Crystalline sheets of tropomyosin.

In the presence of spermine tropomyosin forms sheets having two-dimensional crystallinity and tactoids. The most common form of sheet has cmm symmetry with a = 80 nm and b = 5 nm. The structure of this sheet has been solved in projection to a nominal resolution of 1.5 nm by combining data from electron diffraction and electron microscopy. Analysis of this pattern and that of rarely observed sheets having p2 symmetry (a = 40 nm, b = 5 nm and gamma = 80 degrees) indicated that the cmm structure was formed by superposition of two p2 sheets. The tropomyosin molecules in each p2 sheet were arranged in rows directed along the p2 (0,1) lattice lines, with all the molecules in one row having the same polarity and lying antiparallel to the molecules in adjacent rows. These rows associated in pairs, possibly by the supercoiling of the molecules in one row about those in the neighbouring row.