title: Conformation of spin-labeled tropomyosin in reconstituted muscle thin filaments in response to calcium ion and heavy meromyosin.
abstract: Tropomyosin (TM) exists in thermal equilibrium between a highly structured N state, a partially unfolded X state, and a completely unfolded D state, i.e., N in equilibrium X in equilibrium D. The strongly immobilized electron spin resonance (ESR) spectral component of spin-labeled TM corresponds to TM in the N state and the weakly immobilized component to TM in the X state below the main unfolding transition and to TM in the D state above this transition [Graceffa, P., & Lehrer, S. S. (1984) Biochemistry 23, 2606-2612]. The addition of actin, troponin (TN), and heavy meromyosin (HMM) to spin-labeled TM reduces the ratio of weakly to strongly immobilized labels, indicating a shift in the N in equilibrium X in equilibrium D equilibrium toward the N state. At 37 degrees C, for spin-labeled TM alone K (=X/N) greater than 1.0 with some TM in the D state, K = 0.8 for spin-labeled TM bound to actin, and K less than 0.05 for spin-labeled TM bound to actin + TN +/- Ca2+, actin + HMM + TN +/- Ca2+, and actin + HMM. Thus, actin + TN dramatically shifts the TM structure to the N conformation with little further effect upon addition of Ca2+ or HMM. The temperature at which spin-labeled TM begins to dissociate from a protein complex was determined from the temperature dependence of the ESR spectra.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.forename: P
authors.initials: P
authors.lastname: Graceffa
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D003599:Cytoskeleton / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D007700:Kinetics; D009132:Muscles / Q000378:metabolism*; D015879:Myosin Subfragments / Q000502:physiology*; D011487:Protein Conformation; D011817:Rabbits; D013113:Spin Labels; D013816:Thermodynamics; D014335:Tropomyosin / Q000378:metabolism*
chemical_list: D015879:Myosin Subfragments; D013113:Spin Labels; D014335:Tropomyosin; D000251:Adenosine Triphosphatases; D002118:Calcium