CK2 Forms a Stable Complex with TFIIIB and Activates RNA Polymerase III Transcription in Human Cells

Abstract

CK2 is a highly conserved protein kinase with growth-promoting and oncogenic properties. It is known to activate RNA polymerase III (PolIII) transcription in Saccharomyces cerevisiae and is shown here to also exert a potent effect on PolIII in mammalian cells. Peptide and chemical inhibitors of CK2 block PolIII transcription in human cell extracts. Furthermore, PolIII transcription in mammalian fibroblasts is decreased significantly when CK2 activity is compromised by chemical inhibitors, antisense oligonucleotides, or kinase-inactive mutants. Coimmunoprecipitation and cofractionation show that endogenous human CK2 associates stably and specifically with the TATA-binding protein-containing factor TFIIIB, which brings PolIII to the initiation site of all class III genes. Serum stimulates TFIIIB phosphorylation in vivo, an effect that is diminished by inhibitors of CK2. Binding to TFIIIC2 recruits TFIIIB to most PolIII promoters; this interaction is compromised specifically by CK2 inhibitors. The data suggest that CK2 stimulates PolIII transcription by binding and phosphorylating TFIIIB and facilitating its recruitment by TFIIIC2. CK2 also activates PolI transcription in mammals and may therefore provide a mechanism to coregulate the output of PolI and PolIII. CK2 provides a rare example of an endogenous activity that operates on the PolIII system in both mammals and yeasts. Such evolutionary conservation suggests that this control may be of fundamental importance.Protein kinase CK2 (formerly known as casein kinase II) is ubiquitous and highly conserved in eukaryotes (reviewed in references 1 and 29). It phosphorylates proteins on serine and threonine in both the nucleus and the cytoplasm. CK2 exists as a tetramer, composed of two isozymic catalytic subunits, ␣ and ␣Ј, and two copies of a regulatory ␤ subunit or one copy each of ␤ and the closely related ␤Ј. The CK2␣ and CK2␣Ј subunits are nearly 90% identical and can compensate for each other, but there is also some functional specialization (57, 69). The ␤ subunits allow optimal kinase activity and can regulate substrate specificity; they form a stable dimer linking the two catalytic subunits, which do not contact each other (35).Although its signaling function has long remained obscure, CK2 has been shown recently to form part of the Wnt pathway in both Drosophila and mammals (54, 65). Many studies have found that increases in the level and/or activity of CK2 are associated with cell growth and proliferation (for example, references 3, 4, 7, 25, 30, 34, and 39). Thus, CK2 expression can be increased by mitogens (7, 39), and CK2 is most abundant in cells with high mitotic activity, such as transformed cells and normal colorectal mucosa (34). Indeed, microinjection of CK2 can induce immediate-early gene expression in the absence of growth factors (11). Conversely, inactivation of CK2 by specific antibodies or antisense oligonucleotides can arrest the proliferation of primary human fibroblasts (41,42). Similarly, cell cycle progression is blocked in Saccharomyces ...

Casein kinase II-mediated phosphorylation of general repressor Maf1 triggers RNA polymerase III activation

“…CK2 is also present in the human Pol III complex and phosphorylates the TFIIIB subunits Brf1 (24) and Bdp1 (26). Our results indicate that human Maf1 can also be phosphorylated by CK2 in vitro, although we have not tested this in vivo.…”

“…CK2 is stably associated with TFIIIB (23,24) and phosphorylates subunits TBP in yeast, and Brf1 and Bdp1 in humans (24,25). CK2 activity is required for assembly of Pol III transcription initiation complex on gene promoters in human cells (24,26).…”

Akt activation enhances ribosomal RNA synthesis through casein kinase II and TIF-IA

“…CK2 resides in the nucleolus, as well as in the nucleus and cytoplasm, regulates a large number of nucleolar proteins (25) and plays a major role in nucleolar compartmentalization (26). CK2 also regulates the transcriptional activities of all three RNA polymerases (25,(27)(28)(29). In the case of Pol I, CK2 has been found in the rRNA promoter region by ChIP assay and coimmunoprecipitates with the Pol I transcription initiation complex (30,31).…”

RNA polymerase III transcription and cancer

“…CK2 is ubiquitous and highly conserved through evolution (Litchfield and Luscher, 1993; Allende and Allende, 1995). It binds and phosphorylates TFIIIB in both yeast and mammals, and can increase pol III transcription dramatically by facilitating assembly of the initiation complex (Hockman and Schultz, 1996;Schultz, 1997, 2001;Johnston et al, 2002). It induces lymphomas in transgenic mice and is hyperactive in some human malignancies (Faust et al, 1996;Munstermann et al, 1990;Notterman et al, 2001;Seldin and Leder, 1995).…”