Fluorescence study of the effect of cholesterol on spectrin-aminophospholipid interactions

The ability of the membrane skeletal protein spectrin to interact with phospholipids, and aminophospholipids in particular, in both natural and model membranes, is well documented. The present study involves phospholipid-induced quenching of tryptophan fluorescence to probe spectrin-membrane interactions in the presence and absence of cholesterol. We performed the experiments on small unilamellar vesicles of phospholipids made of DMPC and DMPC/DMPE and of DOPC and DOPC/DOPE with and without cholesterol at two different temperatures, one below at 15 A degrees C and another above, at 50 A degrees C, the main phase transition temperature (T (m)) of the bulk phospholipid. Results indicate that erythroid and brain spectrin binds DMPC/DMPE membranes by tenfold and 40-fold stronger, respectively, in the presence of 20 % cholesterol, up to which both gel (L-beta) and liquid crystalline (L-alpha) phases coexists, at 15 A degrees C particularly in DMPC-based membranes containing saturated fatty acyl chains and not in DOPC-based membranes with appreciably lower T (m). Time-resolved fluorescence and circular dichroism spectroscopic studies indicated no significant change in the mean lifetime of the tryptophan residues in spectrin and in the secondary structures of the proteins upon binding to the phospholipid SUVs.