Interaction of erythrocyte protein 4.1 with phospholipids. A monolayer and liposome study

We have studied the interaction of purified human erythrocyte protein 4.1 with phospholipid membranes by monitoring both the increase in surface pressure of monolayers at the air/water interface and the change in permeability in liposomes to fluorescent molecules, in the presence of protein 4.1. Protein 4.1 penetrated into monolayers of brain phosphatidylserine (PS) and egg phosphatidylcholine (PC), even above surface pressures of 30 mN/m. Protein 4.1 increased the permeability of negatively charged PS, but not PC, liposomes, measured as the increase in fluorescence when encapsulated 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and p-xylenebispyridinium bromide (DPX) or carboxyfluorescein were released into the medium. The interaction of protein 4.1 with PS large unilamellar vesicles (LUV) was increased as the pH and the ionic strength were lowered, and decreased as the Ca2+ or Mg2+ concentrations and ionic strength were raised. In order to study the relevance of these measurements to the erythrocyte, we prepared LUV of synthetic lipid mixtures characteristic of both the inner and the outer membrane leaflets. Protein 4.1 increased the permeability of inner, but not outer, leaflet LUV at both pH 6.0 and 7.4. These observations suggest that negatively charged phospholipid domains around the protein 4.1 high-affinity protein-binding site(s) may contribute to the anchoring of protein 4.1 to the cytoplasmic surface of the red cell membrane.