Protein kinase CK2 is a highly conserved Ser/Thr protein kinase that is ubiquitous among eucaryotic organisms and appears to play an important role in many cellular functions. This enzyme in yeast has a tetrameric structure composed of two catalytic (alpha and/or alpha') subunits and two regulatory beta and beta' subunits. Previously, we have reported isolation from yeast cells four active forms of CK2, composed of alphaalpha'betabeta', alpha2betabeta', alpha'2betabeta' and a free alpha'-catalytic subunit. Now, we report that in Saccharomyces cerevisiae CK2 holoenzyme regulatory beta subunit cannot substitute other beta' subunit and only both of them can form fully active enzymatic unit. We have examined the subunit composition of tetrameric complexes of yeast CK2 by transformation of yeast strains containing single deletion of the beta or beta' regulatory subunits with vectors carrying lacking CKB1 or CKB2 genes. CK2 holoenzyme activity was restored only in cases when both of them were present in the cell. Additional, co-immunoprecypitation experiments show that polyadenylation factor Fip1 interacts with catalytic alpha subunits of CK2 and interaction with beta subunits in the holoenzyme decreases CK2 activity towards this protein substrate. These data may help to elucidate the role of yeast protein kinase CK2beta/beta' subunits in the regulation of holoenzyme assembly and phosphotransferase activity.