Orientation of membrane-bound melittin studied by a combination of HPLC and liquid secondary ion mass spectrometry (LSIMS)

Combining two analytical techniques, HPLC and liquid secondary ion mass spe ctrometry, the orientation of liposomal membrane-hound melittin was analyze d through its trypsin-digested products. We found that trypsin can access a ll proteolytic sites of the membrane-bound melittin when the liposomes have no transmembrane potential, whereas the proteolytic site near the N termin us of melittin is blocked when the liposomes have a negative transmembrane potential. The results suggest that the negative transmembrane potential ma y induce the melittin molecules to insert into the membrane perpendicularly , whereas melittin lies flat on the membrane surface in the absence of a ne gative potential.