine is shown directly by the use of spinlabelled polypeptide. Upon binding to phosphatidic acid a shift in the hyperfine coupling constant from 16.5 to 14.6 Oe is observed. The spectrum of the lipid-bound peptide is superimposed on the spectrum of polylysine in solution. Half of the lysine groups are bound to the charged membranes. A change in the conformation of polylysine from a random coil to a partially ordered configuration is suggested. 2. Spin labelling of the lipid component gives evidence concerning the molecular organization of a lipid mixture containing charged phosphatitid acid. Addition of polylysine induces the formation of crystalline patches of bound phosphatidic acid. 3. Excimer forming pyrene decanoic acid has been employed. Addition of positively charged polylysine (pH 9.0) to phosphatidic acid membranes increases the transition temperature of the lipid from Tt = 50 to Tt = 62 degrees C. Thus, a lipid segregation of lipid into regions of phosphatidic acid bound to the peptide which differ in their microviscosity from the surrounding membrane is induced. One lysine group binds one phosphatidic acid molecule, but only half of the phosphatidic acid is bound. 4. Direct evidence for charge induced domain formation in lipid mixtures containing phosphatidic acid is given by electron microscopy. Addition of polylysine leads to a change in the surface curvature of the bound charged lipid. The domain size is estimated from the electron micrographs. The number of domains present is dependent on both the ratio of charged to uncharged lipids as well as on the amount of polylysine added to the vesicles. The size of the domains is not dependent on membrane composition. However, the size seems to increase in a stepwise manner that is correlated with a multiple of the area covered by one polylysine molecule.