CASEIN KINASE-2 PHOSPHORYLATES RECOMBINANT HUMAN SPERMIDINE SPERMINE N-1-ACETYLTRANSFERASE ON BOTH SERINE AND THREONINE RESIDUES/

Casein kinase 2 purified from human erythrocyte cytosol has been found to phosphorylate human spermidine/spermine N-1-acetyltransferase (SSA T) expressed as a fusion protein in E. coli and purified to homogeneit y with a specific activity similar to that reported for pure human SSA T. The amino acid sequence of the protein revealed not less than four phosphorylable residues, optimal target for protein kinase 2 phosphory lation being flanked by acid residues in position +1 and +3. Our resul ts indicate that most P-32-phosphate is taken up by Ser residues, as e videnced by HCl hydrolysis and electrophoresis and that the phosphoryl ation 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 i n the C-terminus of the protein, probably Ser 146 and 149. (C) 1996 Ac ademic Press.