Time-resolved fluorescence of S-100a protein in the absence and presence of calcium and phospholipids

Abstract

We have used phase-modulation fluorescence lifetime measurements to study the single Trp residue of the Ca 2+-binding protein S-100a. Trp fluorescence decay was not exponential for the protein irrespective of the absence or presence of Ca 2+. Fluorescence decay was best described by Lorentzian lifetime distributions centered around two components (approx. 3 and 0.7 ns) for protein in absence of Ca 2+ and one component (approx. 2.9 ns) for the protein in presence of 2 mM Ca 2+. Similar studies were performed with S-100a interacting with cardiolipin, phosphatidylserine or egg phosphatidylcholine, both in absence and in presence of 2 mM Ca 2+. Out data suggest that the conformation of the protein and its Ca 2+-binding properties vary depending on the characteristics of charge and structure of phospholipids.