The shapes and sizes of two domains of tropomodulin, the P-end-capping protein of actin-tropomyosin.

Tropomodulin, the P-end (slow-growing end)-capping protein of the actin-tropomyosin filament, and its fragment (C20) of the C-terminal half were studied by synchrotron small-angle X-ray scattering, restoring low-resolution shapes using an ab initio shape-determining procedure. Tropomodulin is elongated (115 A long) and consists of two domains, one of 65 A in length and the other being similar to C20 in shape and size if the long axes of the two are tilted by about 40 degrees relative to each other. We propose a model for tropomodulin in association with tropomyosin and actin: the N-terminal half of tropomodulin, a rod, binds to the N-terminus of tropomyosin and the C-terminal triangle domain protrudes from the P-end being slightly bent towards the actin subunit at the end, thereby blocking the P-end.