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Catalytic domain of the Serine/Threonine Kinase, Homeodomain-Interacting Protein Kinase STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. HIPKs, originally identified by their ability to bind homeobox factors, are nuclear proteins containing catalytic kinase and homeobox-interacting domains as well as a PEST region overlapping with the speckle-retention signal (SRS). They show speckled localization in the nucleus, apart from the nucleoles. They play roles in the regulation of many nuclear pathways including gene transcription, cell survival, proliferation, differentiation, development, and DNA damage response. Vertebrates contain three HIPKs (HIPK1-3) and mammals harbor an additional family member HIPK4, which does not contain a homeobox-interacting domain and is localized in the cytoplasm. HIPK2, the most studied HIPK, is a coregulator of many transcription factors and cofactors and it regulates gene transcription during development and in DNA damage response. The HIPK subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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