Phosphorylation of Recombinant Human Spermidine/Spermine N1-Acetyltransferase by CK1 and Modulation of Its Binding to Mitochondria: A Comparison with CK2

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.