High mobility group I (Y) proteins bind HIPK2, a serine-threonine kinase protein which inhibits cell growth

The HMGI proteins (HMGI, HMGY and HMGI-C) have an important role in the chr omatin organization and interact with different transcriptional factors. Th e HMGI genes are expressed at very low levels in normal adult tissues, wher eas they are very abundant during embryonic development and in several expe rimental and human tumours. In order to isolate proteins interacting with t he HMGI(Y) proteins, a yeast two-hybrid screening was performed using the H MGI(Y) protein as bait. This analysis led to the isolation of homeodomain-i nteracting protein kinase-2 (HIPK2), a serine/threonine nuclear kinase. HIP K2 co-immunoprecipitates with the HMGI(Y) protein in 293T cells. The intera ction between HIPK2 and HMGI(Y) occurs through the PEST domain of HIPK2 and it is direct because in vitro translated HIPK2 binds HMGI(Y). We also show that HIPK2 is able to phosphorylate the HMGI(Y) protein by an in vitro kin ase assay. In order to understand a possible role of HIPK2 gene in cell gro wth we performed a colony assay which showed an impressive HIPK2 inhibitory effect on normal thyroid cells. Flow cytometric analysis would indicate th e block of cell growth at the G2/M phase of the cell cycle. Since normal th yroid cells do not express detectable HMGI(Y) protein levels, we assume tha t the HIPK2 inhibitory effect is independent from the interaction with the HMGI(Y) protein.