Structure of the Tropomyosin Overlap Complex from Chicken Smooth Muscle: Insight into the Diversity of N-Terminal Recognition

摘要

Tropomyosin is a stereotypical R-helical coiled coil that polymerizes to form a filamentous macro-nmolecular assembly that lies on the surface of F-actin. The interaction between the C-terminal and N-terminalnsegments on adjacent molecules is known as the overlap region.We report here two X-ray structures of the chickennsmoothmuscle tropomyosin overlap complex.Anovel approachwas used to stabilize theC-terminal andN-terminalnfragments. Globular domains from both the human DNA ligase binding protein XRCC4 and bacteriophage φ29nscaffolding protein Gp7 were fused to 37 and 28 C-terminal amino acid residues of tropomyosin, respectively,nwhereas the 29 N-terminal amino acids of tropomyosin were fused to the C-terminal helix bundle of microtubulenbinding protein EB1. The structures of both the XRCC4 and Gp7 fusion proteins complexed with the N-terminalnEB1 fusion contain a very similar helix bundle in the overlap region that encompasses ∼15 residues. The C-terminalncoiled coil opens to allow formation of the helix bundle, which is stabilized by hydrophobic interactions. Thesenstructures are similar to that observed in the NMR structure of the rat skeletal overlap complex [Greenfield, N. J.,net al. (2006) J.Mol. Biol. 364,80-96]. The interactions between theN- and C-terminal coiled coils of smooth musclentropomyosin show significant curvature, which differs somewhat between the two structures and implies flexibility innthe overlap complex, at least in solution. This is likely an important attribute that allows tropomyosin to assemblenaround the actin filaments. These structures provide a molecular explanation for the role of N-acetylation in thenassembly of native tropomyosin.