Three-dimensional structure of the complex of skeletal muscle actin and bovine pancreatic DNAse I at 6-A resolution.

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Abstract
The structure of rabbit skeletal muscle actin complexed with bovine pancreatic DNase I was determined by X-ray crystallographic methods at 6 .ANG. resolution. The analysis was based on a new orthorhombic crystal form, space group P212121, with 1 complex in the asymmetric unit. Six isomorphous heavy-atom derivatives yielding an overall figure of merit of 0.72 were used to calculate the electron-density map. Molecular models for actin and DNase I derived from this map have dimensions 67 .times. 40 .times. 37 .ANG. and 50 .times. 40 .ANG., respectively. The actin molecular is elongated and consists of a larger and a smaller domain, each containing density regions resembling a central .beta.-pleated sheet surrounded by .alpha.-helices. The highest electron-density peak is found in the cleft between the 2 domains, perhaps indicating the bound ATP. Observed crystal contacts between actin molecules and a model for the F-actin filament are discussed. Two high-affinity Ca2+-binding sites which also bind Ba2+ were located at the surface of the DNase I molecule.