Cation effects on the conformations of muscle and non-muscle alpha-actinins

We examined the effects of changing KCl concentration on the secondary structures of alpha-actinins using circular dichroism (CD), 1,1'-bis(4-anilino) naphthalene-5,5'-disulfonic acid (bisANS) fluorescence and proteolysis experiments. Under near-physiological conditions, divalent cations also were added and changes in conformation were investigated. In 25 mM KH2PO4, pH 7.5, increasing KCl from 0 to 120 mM led to decreases in alpha-helix conformation for brain, platelet and heart alpha-actinins (40.5-30.2%, 65.5-37.8% and 37.5-27.8%, respectively). In buffered 120 mM KCl, 0.65 mM calcium produced small changes in the CD spectra of both brain and platelet alpha-actinin, but had no effect on heart alpha-actinin. bisANS fluorescence of all three alpha-actinins also showed significant changes in conformation with increasing KCl. However, in buffered 120 mM KCl increasing concentrations of Ca2+ or Mg2+ did not have significant effects on the bisANS fluorescence of any alpha-actinin. Digestion of brain, platelet and heart alpha-actinins with alpha-chymotrypsin showed an increase of proteolytic susceptibility in 120 mM KCl. These experiments also showed that increasing the concentration of Ca2+ or Mg2+ led to greater changes in digestion fragment patterns in the absence of KCl than in the presence of 120 mM KCl. The results suggest that alpha-actinins exist in different conformations depending on the ionic strength of the medium, which could explain the differences in calcium and F-actin binding results obtained from different alpha-actinins.