Preliminary X-ray characterization and phasing of a type II cohesin domain from the cellulosome of Acetivibrio cellulolyticus

The N-terminal type II cohesin from the cellulosomal ScaB subunit of Acetivibrio cellulolyticus was crystallized in two different crystal systems: orthorhombic (space group P2₁2₁2₁), with unit-cell parameters a = 37.455, b = 55.780, c = 87.912 Å, and trigonal (space group P3₁21), with unit-cell parameters a = 55.088, b = 55.088, c = 112.553 Å. The two crystals diffracted to 1.2 and 1.9 Å, respectively. A selenomethionine derivative was also crystallized and exhibited trigonal symmetry (space group P3₁21), with unit-cell parameters a = 55.281, b = 55.281, c = 112.449 Å and a diffraction limit of 1.97 Å. Initial phasing of the trigonal crystals was successfully performed by the SIRAS method using Cu Kα radiation with the selenomethionine derivative as a heavy-atom derivative. The structure of the orthorhombic crystal form was solved by molecular replacement using the coordinates of the trigonal form.