Fluorescence of fluorescein attached to myosin SH1 distinguishes the rigor state from the actin-myosin-nucleotide state

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Abstract
The fluorescence intensity of 5-(iodoacetamido)fluorescein (5-IAF) attached to the SH1 of [rabbit] myosin subfragment 1 (S-1) increases 3-fold on formation of the rigor complex. On adding Mg2+-ADP, light scattering indicates no dissociation, but the fluorescence increment disappears. This fluorescence signal can distinguish the rigor state from other states, especially from ternary complexes such as actin-myosin-nucleotide. By using this signal one can measure spectroscopically several kinetic paramters of acto-S-1-nucleotide interaction: In the presence of 20 mM KCl, 2 mM MgSO4, and 10 mM TES [trismethylaminoethane sulfonic acid] (pH 7.5) at 22.degree. C, Mg2+-ADP binds to acto-S-15* (S-15* denotes 5-IAF-labeled S-1) with a Ka = 2 .times. 106 M-1 and Mg2+-PPi binds to acto-S-15* with 2 apparent affinities, Ka = 8 .times. 104 M-1 and 1.4 .times. 103 M-1; the association rates of Mg2+-ADP and Mg2+-ATP for acto-S-15* are 107 s-1 M-1 and 4 .times. 106 s-1 M-1, respectively, and the dissociation rate of Mg2+-ADP for acto-S-15* is 5 s-1. In contrast to the fluorescence intensity of the dye, the lifetime and the absorbance are essentially unaffected by complex formation with F-actin or nucleotides. There must be a static quencher such as Trp, Tyr or Met in the neighborhood of the attached dye and that the contact between dye and quencher is modulated by actin-induced or nucleotide-induced conformational changes in S-1.
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