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Myosin-Induced Movement of , ß, and ßß Smooth Muscle Tropomyosin on Actin Observed by Multisite FRET

Muscle and Motility Group, Boston Biomedical Research Institute, Watertown, Massachusetts 02472

Correspondence: Address reprint requests to Corrado Bacchiocchi, E-mail: bacchio{at}bbri.org.

The interaction of the , ßß, and ß smooth muscle tropomyosin (Tm) isoforms with F-actin was systematically studied in the absence and in the presence of myosin subfragment 1 (S1) using multifrequency phase/modulation Förster resonance energy transfer (FRET). A Gaussian double distance distribution model was adopted to fit FRET data between a 5-(2-iodoacetyl-amino-ethyl-amino)naphthalene-1-sulfonic acid donor at either Cys-36 of the ß-chain or Cys-190 of the -chain and a 4-dimethylaminophenylazophenyl 4'-maleimide acceptor at Cys-374 of F-actin. Experimental data were obtained for singly and doubly labeled ß Tm (donor only at , only at ß, or both) and for doubly labeled or ßß Tm. Data for singly labeled ßTm were combined in a global analysis with doubly labeled ßTm. In all doubly labeled isoforms, upon S1 binding, one donor-acceptor "apparent" distance increased slightly by 0.5–2 Å, whereas the other decreased by 6–9 Å. These changes are consistent with a uniform "rolling" motion of Tm over the F-actin surface. The analysis indicates that Tm occupies relatively well-defined positions, with some flexibility, in both the predominantly closed (-S1) and open (+S1) thin-filament states. The results for the ßTm heterodimer indicate that the local twofold symmetry of or ßß Tm is effectively broken in ßTm bound to F-actin, which implies a difference between the - and ß-chains in terms of their interaction with F-actin.

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