Fluorescence labeling and computational analysis of the strut of myosin’s 50 kDa cleft

A new fluorescent labeling procedure specific for the strut sequence of myosin subfragment-1’s 50 kDa cleft was developed using CY3 N-hydroxy succinimidyl ester as a hydrophobic tag and hydrophobic interaction chromatography to purify the major labeled species which retained actin-activated ATPase activity. Stern–Volmer analysis suggests that the CY3 is in close proximity to basic residues, consistent with inspection of the mapped labeling site in the atomic model. Fluorescence polarization indicates that the CY3 becomes more mobile upon actin binding, supporting a location near the actomyosin interface. In contrast, nucleotide binding to myosin had little impact on the CY3. Molecular mechanics and stochastic dynamics simulations suggest that this labeling site is sensitive to forced cleft opening and closure, but the upper 50 kDa cleft does not move easily. In addition, there appear to be some long-range effects of forced cleft opening and closing that could impact the lever arm position.