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J Mol Biol. 2005 Jan 14;345(2):363-73.

Assembly of Acanthamoeba myosin-II minifilaments. Model of anti-parallel dimers based on EM and X-ray diffraction of 2D and 3D crystals.

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Structural Biology Laboratory, Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, CA 92037, USA.


Current models suggest that the first step in the assembly of Acanthamoeba myosin-II is anti-parallel dimerization of the coiled-coil tails with an overlap of 15 nm. Sedimentation equilibrium experiments showed that a construct containing the last 15 heptads and the non-helical tailpiece of the myosin-II tail (15T) forms dimers. To examine the structure of the 15T dimer, we grew 3D and 2D crystals suitable for X-ray diffraction and electron image analysis, respectively. For both conditions, crystals formed in related space and plane groups with similar unit cells (a=87.7 A, b=64.8 A, c=114.9 A, beta=108.0 degrees). Inspection of the X-ray diffraction pattern and molecular replacement analysis revealed the orientation of the coiled-coils in the unit cell. A 3D density map at 15A in-plane resolution derived from a tilt series of electron micrographs established the solvent content of the 3D crystals (75%, v/v), placed the coiled-coil molecules at the approximate translation in the unit cell, and revealed the symmetry relationships between molecules. On the basis of the low-resolution 3D structure, biochemical constraints, and X-ray diffraction data, we propose a model for myosin interactions in the anti-parallel dimer of coiled-coils that guide the first step of myosin-II assembly.

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