Packing contacts in orthorhombic and monoclinic crystals of a thermophilic aspartyl-tRNA synthetase favor the hydrophobic regions of the protein

Abstract

The thermostable aspartyl-tRNA synthetase (AspRS-1) from Thermus thermophilus is a 132 kDa homodimer with a subunit composed of 580 amino acids. It catalyses the aminoacylation of tRNA Asp with aspartic acid in the process of translating genetic information. Here we present data on crystals grown in the presence of two different crystallizing agents. A first crystal form (form A) grows in the presence of 0.8 M ammonium sulfate and exhibits the orthorhombic space group P2 12 12 1. Monoclinic plates (form B) grow in an aqueous solution of 6% (m/v) PEG-8000. In this study, the monoclinic crystal structure (form B) was solved by molecular replacement using the orthorhombic crystal structure as a model and refined to a 2.65 Å resolution limit. The contacts between molecules in both crystalline lattices are compared. Although the overall-accessible surface of the protein is more hydrophilic than average, the packing contacts in both lattices comprise mainly hydrophobic van der Waals interactions and only a few salt bridges and hydrogen bonds. Interaction areas are much larger in the orthorhombic than in the monoclinic lattice, and only 6 contact residues out of 134 are common.