Freezing of actin. Reversible oxidation of a sulfhydryl group and structural change

It was found that the essential change in actin (whether G- or F-actin) on freeze-thawing was the specific oxidation of one of five sulfhydryl (SH) groups, i.e. the SH group of Cys 373 in the amino acid sequence. Oxidized SH groups formed an inter-molecular disulfide (SS) bond to yield an actin dimer. F-actin, subjected to freeze-thawing (or F-actin obtained by the transformation of once frozen G-actin which is essentially a dimer), has anomalous physico-chemical properties and a different conformation from normal F-actin, as determined by optical and electron microscopic observations, as well as high steady ATP-splitting activity in the presence of Mg2+. However, it was found that those peculiarities disappeared and normal actin was reformed on reducing the oxidized SH group with dithiothreitol (DTT). It was also found that the normal characteristics of actin were preserved for more than four months on freezing in the presence of a sufficient amount of DTT.