- Casein kinase 2 phosphorylates recombinant human spermidine/spermine N1-acetyltransferase on both serine and threonine residues. Bordin, L., Vargiu, C., Colombatto, S., Clari, G., Testore, G., Toninello, A., Grillo, M.A. Biochem. Biophys. Res. Commun. (1996)