Protein kinase CK2 is a ubiquitous eukaryotic ser/thr protein kinase. The active holoenzyme is a heterotetrameric protein composed of catalytic (โฃ and โฃะ) and regulatory (โค) subunits that phosphorylates many different protein substrates and appears to be involved in the regulation of cell division. Despite important structural studies, the intimate details of the interactions of the โฃ catalytic subunits with the โค regulatory subunits are unknown. Recent evidence that indicates that both CK2 subunits can interact promiscuously with other proteins in a manner that excludes the binding of their complementary CK2 partners has opened the possibility that the phosphorylating activity of this enzyme may be regulated in a novel way. These alternative interactions could limit the in vivo availability of CK2 subunits to generate fully active holoenzyme CK2 tetramers. Likewise, variations in the ratio of โฃand โค-subunits could determine the activity of several phosphorylating and dephosphorylating activities. The promiscuity of the CK2 subunits can be extrapolated to a more widespread phenomenon in which ''wild-card'' proteins could act as general switches by interacting and regulating several catalytic activities.