Detection of protein conformation under stress conditions using liposomes as sensor materials

Calcein release from liposomes was analyzed kinetically in the presence of various proteins under the stress condition. In the case of bovine carbonic anhydrase (CAB), a significant release of calcein was observed at pH 4. The conformational change of CAB from the native to the molten-globule (MG) state was confirmed using an aqueous two-phase partitioning method and immobilized liposome chromatography. The CAB-liposome interaction was maximum at a specific pH (4.0), where the CAB conformation was an MG-like state. The results may show that the hydrophobic interaction between CAB and liposomes enhances the perturbation of the liposome membrane, leading to a significant release of calcein from liposomes. These phenomena depended not only on the characteristics of proteins (local hydrophobicity, LH_pr) but also on the dynamic properties of liposomes (membrane fluidity). These results obtained with CAB may be extended to other proteins. The manner of protein-induced calcein release was classified into at least two types. Calcein release due to the addition of proteins having either disulfide bonds or being rich in beta-sheet structure was not observed. On the contrary, a significant release of calcein was observed in the case of a reduced protein with a cleaved disulfide bond and in the case of a protein with relatively a low content of beta-sheet structure. These findings suggest that a liposome containing calcein can be used as an effective sensor element for the detection of proteins having large structural fluctuations.