RGFGIGS IS AN AMINO-ACID-SEQUENCE REQUIRED FOR ACETYL-COENZYME-A BINDING AND ACTIVITY OF HUMAN SPERMIDINE SPERMINE N(1)ACETYLTRANSFERASE/

Polyamine catabolism is rate limited by spermidine/spermine N-1-acetyl transferase (SSAT), Although the amino acid sequence of SSAT is known, the substrate binding and catalytic sites are not, The goal of this s tudy was to define the region responsible for acetyl coenzyme A bindin g, Human SSAT contains a region of 20 amino acids homologous to severa l microbial antibiotic N-acetyltransferases. The highest;homology is r epresented in the Campylobacter coli streptothricin acetyltransferase sat4 gene, where 16 identical or highly conserved amino acids exist in a 20-residue stretch, The most conserved residues within this region are RGF-GIGS beginning at Arg-101 in the human SSAT, Site-directed mut ations to Arg-101, Gly-104, and Gly-106 resulted in proteins with no m easurable activity, The G102D mutation produced a partially active pro tein with a decreased affinity for acetyl coenzyme A and with a K-m >1 0-fold that of the wild-type protein, Analysis using the PredictProtei n program suggests a common structure among the microbial and eukaryot ic N-acetyltransferases in the region corresponding to the RGF-GIGS of human SSAT consisting of an cu-helix usually preceded by a glycine lo op, Our data are consistent with the hypothesis that Arg-101 and the p roximal glycine loop are necessary for the activity of human SSAT.