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GFP-Tagged Regulatory Light Chain Monitors Single Myosin Lever-Arm Orientation in a Muscle Fiber

Thomas P. Burghardt ^* , Katalin Ajtai ^*, Daniel K. Chan ^*, Miriam F. Halstead ^*, Jinhui Li ^* and Ye Zheng ^*

^* Biochemistry and Molecular Biology, and Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, Minnesota

Correspondence: Address reprint requests to T. P. Burghardt, E-mail: burghardt{at}mayo.edu.

Myosin is the molecular motor in muscle-binding actin and executing a power stroke by rotating its lever arm through an angle of 70° to translate actin against resistive force. A green fluorescent protein (GFP)-tagged human cardiac myosin regulatory light chain (HCRLC) was constructed to study in situ lever arm orientation one molecule at a time by polarized fluorescence emitted from the GFP probe. The recombinant protein physically and functionally replaced the native RLC on myosin lever arms in the thick filaments of permeabilized skeletal muscle fibers. Detecting single molecules in fibers where myosin concentration reaches 300 µM is accomplished using total internal reflection fluorescence microscopy. With total internal reflection fluorescence, evanescent field excitation, supercritical angle fluorescence detection, and CCD detector pixel size limits detection volume to just a few attoliters. Data analysis manages both the perturbing effect of the TIR interface on probe emission and the effect of high numerical aperture collection of light. The natural myosin concentration gradient in a muscle fiber allows observation of fluorescence polarization from C-term GFP-tagged HCRLC exchanged myosin from regions in the thick filament containing low and high myosin concentrations. In rigor, cross-bridges at low concentration at the end of the thick filament maintain GFP dipole moments at two distinct polar angles relative to the fiber symmetry axis. The lower angle, where the dipole is nearly parallel to fiber axis, is more highly populated than the alternative, larger angle. Cross-bridges at higher concentration in the center of the thick filament are oriented in a homogeneous band at 45° to the fiber axis. The data suggests molecular crowding impacts myosin conformation, implying mutual interactions between cross-bridges alter how the muscle generates force. The GFP-tagged RLC is a novel probe to assess single-lever-arm orientation characteristics in situ.