Brain spectrin (fodrin) interacts with phospholipids as revealed by intrinsic fluorescence quenching and monolayer experiments

We demonstrate that phospholipid vesicles affect the intrinsic fluorescence of isolated brain spectrin, In the present studies we tested the effects o f vesicles prepared from phosphatidylcholine (PtdCho) alone, in addition to vesicles containing PtdCho mixed with other phospholipids [phosphatidyleth anolamine (PtdEtn) and phosphatidylserine] as well as from total lipid mixt ure extracted from brain membrane. The largest effect was observed with Ptd Etn/PtdCho (3:2 molar ratio) vesicles; the effect was markedly smaller when vesicles were prepared from egg yolk PtdCho alone. Brain spectrin injected into a subphase induced a substantial increase in the surface pressure of monolayers prepared from phospholipids. Results obtained with this techniqu e indicated that the largest effect is again observed with monolayers prepa red from a PtdEtn/PtdCho mixture. The greatest effect was observed when the monolayer contained 50-60 % PtdEtn in a PtdEtn/PtdCho mixture. This intera ction occurred at salt and pH optima close to physiological conditions (0.1 5 M NaCl, pH 7.5). Experiments with isolated spectrin subunits indicated th at the effect of the beta subunit on the monolayer surface pressure resembl ed that measured with the whole molecule. Similarly to erythrocyte spectrin -membrane interactions, brain spectrin interactions with PtdEtn/PtdCho mono layer were competitively inhibited by isolated erythrocyte ankyrin. This al so suggests that the major phospholipid-binding site is located in the beta subunit and indicates the possible physiological significance of this inte raction.