Conformational change of skeletal muscle alpha-actinin induced by salt

We examined the effect of KCl concentration on conformation of skeletal muscle alpha-actinin. One-dimensional peptide maps of alpha-actinin digested with chymotrypsin indicated that alpha-actinin can take on at least three different conformations depending on the KCl concentration of the solvent, i.e., at low (0-0.02 M KCl), intermediate (0.05-0.2 M KCl), and high (0.3-0.6 M KCl) salt concentration. Viscosity measurement and gel-filtration chromatography of alpha-actinin at these three salt ranges indicated that the axial ratio of alpha-actinin increased as the ionic strength of the solvent decreased. By assuming 45% hydration of the alpha-actinin molecule and using a molecular weight of 210,000, dimensions of alpha-actinin were calculated from viscosity data. The size estimated under the low-salt conditions was 3.2 x 74.2 nm. They were 3.4 x 51.3 nm and 4.5 x 40.1 nm, respectively, in the intermediate and high salt ranges. The result of the gel-filtration chromatography showed that the conformational change was reversible and that the change took place through the elongation and/or shortening of the rod domain of the molecule. We explain the salt-induced length change of alpha-actinin by the twisted-coiling model proposed by McGough and Josephs for erythrocyte spectrin (McGough, A. and Josephs, R. (1990) Proc. Natl. Acad. Sci. USA 87, 5208-5212). Pelleting experiments indicated that the conformational change affected the binding ratio between alpha-actinin and actin.