Introduction of a leucine half-zipper engenders multiple high-quality crystals of a recalcitrant tRNA synthetase.

Although Escherichia coli alanyl-tRNA synthetase was among the first tRNA synthetases to be sequenced and extensively studied by functional analysis, it has proved to be recalcitrant to crystallization. This challenge remained even for crystallization of the catalytic fragment. By mutationally introducing three stacked leucines onto the solvent-exposed side of an alpha-helix, an engineered catalytic fragment of the synthetase was obtained that yielded multiple high-quality crystals and cocrystals with different ligands. The engineered alpha-helix did not form a leucine zipper that interlocked with the same alpha-helix from another molecule. Instead, using the created hydrophobic spine, it interacted with other surfaces of the protein as a leucine half-zipper (LHZ) to enhance the crystal lattice interactions. The LHZ made crystal lattice contacts in all crystals of different space groups. These results illustrate the power of introducing an LHZ into helices to facilitate crystallization. The authors propose that the method can be unified with surface-entropy reduction and can be broadly used for protein-surface optimization in crystallization.