Crystal Structure and Enantiomer Selection by D-Alanyl Carrier Protein Ligase DltA from Bacillus cereus

Ubiquitous D-alanylation of lipoteichoic acids modulates the surface charge and ligand binding of the Gram-positive cell wall. Disruption of the bacterial DltABCD gene involved in teichoic acid alanylation, as well as inhibition of the DltA protein, has been shown to increase a Gram-positive bacterium's susceptibility to antibiotics. The DltA D-alanyl carrier protein ligase promotes a two-step process starting with adenylation of D-alanine. We have determined the 2.0 angstrom resolution crystal structure of a DltA protein from Bacillus cereus in complex with the D-alanine adenylate intermediate of the first reaction. Despite the low level of sequence similarity, the DltA structure resembles known structures of adenylation domains such as the acetyl-CoA synthetase. The enantiomer selection appears to be enhanced by the medium-sized side chain of Cys-269. The Ala-269 mutant protein shows marked loss of such selection. The network of noncovalent interactions between the D-alanine adenylate and DltA provides structure-based rationale for aiding the design of tight-binding DltA inhibitors for combating infectious Gram-positive bacteria such as the notorious methicillin-resistant Staphylococcus aureus.