Crystal structure of an aminoglycoside 6′-N-acetyltransferase: defining the GCN5-related N-acetyltransferase superfamily fold

The predominant mechanism of antibiotic resistance employed by pathogenic bacteria against the clinically used aminoglycosides is chemical modification of the drug. The detoxification reactions are catalyzed by enzymes that promote either the phosphorylation, adenylation or acetylation of aminoglycosides. Structural studies of these aminoglycoside-modifying enzymes may assist in the development of therapeutic agents that could circumvent antibiotic resistance. In addition, such studies may shed light on the development of antibiotic resistance and the evolution of different enzyme classes.The crystal structure of the aminoglycoside-modifying enzyme aminoglycoside 6'-N-acetyltransferase type li (AAC(6')-li) in complex with the cofactor acetyl coenzyme A has been determined at 2.7 A resolution. The structure establishes that this acetyltransferase belongs to the GCN5-related N-acetyltransferase superfamily, which includes such enzymes as the histone acetyltransferases GCN5 and Hat1.Comparison of the AAC(6')-li structure with the crystal structures of two other members of this superfamily, Serratia marcescens aminoglycoside 3-N-acetyltransferase and yeast histone acetyltransferase Hat1, reveals that of the 84 residues that are structurally similar, only three are conserved and none can be implicated as catalytic residues. Despite the negligible sequence identity, functional studies show that AAC(6')-li possesses protein acetylation activity. Thus, AAC(6')-li is both a structural and functional homolog of the GCN5-related histone acetyltransferases.