Tropomyosin is a long protein (~40 nm) with two [alpha]-helical polypeptide chains that form a coiled coil in a parallel orientation by winding around each other in a left-handed
superhelix. Tropomyosin molecules lie along the actin helix and successive molecules interact in a head-to-tail manner, and the junction is formed between the COOH-terminus of one tropomyosin and the NH-terminus of the next.
Three of these helical strands then twist around on one another, like the strands of a rope, in a right-handed
superhelix, to make up the complete molecule.
Domain III has a sandwich topology and domain IV is a
superhelix. The APN have conserved structures, with slightly mutations just like result of coevolution with Cry proteins.