Fibroblast Growth Factor-2 Binds to the Regulatory β Subunit of CK2 and Directly Stimulates CK2 Activity toward Nucleolin

Abstract

The presence of fibroblast growth factor-2 (FGF-2) in the nucleus has now been reported both in vitro and in vivo, but its nuclear functions are unknown. Here, we show that FGF-2 added to nuclear extract binds to protein kinase CK2 and nucleolin, a CK2 natural substrate. Added to baculovirus-infected cell extracts overexpressing CK2 or its isolated subunits, FGF-2 binds to the enzyme through its regulatory ␤ subunit. Using purified proteins, FGF-2 is shown to directly interact with CK2 and to stimulate CK2 activity toward nucleolin. Furthermore, a mitogenic-deficient FGF-2 mutant protein has an impaired ability to interact with CK2 and to stimulate CK2 activity using nucleolin as substrate. We propose that in growing cells, one function of nuclear FGF-2 is to modulate CK2 activity through binding to its regulatory ␤ subunit.The fibroblast growth factors (FGFs) 1 family includes nine polypeptides of which FGF-1 and FGF-2 (acidic and basic FGF) are prototype members (reviewed in Refs. 1 and 2). FGF-2 exerts its pleiotropic effects in cell growth and differentiation through a dual receptor system consisting of four different high-affinity transmembrane receptor tyrosine kinases and low-affinity binding sites corresponding to heparan sulfate proteoglycans (reviewed in Refs. 2 and 3). In addition, FGF-1 and FGF-2 are translocated from outside the cell to the nucleus (reviewed in Ref.2). Many other growth factors have been detected in the cell nucleus (reviewed in Refs. 4 and 5). In the case of Schwannomaderived growth factor and FGF-1, nuclear localization is necessary for mitogenic activity (6 -8). For FGF-2, nuclear translocation is correlated to cell proliferation, since it is no longer recovered in the nucleus of confluent cells (9, 10). Therefore, these data strongly suggest that FGF-2, as well as other growth factors, plays specific, but still unknown, nuclear functions in addition to classical signaling through cell surface receptors.We reported previously that in synchronized ABAE growing cells, a large increase of ribosomal genes transcription is tightly correlated to the nuclear translocation of FGF-2 and especially to an accumulation of the growth factor in the nucleolus. In contrast, in quiescent cells ribosomal genes transcription is 20-fold lower, and FGF-2 is exclusively found in the cytoplasm (9, 10). Upon addition of FGF-2 to nuclei of serum-starved cells, a 6-fold increase of ribosomal genes transcription is observed together with an increase in phosphorylation of nuclear proteins, essentially of nucleolin (9, 11); These data suggest that a nuclear function of FGF-2 could be to regulate ribosomal genes transcription by modulating the phosphorylation level of nuclear and nucleolar proteins as nucleolin. Nucleolin that is the major component of nucleolus is also a major substrate for protein kinase CK2 in rapidly proliferating tissues, and its phosphorylation level is correlated to the rate of ribosome biogenesis (reviewed in Ref. 12). CK2 is a serine/threonine protein kinase present in both the cyt...

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