Criticality of a conserved tyrosine residue in the SpeG protein from Escherichia coli.

The SpeG spermidine/spermine N-acetyltransferase (SSAT) from Escherichia coli belongs to the Gcn5-related N-acetyltransferase (GNAT) superfamily of proteins. In vitro characterization of this enzyme shows it acetylates the polyamines spermine and spermidine, with a preference toward spermine. This enzyme has a conserved tyrosine residue (Y135) that is found in all SSAT proteins and many GNAT functional subfamilies. It is located near acetyl coenzyme A in the active center of these proteins and has been suggested to act as a general acid in a general acid/base chemical mechanism. In contrast, a previous study showed this residue was not critical for E. coli SpeG enzymatic activity when mutated to phenylalanine. This result was quite different from previous studies with a comparable residue in the human and mouse SSAT proteins, which also acetylate spermine and spermidine. Therefore, we constructed several mutants of the E. coli SpeG Y135 residue and tested their enzymatic activity. We found this conserved residue was indeed critical for E. coli SpeG enzyme activity and may behave similarly in other SSAT proteins.