Synergistic effects of a membrane protein (spectrin) and Ca 2+ on the Na + permeability of phospholipid vesicles

Abstract

1. 1.|Spectrin increases the surface pressure of phosphatidylserine and phosphatidylcholine monolayers when injected below the surface, to a greater extent at acid pH than at neutral pH. This effect is largest for phosphatidylserine monolayers, when increases in surface pressure are seen even at high initial film pressures (30–45 dynes/cm). Albumin shows similar effects. 2. 2.|Sonicated phosphatidylserine vesicles show very low 22Na + diffusion rates of 0.05 %/h, equivalent to 9·10 −13 cm/sec. The addition of spectrin and albumin increases this rate several orders of magnitude, to 20 and 75 %/h, respectively, but only at acid pH values (3.5–4.5). Spectrin also increases the diffusion rate of phosphatidylcholine vesicles 20-fold at pH 3.5, but has no effect at pH 7.4. 3. 3.|Ca 2+ and protein exhibit a synergistic effect on the permeability of phosphatidylserine and mixed phosphatidylserine/phosphatidylcholine vesicles. In contrast to protein alone, the effect of calcium plus protein is seen at both neutral and acid pH. Ca 2+ does not increase the effect of spectrin on phosphatidylcholine vesicles nor does it alter the penetration of spectrin into phosphatidylserine monolayers. Neither spectrin nor spectrin plus calcium have any effect on positively charged stearyl amine/phosphatidylcholine vesicles at pH 7.4. 4. 4.|The results are interpreted as indicating that both electrostatic attractions between protein and lipid, and conformational changes in the protein mediate penetration of the protein into the phospholipid bilayer, resulting in the large permeability increases observed. The role of Ca 2+ in the interactions is discussed. It is suggested that the binding of protein to phospholipid vesicles could produce areas of high negative charge density. Ca 2+ binding would then be enhanced leading to perturbations of the bilayer structure and to large increases in permeability.