The influence of divalent cations on the dynamic properties of actin filaments: A spectroscopic study

The principal aim of this investigation was to study the change of the prot ein flexibility and/or conformational properties of actin filaments upon th e replacement of Ca2+ by Mg2+. The temperature dependence of the fluorescen ce lifetime and the anisotropy decay of N-(iodoacetyl)-N'-(5-sulfo-1-naphth yl)ethylenediamine (IAEDANS) attached covalently to the Cys(374) residue of actin were measured. Saturation transfer electron paramagnetic resonance ( ST-EPR) experiments were also carried out using N-(1-oxyl-2,2,6,6-tetrameth yl-4-piperidinyl)-maleimide (MSL) attached to the same residue (Cys374). Th e Arrhenius analysis of the temperature dependence of the fluorescence life times shows that for Mg-F-actin, both the activation energy (E*) and the fr equency factor (A) are smaller than they are for Ca-F-actin. The longer rot ational correlation times resolved in the fluorescence experiments are larg er in the Mg2+-loaded form of the actin filament between 6 degrees C and 28 degrees C, but this difference becomes negligible above 28 degrees C. The results of saturation transfer electron paramagnetic resonance measurements on maleimide spin-labeled actin filaments indicate that the replacement of Ca2+ by Mg2+ induced a decrease of the mobility of the label on the sub-mi llisecond time scale. Based upon these results, we concluded that the filam ents polymerized from Ca-actin are more flexible than the filaments of Mg-a ctin.