Structural differences between C-terminal regions of tropomyosin isoforms

Tropomyosins are actin-binding regulatory proteins which overlap end-to-end along the filament. High resolution structures of the overlap regions were determined for muscle and non-muscle tropomyosins in the absence of actin. Conformations of the junction regions bound to actin are unknown. In this work, orientation of the overlap on actin alone and on actin-myosin complex was evaluated by measuring FRET distances between a donor (AEDANS) attached to tropomyosin and an acceptor (DABMI) bound to actin's Cys374. Donor was attached to the Cys residue introduced by site-directed mutagenesis near the C-terminal half of the overlap. The recombinant alpha-tropomyosin isoforms used in this study - skeletal muscle skTM, non-muscle TM2 and TM5a, and chimeric TM1b9a had various amino acid sequences of the N- and C-termini involved in the end-to-end overlap. The donor-acceptor distances calculated for each isoform varied between 36.4 Å and 48.1 Å. Rigor binding of myosin S1 increased the apparent FRET distances of skTM and TM2, but decreased the distances separating TM5a and TM1b9a from actin. The results show that isoform-specific sequences of the end-to-end overlaps determine orientations and dynamics of tropomyosin isoforms on actin. This can be important for specificity of tropomyosin in the regulation of actin filament diverse functions.