Subunit symmetry of tetrameric phosphorylase a

Native crystallographic data of tetrameric phosphorylase a crystals, space group P21, have been collected photographically to 3 A resolution. These data have been used in Patterson search methods in reciprocal and real space. The tetramers were found to exhibit molecular 222 symmetry. The cross vector between the centres of the two symmetry related tetramers in the unit cell was determined by two different translation function methods. On the basis of these rotation and translation function results a model for the arrangement of monomers within the tetramer and of tetramers in the unit cell is proposed; The 222 symmetry of the tetrameric molecule is found only when high resolution diffraction data are included (i.e. higher than 6 A). At lower resolution other symmetries dominate. Calculations with the proposed model have shown that these spurious symmetries result from the nonspecific overlap of protein-protein and solvent-solvent cross vectors. These results emphasize the importance of high resolution data when noncrystallographic symmetry of globular proteins is studied.