Crystallization and initial X-ray diffraction of BtuB, the integral membrane cobalamin transporter of Escherichia coli

BtuB, the cobalamin transporter from Escherichia coli, has been overexpressed, purified and crystallized. The purified protein was solubilized in n-octyl tetraoxyethylene (C₈E₄) and was crystallized using sitting-drop vapor diffusion with PEG 3350 and magnesium acetate as precipitants (pH 6.5). Two crystal forms have been obtained. Crystal type I belongs to space group P3₁21, with unit-cell parameters a = b = 81.6, c = 210.0 Å, α = β = 90, γ = 120°. Crystal type II belongs to space group P3₁21, with unit-cell parameters a = b = 81.6, c = 226.0 Å, α = β = 90, γ = 120°. Each crystal form contains a monomer in the asymmetric unit. Diffraction for crystal type I extends to 2.0 Å and diffraction for crystal type II extends to 2.7 Å. Both crystal forms are suitable for structure determination.