✍ Scribed by Francisco Romero-Oliva; Germaine Jacob; Jorge E. Allende
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Protein kinase CK2 (casein kinase II) is normally a heterotetramer composed of catalytic (α, α′) and regulatory subunits (β). CK2 is able to phosphorylate a large number of protein substrates but the physiological mechanisms of its regulation are still unresolved. Lysine‐rich peptides such as polylysine and histone H1 are known to stimulate the catalytic activity of the holoenzyme. This activation is mediated through the CK2β regulatory subunit. In this communication, we report that the same concentrations of lysine‐rich peptides or proteins that activate the holoenzyme cause strong inhibition of the phosphorylation of proteins catalyzed by the free catalytic CK2α subunit. The inhibitory effect of polylysine and histone H1 is observed with several protein substrates of CK2α (casein, adeno E1A, transcription factor II A, and CK2β itself). With calmodulin, however, the inhibition of CK2α phosphorylation caused by polylysine is much lower while with a model peptide substrate of CK2 the inhibition caused by this polycation is negligible. The inhibition of CK2α by polylysine is observed only at limiting concentrations of the target substrate proteins. The dual effect of polylysine and of histone H1, which results in the inhibition of CK2α and stimulation of the CK2 α~2~β~2~ tetrameric holoenzyme, has the consequence that the addition of the CK2β, in the presence of polylysine and low concentrations of substrate protein, can cause a 242‐fold stimulation of the activity of CK2α. Other polycationic compounds such as polyarginine and spermine do not inhibit the phosphorylation of casein by CK2α, indicating that the effect is specific for lysine‐rich peptides. Since there is evidence that there may be free CK2α subunits in the nuclei of cells, where there is abundant histone H1, the inhibition of CK2α by this lysine‐rich protein may have physiological relevance. J. Cell. Biochem. 89: 348–355, 2003. © 2003 Wiley‐Liss, Inc.
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