AMINO-TERMINAL BASIC RESIDUES OF SRC MEDIATE MEMBRANE-BINDING THROUGHELECTROSTATIC INTERACTION WITH ACIDIC PHOSPHOLIPIDS

Membrane targeting of pp60(src) (Src) is mediated by its myristoylated amino terminus. We demonstrate that, in addition to myristate, six ba sic residues in the amino terminus are essential for high-affinity bin ding to the lipid bilayer via electrostatic interaction with acidic ph ospholipids. Specifically, c-Src was shown to bind 2500-fold more stro ngly to vesicles composed of the physiological ratio of 2:1 phosphatid ylcholine (PC)/phosphatidylserine (PS) than to neutral PC bilayer vesi cles. The apparent K-d for binding of c-Src to the PC/PS bilayer was 6 X 10(-7) M. This interaction is sufficiently strong to account for c- Src membrane targeting. Mutants of c-Src in which the amino-terminal b asic residues were replaced by neutral asparagine residues exhibited b inding isotherms approaching that of wild-type binding to neutral bila yers (apparent K-d of 2 X 10(-3) M). The transforming v-Src and activa ted c-Src (Y527F) proteins also bound more strongly to PC/PS bilayers (apparent K-d of approximate to 1 X 10(-5) M) than to neutral PC bilay ers. In vivo experiments with Src mutants confirmed the role of positi ve charge in mediating membrane binding and cellular transformation.