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The tropomyosin domain is flexible and disordered in reconstituted thin filaments
1Institute of Molecular Biophysics, Florida State University, Tallahassee 32306-3015.
Abstract:
We have used EPR spectroscopy to study the rotational motion and orientation of tropomyosin labeled with maleimide spin-label, in skeletal muscle fibers. Fibers depleted of intrinsic myosin, troponin, and tropomyosin were reconstituted with labeled tropomyosin. The 3-7 ns mobility of the labeled domains was only slightly (2-fold) inhibited by reconstitution into fibers. No motional changes were observed on addition of troponin, irrespective of the presence of Ca2+; however, the binding of extrinsic myosin heads increased the rate of domain motion to that observed in solution. Orientational studies demonstrate a broad angular distribution of the labeled domain of tropomyosin, with respect to the fiber axis. Troponin reduces the orientational disorder, while the binding of Ca2+ to troponin partially reverses this ordering effect. Myosin S1 has no effect on the orientational distribution of tropomyosin. Overall, the observed changes are very small, implying a loose association of the probed domain of tropomyosin with the thin filament.
Insights
Electron paramagnetic resonance (EPR) spectroscopy reveals tropomyosin
Area of Science:
- Muscle physiology
- Biophysics
- Structural biology
Background:
- Tropomyosin is a key protein in muscle contraction, regulating actin-myosin interactions.
- Understanding tropomyosin's dynamics and orientation is crucial for elucidating muscle function.
- Previous studies have focused on structural aspects, but dynamic information in situ is limited.
Purpose of the Study:
- To investigate the rotational motion and orientation of tropomyosin within skeletal muscle fibers using EPR spectroscopy.
- To determine how troponin and myosin binding affects tropomyosin dynamics and positioning.
- To assess the association strength of tropomyosin with the thin filament.
Main Methods:
- Utilized electron paramagnetic resonance (EPR) spectroscopy on maleimide spin-labeled tropomyosin.
- Reconstituted skeletal muscle fibers depleted of native proteins with labeled tropomyosin.
- Performed motional and orientational analyses of labeled tropomyosin under various conditions (troponin, Ca2+, myosin S1 addition).
Main Results:
- Tropomyosin domain mobility was only slightly reduced upon reconstitution into muscle fibers.
- Troponin binding had no effect on mobility, but myosin S1 increased mobility to solution-like rates.
- Tropomyosin exhibited broad orientational distribution, which troponin reduced, with Ca2+ partially reversing this.
- Myosin S1 did not alter tropomyosin's orientational distribution.
Conclusions:
- The probed tropomyosin domain exhibits dynamic and orientational flexibility within the muscle fiber.
- The association of tropomyosin with the thin filament appears to be relatively loose.
- Myosin binding influences tropomyosin dynamics, suggesting functional interplay during muscle contraction.