Thermal stability of F-actin as studied by spin labelling

An analysis of the EPR spectra of maleimide spin-labelled actin was undertaken. We estimated a rotational correlation time of (13 +/- 2) nsec for the five-membered maleimide spin label bound to G-actin. The polarity of the environment of the bound labels indicated a strong polar character. The temperature dependence of the EPR spectral parameters of the attached label for F-actin exhibited rapid changes between 60-70 degrees C, which might be due to changes of protein structure. The conformational changes were reversible below 65 degrees C. The spin label spectra showed that the polymerization and depolymerization could be accomplished on actin thermally treated in F-form for 10 minutes at a temperature not higher than 60 degrees C. The findings suggest thermal stability of the spin-labelled sites in F-actin below 65 degrees C.