The flexibility of actin filaments as revealed by fluorescence resonance energy transfer - The influence of divalent cations

The temperature profile of the fluorescence resonance energy transfer effic iency normalized by the fluorescence quantum yield of the donor in the pres ence of acceptor, f, was measured in a way allowing the independent investi gation of (i) the strength of interaction between the adjacent protomers (i ntermonomer flexibility) and (ii) the flexibility of the protein matrix wit hin actin protomers (intramonomer flexibility). In both cases the relative increase as a function of temperature in f is larger in calcium-F-actin tha n in magnesium-F-actin in the range of 5-40 degrees C, which indicates that both the intramonomer and the intermonomer flexibility of the actin filame nts are larger in calcium-F-actin than those in magnesium-F-actin. The inte rmonomer flexibility was proved to be larger than the intramonomer one in b oth the calcium-F-actin and the magnesium-F-actin, The distance between Gln (41) and Cys(374) residues was found to be cation-independent and did not c hange during polymerization at 21 degrees C, The steady-state fluorescence anisotropy data of fluorophores attached to the Gln(41) or Cys(374) residue s suggest that the microenvironments around these regions are more rigid in the magnesium-loaded actin filament than in the calcium-loaded form.