Pim-1, a protooncogene product, is a serine/threonine kinase and is thought to play a role in signal transduction in blood cells. Few phosphorylated target proteins for Pim-1, however, have been identified. In the present study, two-hybrid screening to clone cDNAs encoding proteins binding to Pim-1 was carried out, and a cDNA for heterochromatin protein 1γ (HP1γ) was obtained. Binding assays both in yeast and in vitro pull-down using the purified HP1γ and Pim-1 expressed in Escherichia coli showed that Pim-1 directly bound to the chromo shadow domain of HP1γ. HP1γ was also associated with Pim-1 in human HeLa cells and the serine clusters located at the center of HP1γ were phosphorylated by Pim-1 in vitro. Furthermore, a transcription repression activity of HP1γ was further stimulated by the deletion of the serine clusters targeted by Pim-1. These results suggest that Pim-1 affects the structure or silencing of chromatin by phosphorylating HP1.
