Purification, crystallization and preliminary X-ray investigation of aqualysin I, a heat-stable serine protease

Aqualysin I, a thermostable protease found in the culture medium of Thermus aquaticus YT-1, has been purified to homogeneity using a combination of ion-exchange and affinity chromatography. It is a polypeptide with a molecular weight of 28 350 [Kwon, Terada, Matsuzawa & Ohta (1988). Eur. J. Biochem. 173, 491–497] and is most active at 343–353 K and pH about 10.0 [Matsuzawa, Tokugawa, Hamaoki, Mizoguchi, Taguchi, Terada, Kwon & Ohta (1988). Eur. J. Biochem. 171, 441–447]. Crystals of the enzyme are monoclinic, space group P2₁, with cell dimensions a = 40.80 (5), b = 64.39 (6), c = 45.51 (6) Å and β = 109.1 (1)°. The asymmetric unit consists of a single molecule (V_m = 1.99 ųDa⁻¹). The crystals are stable to X-radiation and scatter to at least 2.8 Å resolution.