Perfect hemihedral twinning in crystals of the γ-subunit of translation initiation factor 2 from Sulfolobus solfataricus: Cause and effect

The crystal structure of the gamma-subunit of translation initiation factor 2 from the archaeon Sulfolobus solfataricus (SsoIF2 gamma) has been solved based on perfectly hemihedral twinned data. The protein was cocrystallized with the 10-fold molar excess of GTP analog (GDPCP) over protein. However, no nucleotide was found in the structure, and the model demonstrated the apo form of the protein. Two slightly different molecules in the asymmetric unit of the crystal are related by the non-crystallographic 2-fold axis and form a tightly associated dimer. This dimer is stabilized by an intermolecular hydrophobic core and hydrogen bonds. Lack of GDPCP in the nucleotide-binding pocket of the gamma-subunit and significant excess of dimers over monomers in the crystallization solution suggest that these dimers are the building blocks of the crystal. Contrary to SsoIF2 gamma monomers, these dimers are able to crystallize in two oppositely oriented slightly different crystal domains, thus forming a twinned crystal. Comparison of crystallization conditions for the twinned and untwinned crystals of apo SsoIF2 gamma showed that stabilization of the dimers in the solution may be caused by higher sodium salt concentration. Since amino acid residues involved in intermolecular contacts in the dimer are responsible for binding of the gamma and alpha-subunits within SsoIF2, increase in sodium salt concentration may prevent functioning of SsoIF2 in the cell.