THERMAL UNFOLDING OF THE CYTOTOXIN ALPHA-SARCIN - PHOSPHOLIPID-BINDING INDUCES DESTABILIZATION OF THE PROTEIN-STRUCTURE

The effect of membrane binding on the structure and stability of the cytotoxin alpha-sarcin has been studied by differential scanning calorimetry, Fourier-transform infrared and fluorescence spectroscopic techniques. The thermal unfolding of alpha-sarcin in aqueous solution fits into a two-state transition characterized by a transition temperature (T-m) of 52.6 degrees C and a calorimetric enthalpy (Delta H-cal of 136 kcal/mol. Upon interaction with phosphatidylglycerol vesicles, alpha-sarcin undergoes conformational changes, as deduced from the FTIR and fluorescence emission spectra. These changes result in a decreased T-m and Delta H-cal values for the thermal unfolding of phospholipid-bound alpha-sarcin. The lower T-m value for lipid-bound alpha-sarcin is also observed at the level of secondary and tertiary structures, based on analyses of both the amide I' infrared spectrum and the tryptophan emission of the protein as a function of temperature, respectively. The results obtained indicate a protein destabilization promoted by the phospholipid interaction.