Unique activation mechanism of protein kinase CK2 - The N-terminal segment is essential for constitutive activity of the catalytic subunit but not of the holoenzyme

CK2 is an essential, ubiquitous, and highly pleiotropic protein kinase whose catalytic subunits (alpha and alpha') and holoenzyme (composed by two catalytic and two regulatory beta-subunits) are both constitutively active, a property that is suspected to contribute to its pathogenic potential. Extensive interactions between the N-terminal segment and the activation loop are suspected to underlie the high constitutive activity of the isolated catalytic subunit. Here we show that a number of point mutations (Tyr(26) --> Phe, Glu(180) --> Ala, Tyr(182) --> Phe) and deletions (Delta2-6, Delta2-12, Delta2-18, Delta2-24, Delta2-30) expected to affect these interactions are more or less detrimental to catalytic activity of the a-subunit of human CK2, the deleted mutants Delta2-24 and Delta2-30 being nearly inactive under normal assay conditions. Kinetic analyses showed that impaired catalytic activity of mutants Delta2-12, Delta2-18, Delta2-24, and Y182F is mainly accounted for by dramatic increases in the K-m values for ATP, whereas a drop in K-cat with K-m values almost unchanged was found with mutants Y26F and E180A. Holoenzyme reconstitution restored the activity of mutants Delta2-12, Delta2-18, Y26F, E180A, and Y182F to wild type level and also conferred catalytic activity to the intrinsically inactive mutants, Delta2-24 and Delta2-30. These data demonstrate that specific interactions between the N-terminal segment and the activation loop are essential to provide a fully active conformation to the catalytic subunits of CK2; they also show that these interactions become dispensable upon formation of the holoenzyme, whose constitutive activity is conferred by the beta-subunit through a different mechanism.