Ca2+-induced distance change between points on actin and troponin in skeletal muscle thin filaments estimated by fluorescence energy transfer spectroscopy.

Fluorescence resonance energy transfer spectroscopy has been used to study the spatial relationships between probes attached to actin and troponin in the reconstituted skeletal muscle thin filament in the presence and absence of Ca2+ ions. Gln-41 and the nucleotide-binding site of actin were selectively labeled with the acceptor probe: fluorescein cadaverine and 2'(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate (TNP-ADP), respectively. Troponin was selectively labeled at positions 9 or 133 of troponin-I and 98 of troponin-C with a donor probe; 5-(2-iodoacetylaminoethyl)aminonaphthalene 1-sulfonic acid (IAEDANS). The distances between probes attached to position 133 of TnI and Gln-41 or the nucleotide site of actin were determined to be 51.6+/-1.2 and 42.7+/-0.9 A respectively in the presence of Ca2+, and these distances decreased by 11.5 and 9.3 A respectively in the absence of Ca2+ ions. The distances between the probes attached to position 9 of TnI and Gln-41 or the nucleotide site of actin were determined to be 59.1+/-2.0 or 49.3+/-1.5 A respectively in the presence of Ca2+, and the distances decreased by 5.3 or 3.7 A in the absence of Ca2+. The distances between probes attached to position 98 of TnC and Gln-41 or the nucleotide site of actin were determined to be 55.1+/-1.7 and 57+/-5 A in the presence of Ca2+ and the distances increased slightly by approximately 1 A in the absence of Ca2+. The results suggest that the C-terminal domain of troponin I moves to the outer domain of actin during inhibition, while the C-terminal domain of TnC does not move much.