FLUORESCENCE RESONANCE ENERGY-TRANSFER BETWEEN POINTS ON TROPOMYOSIN AND ACTIN IN SKELETAL-MUSCLE THIN-FILAMENTS - DOES TROPOMYOSIN MOVE

Fluorescence resonance energy transfer (FRET) spectroscopy has been us ed to determine spatial relationships between residues on tropomyosin and actin in reconstituted muscle thin filament, and to detect a posit ional change of tropomyosin relative to actin on the thin filament in the presence and absence of Ca2+ ions. In addition to Cys-190 which is a single cysteine residue in rabbit skeletal muscle alpha-tropomyosin , a new site, Cys-87 which is a unique cysteine residue in a mutant al pha-tropomyosin, was labeled with a resonance energy donor molecule, 5 -(2-iodoacetylaminoethyl)aminonaphthalene 1-sulfonic acid (IAED-ANS). On the other hand, Gln-41, Lys-61, Cys-374, and the ATP-binding site o f actin were selectively labeled with acceptor probes: fluorescein cad averine, fluorescein 5-isothiocyanate, 3-dimethyl-aminophenylazophenyl 4'-maleimide, and TNP-ATP (or TNP-ADP), respectively. The distances b etween probes attached to position 87 of the mutant tropomyosin and Gl n-41, Lys-61, Cys-374, or the nucleotide-binding site of actin on the reconstituted thin filament in the presence of Ca2+ ion were measured to be 43.2, 49.7, 45.4, and 35.2 Angstrom, respectively, and the dista nce between probes attached to position 190 of tropomyosin and Gln-41 or the nucleotide-binding site of actin were 51.6 and 43.1 Angstrom, r espectively. The transfer efficiencies between these donor and accepto r molecules were large, so that the efficiency should be very sensitiv e to changes in distance between probes attached to tropomyosin and ac tin. However, the transfer efficiency did not change appreciably upon removal of Ca2+ ions, suggesting that tropomyosin does not change its position on the reconstituted thin filament in response to a change in Ca2+ ion concentration. The present results do not support the notion of tropomyosin movement on skeletal muscle thin filaments as proposed in the steric blocking theory.