Evidence for an altered structure of actin-S1 complexes when Mg-adenylylimidodiphosphate binds

Simultaneous measurements of actin-S1 binding and light scatter (or turbidity) were made in the presence and absence of saturating Mg-adenylylimidodiphosphate (Mg.AMP.PNP). It was found that if the actin was covalently labelled on cysteine 363 by dansyl aziridine (DAZ) the ternary complex DAZactin-S1-Mg.AMP.PNP scattered 36% less light than the binary complex DAZactin-S1. If the actin was not labelled there was no difference in scatter. The decrease in scatter of DAZactin-S1 was dependent on Mg.AMP.PNP concentration in a hyperbolic manner with half maximal decrease at 40 micrometers. No change in light scatter by DAZactin-S1 was found when H.AMP.PNP or Mg.ADP bound. The change in light scatter is evidence that the structure of DAZactin-S1 is altered when Mg.AMP.PNP binds. The possible nature of the structural change and its role in chemo-mechanical transduction is discussed.