Arabidopsis N-acetyltransferase activity 2 preferentially acetylates 1,3-diaminopropane and thialysine

Polyamine acetylation has an important regulatory role in polyamine metabolism. It is catalysed by GCN5- related N-acetyltransferases, which transfer acetyl groups from acetyl-coenzyme A to the primary amino groups of spermidine, spermine (Spm), or other polyamines and diamines, as was shown for the human Spermidine/ Spermine N1-acetyltransferase 1 (HsSSAT1). SSAT homologues specific for thialysine, a cysteine-derived lysine analogue, were also identified (e.g., HsSSAT2). Two HsSSAT1 homologues are present in Arabidopsis, namely N-acetyltransferase activity (AtNATA) 1 and 2. AtNATA1 was previously shown to be specific for 1,3-diaminopropane, ornithine, putrescine and thialysine, rather than Spm and spermidine. In the present study, in an attempt to find a plant Spm-specific SSAT, AtNATA2 was expressed in a heterologous bacterial system and catalytic properties of the recombinant protein were determined. Data indicate that recombinant AtNATA2 preferentially acetylates 1,3-diaminopropane and thialysine, throwing further light on AtNATA1 substrate specificity. Structural analyses evidenced that the preference of AtNATA1, AtNATA2 and HsSSAT2 for short amine substrates can be ascribed to different main-chain conformation or substitution of HsSSAT1 residues interacting with Spm distal regions. Moreover, gene expression studies evidenced that AtNATA1 gene, but not AtNATA2, is up-regulated by cytokinins, thermospermine and Spm, suggesting the existence of a link between AtNATAs and N1-acetyl-Spm metabolism. This study provides insights into polyamine metabolism and structural determinants of substrate specificity of non Spm-specific SSAT homologues.