Involvement of asparagine 118 in the nucleotide specificity of the catalytic subunit of protein kinase CK2.

Protein kinase CK2 is a heteromeric enzyme with catalytic (alpha) and regulatory (beta) subunits which form an alpha2beta2 holoenzyme and utilizes both ATP and GTP as nucleotide substrate. Site-directed mutagenesis of CK2alpha subunit was used to study this capacity to use GTP. Deletion of asparagine 118 (alpha(deltaN118)) or the mutant alphaN118E gives a 5-6-fold increase in apparent Km for GTP with little effect on the affinity for ATP. Mutants alphaN118A and alphaD120N did not alter significantly the Km for either nucleotide. CK2alphaN118 has an apparent Ki for inosine 5' triphosphate 5-fold higher than wild-type and is very heat labile. These studies complement recent crystallographic data indicating a role for CK2alpha asparagine 118 in binding the guanine base.