Purification, crystallization, and preliminary X-ray diffraction analysis of the tricorn protease hexamer from Thermoplasma acidophilum

Tricorn protease from Thermoplasma acidophilum is a hexameric enzyme; in vi vo the hexamers assemble further to form large icosahedral capsids of 14.6 MDa. Recombinant Tricorn protease was purified as an enzymatically active h examer of 0.72 MDa that formed crystals of octahedral morphology under low- ionic-strength conditions. These crystals belong to space group C2 with uni t cell dimensions a = 307.5. Angstrom, b = 163.2 Angstrom, c = 220.9 Angstr om, beta = 105.5 degrees and diffract to 2.2-Angstrom resolution using high -brilliance synchrotron radiation. Based on analysis of the self-rotation f unction and the presence of a pseudo-origin peak in the native Patterson ma p, a packing model was derived for the complex, comprising 1.5 hexamers per asymmetric unit with a solvent content of 43%. Due to the ninefold noncrys tallographic symmetry the Tricorn crystals represent an interesting case fo r phasing X-ray crystallographic data by electron microscopic phase informa tion. (C) 2001 Academic Press.