Crystallization and preliminary X-ray investigation of barstar, the intracellular inhibitor of barnase.

Crystals of barstar, the intracellular inhibitor of the extracellular ribonuclease produced by Bacillus amyloliquefaciens (barnase), were obtained through vapor phase equilibration using the hanging drop technique. Three crystal forms have been characterized. Forms I and II, crystallized either in potassium phosphate or sodium citrate, are tetragonal; they exhibit a superstructure along the c-axis. Form III crystals, suitable for a high resolution structure determination, were grown from 55-65% ammonium sulfate. This crystal form is hexagonal and diffracts to at least 2 A resolution at a synchrotron radiation source. It belongs to the hexagonal space group P6, with unit cell dimensions a = b = 143.6 A, c = 35.6 A. There are four molecules of barstar in the asymmetric unit. X-ray data have been collected to 2.2 A Bragg spacing. The structure determination is underway in order to analyze conformational changes of barstar upon complexation with barnase.