Identification of in vivo phosphorylation sites of SET, a nuclear phosphoprotein encoded by the translocation breakpoint in acute undifferentiated leukemia

SET, the translocation breakpoint-encoded protein in acute undifferentiated leukemia (AUL), is identified as a 39-kDa phosphoprotein found predominantly in the cell nuclei 1994, J. Biol. Chem. 269,2258-2262]. SET is fused to a putative oncoprotein, CAN, in AUL and is thought to regulate the transformation potential of SET-CAN by its nuclear localization and phosphorylation. We investigated in detail the in vivo phosphorylation of SET. Phosphorylation of SET occurred in all human cell lines examined in vivo, primarily on serine residues. Endoproteinase Glu-C digestion of phosphorylated SET yielded two phosphopeptides. By radiosequencing, we identified the in vivo phosphorylation sites of SET as Ser⁹ and Ser²⁴. The surrounding sequences of Ser⁹ and Ser²⁴ contained an apparent consensus site sequence for protein kinase C.