Molecular cloning of Trypanosoma brucei CK2 catalytic subunits: The α isoform is nucleolar and phosphorylates the nucleolar protein Nopp44/46

We have demonstrated previously that Nopp44/46, an abundant nucleolar phosphoprotein of Trypanosoma brucei, is associated with a protein kinase. In many organisms multiple nucleolar proteins are phosphorylated by the protein kinase CK2, formerly known as casein kinase II. Here we report the identification of two T. brucei genes, CK2a1 and CK2a2, which encode protein kinases bearing signature motifs common to CK2 catalytic subunits. The protein specified by CK2a1, designated CK2α, was capable of associating with Nopp44/46 as assessed by yeast two-hybrid analysis. An epitope-tagged version of CK2α expressed in T. brucei colocalized with Nopp44/46, with a largely nucleolar localization. This localization contrasts with the predominantly nuclear localization of mammalian CK2. When expressed in Escherichia coli, TbCK2α was catalytically active and phosphorylated Nopp44/46. Together these data demonstrate that TbCK2α is a Nopp44/46-associated kinase. Competition assays revealed that, unlike most CK2s, TbCK2α discriminates highly between ATP and GTP. This distinction may be associated with the substitution of glutamic acid and alanine for the di-asparagine motif thought to participate in purine interaction.