Crystallization and preliminary X-ray studies of a recombinant calcium-binding protein from Entamoeba histolytica.

A calcium-binding protein (CaBP) of Entamoeba histolytica was purified from an E. coli recombinant clone carrying the CaBP gene in a pET-3c expression vector using anion-exchange and size-exclusion chromatography. Examination of the amino-acid sequence of the recombinant protein suggested that it has four independent EF-hand motifs. The protein dissolved in cacodylate buffer was crystallized using the hanging-drop method with 2-methylpentane-2,4-diol (MPD) as the precipitant. X-ray diffraction data have been collected on these crystals using a MAR Research imaging-plate detector system attached to a Rigaku RU200 rotating-anode X-ray generator. The crystals belong to the hexagonal space group P6122 with unit-cell dimensions of a = b = 96.21, c = 65.48 A. Preliminary molecular-replacement computations suggest that the structure of this protein is likely to be similar to that of calmodulin (CAM).