Conformational adaptation of annexin V upon binding to liposomes: a time-resolved fluorescence study

The fluorescence intensity decay of the single tryptophan residue, Trp-187, of free annexin V is described by the sum of three lifetime components (5.4, 1.3, and 0.4 ns), which may be correlated to three ground-state classes of Trp conformers. The two major classes (44 and 48%) are embedded in the protein matrix. When annexin V binds to calcium and liposomes made of dioleoylphosphatidylcholine and dioleoylphosphatidylserine, similar results are obtained whatever the (10-200) lipid ratio. The Trp fluorescence decay is fitted with only two components (6.9-7.2 and 2.0-2.2 ns). Decay-associated spectra reveal that the longest lifetime of bound annexin V can be related to Trp residues (60%) located in a partially polar environment, which could correspond to the protein-membrane interface. The shortest lifetime is attributed to Trp residues (40%) which reside in a hydrophobic surrounding: these Trp residues would penetrate into the phospholipid membrane and contribute to the stabilization of the 2D-array of annexin V molecules.