BTF3 is a potential new substrate of protein kinase CK2

Abstract

BTF3, initially discovered as a factor required for transcription inititation of RNA polymerase II, is expressed in two isoforms, termed a and b. BTF3b, the transcriptionally inactive isoform, was identified as an interaction partner of protein kinase CK2 subunit β employing the interaction trap system for screening of a HeLa cDNA fusion library. We report here on the interaction between the other isoform, BTF3a, and protein kinase CK2. The complete cDNA of BTF3a was cloned by RT-PCR and used for analysis in the two-hybrid system with a three-reporter yeast strain. Interaction of BTF3a with CK2 subunits α, α′ or β was detectable by one of three reporters, whereas the CK2β- BTF3a interaction was activating two reporters. It was also shown that BTF3a is phosphorylatedin vitro by the α2β2 holoenzyme, but not by α or α′ alone, indicating the requirement of β for substrate recognition. Immunoprecipitations of GST-fused BTF3a carried out in vitro resulted in co-precipitation of β. Similarly, GST-BTF3a, but not GST alone isolated with glutathione agarose beads from buffer containing recombinant CK2 subunits was found complexed with α and β, likely representing α2β2 holoenzyme. The data show a weak, nevertheless specific interaction of protein kinase CK2 via subunit β with the putative transcription factor BTF3a in vitro and in vivo, and a role of BTF3a as a potential new substrate for CK2.