Alignment of caldesmon on the actin-tropomyosin filaments

We have reported previously tat each smooth-muscle caldesmon binds predominately to a region within residues 142-227 of tropomyosin, but a weaker binding site also exists at the N-terminal region of tropomyosin. In view of recent evidence for the presence of tropomyosin-binding sites at both the N- and C-terminal domains of caldesmon, we have studied the binding of the N- and C-terminal fragments of human fibroblast caldesmon expressed in Escherichia coli to tropomyosin and its CNBr fragments. The N-terminal fragment, CaD40 (residues 1-152), binds tropomoysin, but the interaction is mostly abolished in the presence of actin. CaD40 binds strongly to Cn1B(142-281) of tropomoysin, but weakly to Cn1A(11-127). The C-terminal fragment, CaD39, which corresponds to residues 443-736 of gizzard caldesmon, binds tropomoysin, and the interaction is enhanced by actin. CaD39 binds to both Cn1A(11-127) and Cn1B(142-281) of tropomyosin Our results suggest that the N-terminal domain of caldesmon interacts with the C-terminal half of one tropomyosin molecule, whereas the C-terminal domain binds to both N- and C-terminal regions of the adjacent tropomyosin molecule along the actin filament. In addition, the binding of the N- terminal domain of caldesmon to the actin-tropomyosin filament is weak, which may allow this domain to project off the thin filament to interact with myosin.