Timemresolved fluorescence of S-100a protein: effect of Ca +, Mg 2+ and unilamellar vesicles of egg phosphatidylcholine

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Phase-modulation fluorescence lifetime measurements were used to study the single Trp residue of the Ca 2+-binding protein S-100a both in the absence and in the presence of Ca 2+ and/or Mg 2+. Trp fluorescence decay for the protein was satisfactorily described by Lorentzian lifetime distributions centered around two components (approximately 4 ns and 0.5 ns). Lifetime values were unchanged by 2 mM Ca 2+, but the fractional intensity associated with longer lifetime increased up to 75%. In the presence of Mg 2+, the Ca 2+ induced increase of the fractional intensity associated with longer lifetime was only 57%. For the protein in buffer, about the 85% of the recovered anisotropy was associated to a rotational correlation time of 6.7 ns. After the addition of Ca 2+, this value was increased to 16.08 ns. In the presence of Mg 2+, Ca +2 increased the rotational correlation time to 33.75 ns. Similar studies were performed with S-100a interacting with egg phosphatidylcholine vesicles (SUV). Our data suggest that the conformation of the protein may be influenced by structural features of the lipidic membrane. Moreover, data obtained in the presence of Mg 2+ indicate some interaction between lipids and S-100, likely mediated by this ion.