Dihydrofolate reductase (DHFR) is a prominent molecular target in antitumor, antibacterial, antiprotozoan, and immunosuppressive chemotherapies, and CK2 protein kinase is an ubiquitous enzyme involved in many processes, such as tRNA and rRNA synthesis, apoptosis, cell cycle or oncogenic transformation. We show for the first time that CK2 alpha subunit strongly interacted with and phosphorylated DHFR in vitro. Using quartz crystal microbalance with dissipation monitoring (QCM-D) we determined DHFR-CK2 alpha binding kinetic parameters (K-d below 0.5 mu M, k(on) = 10.31 x 10(4) M(-1)s(-1) and k(off) = 1.40 x 10(-3)s(-1)) and calculated Gibbs free energy (-36.4 kJ/mol). In order to identify phosphorylation site(s) we used site-directed mutagenesis to obtain several DHFR mutants with predicted CK2-phosphorylable serine or threonine residues substituted with alanines. All enzyme forms were subjected to CK2 alpha subunit catalytic activity and the results pointed to serine 168 as a phosphorylation site. Mass spectrometry analyses confirmed the presence of phosphoserine 168 and revealed additionally the presence of phosphoserine 145, although the latter phosphorylation was on a very low level. (C) 2019 Elsevier Inc. All rights reserved.