PAP-1, a novel target protein of phosphorylation by Pim-1 kinase

Protooncogene, pim-1, has been reported to be a predisposition for lymphoma genesis along with myc, and its protein product, Pim-1, has been shown to b e a serine/threonine protein kinase, whose activity is involved in prolifer ation and differentiation of blood cells. The signal transduction pathways neither to nor from Pim-1, however. have been clarified. We have cloned a c DNA encoding a novel Pim-1 binding protein, PAP-1, comprising 213 amino aci ds with a basic amino-acid cluster near the C-terminus. PAP-1 was colocaliz ed with Pim-1 in human HeLa cell nuclei. The in vitro binding assays using GST fusion proteins of the wild-type and various deletion mutants revealed that the whole molecule of Pim-1 is required for the binding activity to PA P-1 and that Pim-1 binds to the region from amino-acid numbers 1-147 of PAP -1, or to two segments in the region. The association of PAP-1 with Pim-1 w as also shown in vivo in transfected cells. Furthermore, PAP-1 was phosphor ylated in vitro by Pim-1, but not a kinase-negative Pim-1 mutant. The two s erine residues of PAP-1 at amino acids 204 and 206 near the C-terminus were phosphorylated by Pim-1. PAP-1 is thus thought to be a target protein for Pim-1 kinase.