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Crystallization and preliminary X-ray crystallographic studies of the 13-fold symmetric portal protein of bacteriophage SPP1.

Portal proteins are cyclical oligomers which play essential roles in bacteriophage pro-capsid formation, DNA packaging, and in connector formation. Bacteriophage SPP1 portal protein (gp6) is a turbine-like molecule with 13-fold symmetry [Dube et al. (1993) EMBO J. 12, 1303-1309]. The purified protein was crystallized with polyethylene glycol 400 as the precipitating agent using the vapor-diffusion method. Salt conditions were selected based on the properties of gp6 in different ionic environments. X-ray diffraction data up to a resolution of 7.85 A were measured from frozen crystals with orthorhombic space group C2221 and cell dimensions a = 180.5 (5), b = 223.5 (5), c = 417 (1) A. The asymmetric unit contains one tridecameric portal protein with 57.3 kDa subunits. The self-rotation searches confirm the 13-fold symmetry of the crystallized protein.