Characterization of the fluorescein isothiocyanate-reactive site of gizzard myosin ATPase

Abstract

We describe the reaction of fluorescein 5′-isothiocyanate with gizzard myosin, and investigate the effect of this fluorescent modification on ATPase activities. Changes in the ATPase activities upon modification occur rapidly, paralleling the reaction of ‘fast reacting lysine residues’ during the fast phase of the reaction. The loss in the ATPase activity is linearly correlated with the extent of modification. About 90% of the ATPase activity is lost with the incorporation of 2.6 mol of reagent per mol of myosin. The fluorescent label is mainly incorporated into the heavy chain of the myosin molecule. Using limited tryptic digestion of labeled S1, we have shown that the fluorescent dye remains in the 18 kDa fragment. The amino acid composition and the partial sequence of the peptide from the N-terminal end is presented. The results presented here suggest the participation of the 18 kDa peptide in the nucleotide binding domain of gizzard myosin.