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Cytosolic spermidine/spermine acetyltransferase (SSAT) catalyzes the acetylation of the N(1)-propylamino groups of spermine and spermidine. The enzyme has a very short half-life and is rapidly induced by various stimuli. Once acetylated, these polyamines are subjected to the action of polyamine oxidase, which, besides initiating polyamine catabolism, may produce reactive oxygen species that in turn trigger modifications in subcellular compartments such as mitochondria. The present work evaluates the ability of the cAMP-independent Ser/Thr-protein kinase CK1 to phosphorylate SSAT. Results demonstrate that SSAT is phosphorylated by CK1, in sites distinct from those phosphorylated by CK2. Moreover, both phosphorylation processes are involved in the uptake of SSAT into rat liver mitochondria. Although CK2 is less effective than CK1 in phosphorylating SSAT, CK2 phosphorylation is much more powerful in preventing binding of SSAT to mitochondrial structures. These results suggest the involvement of CK1- and CK2-mediated SSAT phosphorylation in regulating the contents of polyamines and SSAT itself within subcellular compartments and implicate SSAT and polyamines as indirect modulators of progression through the cell cycle., ((c)2002 Elsevier Science.)

Casein kinase 2 purified from human erythrocyte cytosol has been found to phosphorylate human spermidine/spermine N1-acetyltransferase (SSAT) expressed as a fusion protein in E. coli and purified to homogeneity with a specific activity similar to that reported for pure human SSAT. The amino acid sequence of the protein revealed not less than four phosphorylable residues, optimal target for protein kinase 2 phosphorylation being flanked by acid residues in position +1 and +3. Our results indicate that most 32P-phosphate is taken up by Ser residues, as evidenced by HCl hydrolysis and electrophoresis and that the phosphorylation extent is modulated by the physiological polyamine concentration. Partial digestion with trypsin at a low concentration for less than one hour preferentially hydrolyzes Lys-Arg-Arg in position 141-143 of the SSAT suggesting that the Ser-phosphorylated residues are located in the C-terminus of the protein, probably Ser 146 and 149.