Resonance energy transfer between points in a reconstituted skeletal muscle thin filament. A conformational change of the thin filament in response to a change in Ca2+ concentration.

The spatial relationships between Lys-61, Cys-374 on actin or SH1 on myosin subfragment-1 (S1) and Cys-190 on tropomyosin or Cys-133 on troponin-I (TnI) in a reconstituted thin filament were studied by fluorescence resonance energy transfer. 5-(2-Iodoacetylaminoethyl)aminonaphthalene 1-sulfonic acid (IAEDANS) attached to Lys-190 on tropomyosin or to Cys-133 on TnI was used as a donor. Fluorescein 5-isothiocyanate (FITC) attached to Lys-61 or 5-(iodoacetoamido)fluorescein (IAF) attached to Cys-374 on actin and 4-dimethylaminophenyl-azophenyl 4'-maleimide (DABMI) attached to SH1 on S1 were used as an acceptor. The transfer efficiency between AEDANS attached to Cys-190 on tropomyosin and FITC attached to Lys-61 on actin was 0.42 in the absence of troponin, 0.46 in the presence of troponin and Ca2+ and 0.55 in the presence of troponin and absence of Ca2+. The corresponding distances between the probes were calculated to be 4.7 nm, 4.6 nm and 4.3 nm respectively, assuming a random orientation factor K2 = 2/3. A large difference in the transfer efficiency from AEDANS attached to Cys-133 on TnI to FITC attached to Lys-61 on actin was observed between in the presence (0.52) and absence (0.70) of Ca2+. The corresponding distances between the probes were calculated to be 4.5 nm in the presence of Ca2+ and 3.9 nm in the absence of Ca2+. The distance between Cys-190 on tropomyosin and Cys-374 on actin was measured to be 5.1 nm and the transfer efficiency (0.35) did not change upon addition of troponin whether Ca2+ is present or not, in agreement with the previous report [Tao, T., Lamkin, M. & Lehrer, S. S. (1983) Biochemistry 22, 3059-3064]. The distance between Cys-133 on TnI and Cys-374 on actin was measured to be 4.4 nm. No detectable change in transfer efficiency (0.58) was observed between values in the presence and absence of Ca2+. These results suggest that a relative movement of the two domains of actin monomer in a reconstituted thin filament occurs in response to a change in Ca2+ concentration. The transfer efficiencies between DABMI attached to SH1 on S1 and AEDANS attached to Cys-190 on tropomyosin or Cys-133 on TnI were too small (less than 2%) for an accurate estimation of the distances, suggesting the distances are longer than 7.3 nm.