Overexpression, purification and crystallization of a choline-binding protein CbpI from Streptococcus pneumoniae

The choline-binding protein CbpI from Streptococcus pneumoniae is a 23.4 kDa protein with no known function. The protein has been successfully purified initially using Ni-NTA chromatography and to homogeneity using Q-Sepharose ion-exchange resin as an affinity column. CbpI was crystallized using PEG 3350 as a precipitant and X-ray crystallographic analysis showed that the crystals belonged to the tetragonal space group P4, with unit-cell parameters a = b = 83.31, c = 80.29 angstroms, alpha = beta = gamma = 90 degrees. The crystal contains two molecules in the asymmetric unit with a solvent content of 55.7% (V(M) = 2.77 angstroms3 Da(-1)) and shows a diffraction limit of 3.5 angstroms.