Homo- and hetero-dimerization of homeodomain-interacting protein kinases (HIPKs)

Homeodomain-interacting protein kinases (HIPKs) constitute a novel family of nuclear protein kinases which function as crucial regulators of apoptosis and cell fate determination during development. Despite the important functions of HIPKs in the context of both UV- and stress-induced apoptosis and cell growth control, there remains little available data with regard to the biochemical properties of HIPKs. Here we show that HIPK1 and HIPK2 are able to both homo- and hetero-dimerize through their C-terminal regions, and to colocalize in the nuclear dots. Treatment of in vitro-translated HIPK1 and HIPK2 with disuccinimidyl suberate (DSS), a chemical cross-linking reagent, was shown to result in the homo-dimerization of HBPK1 and HIPK2, respectively. Yeast two-hybrid domain analysis revealed that the C-terminal portion of HIPK2 was responsible for its dimerization. Furthermore, HIPK2 was also determined to immunoprecipitate with HIPK2 and HIPK1, indicating the homo- and hetero-association of both HIPK2 and HIPK1 in cultured cells. However, the catalytic activities of HIPK1/HIPK2 hetero-dimer with regard to auto-phosphorylation and substrate-phosphorylation were less pronounced than those of the HIPK2 homo-dimer. Consistent with the formation of the HIPK1/2 hetero-dimer, HIPK2 was also found to colocalize with HIPK1 in the nuclear dots. These results suggest that cross-talk takes place between the HIPKs, which exploit a variety of mechanisms in the regulation of HIPK activity.