Frequency distribution of space groups in soluble and membrane proteins and their complexes

The space-group frequency distributions for two types of proteins and their complexes are explored. Based on the incremental availability of data in the Protein Data Bank, an analytical assessment shows a preferential distribution of three space groups, i.e. P2₁2₁2₁ > P12₁1 > C121, in soluble and membrane proteins as well as in their complexes. In membrane proteins, the order of the three space groups is P2₁2₁2₁ > C121 > P12₁1. The distribution of these space groups also shows the same pattern whether a protein crystallizes with a monomer or an oligomer in the asymmetric unit. The results also indicate that the sizes of the two entities in the structures of soluble proteins crystallized as complexes do not influence the frequency distribution of space groups. In general, it can be concluded that the space-group frequency distribution is homogenous across different types of proteins and their complexes.