Inhibition of casein kinase II by dinucleoside polyphosphates

In our search for potential inhibitors of casein kinase II (CKII) in Artemia, we have shown that dinucleoside polyphosphates are a novel class of effectors for this ubiquitous protein kinase. P1,P4-di(guanosine-5')-tetraphosphate (Gp4G) is a better CKII inhibitor than P1,P4-di(adenosine-5')-tetraphosphate (Ap4A). The inhibition by both effectors is more potent when GTP is used as phosphate donor instead of ATP. The inhibition of CKII increases with the number of phosphates linking the guanosine/adenosine moieties (for n = 2-6). Ap4A does not compete with the protein substrate and causes an increase in the apparent KmATP and a decrease in the apparent VmATP, indicating a mixed type of inhibition with respect to ATP.