The structure of augmenter of liver regeneration crystallized in the presence of 50 mM CdCl₂ reveals a novel Cd₂Cl₄O₆ cluster that aids in crystal packing

The crystal structure of the protein augmenter of liver regeneration containing a 14-residue hexahistidine purification tag (hsALR) has been determined to 2.4 Å resolution by Cd-SAD using a highly redundant data set collected on a rotating-anode home X-ray source and processed in 1998. The hsALR crystal structure is a tetramer composed of two homodimers bridged by a novel Cd₂Cl₄O₆ cluster via binding to the side-chain carboxylate groups of two solvent-exposed aspartic acid residues. A comparison with the native sALR tetramer shows that the cluster dramatically changes the hsALR dimer-dimer interface, which can now better accommodate the extra 14 N-terminal residues associated with the purification tag. The refined 2.4 Å resolution structure is in good agreement with both the X-ray data (R_cryst of 0.165, R_free of 0.211) and the expected stereochemistry (r.m.s. deviations from ideality for bond lengths and bond angles of 0.007 Å and 1.15°, respectively).