An analysis of molecular interactions involved in the assembly of tropomyosin tactoids.

Arrangements of parallel and antiparallel tropomyosin molecules are examined for residue interactions which might explain the paracrystal forms observed by electron microscopy. Molecular arrays which produce interaction maxima and corresponding computer graphics-simulated staining patterns are compared with observd electron micrographs. The best correlation of interactions with staining pattern occurs when cationic bridging of acid residues is maximized and supported by favourable ion pair interactions. In the antiparallel case, two main maxima occur and appear to correspond to divalent ion tactoids which have been previously reported. For the Cohen-Longley Mg2+ tactoid the best fit is obtained with a molecular overlap of 201 residues and an end overlap of 17-18 residues (based on a rational 287 peptide sequence). Secondary maxima correspond to other known tactoid forms. Binding of tropomyosin molecules to actin may involve hydrogen bonding to six serine residues which occur at approx. 40-residue intervals.