Phosphorylation of the regulatory beta-subunit of protein kinase CK2 by checkpoint kinase Chk1: identification of the in vitro CK2beta phosphorylation site.

The regulatory beta-subunit of protein kinase CK2 mediates the formation of the CK2 tetrameric form and it has functions independent of CK2 catalytic subunit through interaction with several intracellular proteins. Recently, we have shown that CK2beta associates with the human checkpoint kinase Chk1. In this study, we show that Chk1 specifically phosphorylates in vitro the regulatory beta-subunit of CK2. Chymotryptic peptides and mutational analyses have revealed that CK2beta is phosphorylated at Thr213. Formation of a stable complex between CK2beta and Chk1 is not affected by the modification of Thr213 but it does require the presence of an active Chk1 kinase.