Binding of FGF-1 variants to protein kinase CK2 correlates with mitogenicity

Fibroblast growth factor-1 (FGF-1) has both extra- and intracellular functions. To identify intracellular binding partners for FGF-1, we isolated proteins from U2OS human osteosarcoma cells interacting specifically with FGF-1. One of the isolated proteins was identified as protein kinase CK2 (CK2). We here provide evidence that FGF-1 binds to both the catalytic α-subunit and to the regulatory β-subunit of CK2. The interaction between FGF-1 and CK2α and β was characterized by surface plasmon resonance, giving K(D) values of 0.4 ± 0.3 and 1.2 ± 0.2 µM, respectively. By using a novel assay for intracellular protein interaction, FGF-1 and CK2α are shown to interact in vivo. In vitro, FGF-1 and FGF-2 are phosphorylated by CK2, and the presence of FGF-1 or FGF-2 was found to enhance the autophosphorylation of CK2β. A correlation between the mitogenic potential of FGF-1 mutants and their ability to bind to CK2α was observed. The possible involvement of CK2 in the FGF-induced stimulation of DNA synthesis is discussed.