Maize protein kinase CK2: regulation and functionality of three beta regulatory subunits.

Biochemical and crystallographic data suggest that, in contrast with other organisms, the active maize protein kinase CK2 might be composed simply of a catalytic polypeptide (CK2α), thus lacking CK2β regulatory subunits. To investigate the existence and functionality of CK2β regulatory subunits in Zea mays, we have screened a maize cDNA library using different approaches and have isolated three full-length cDNAs encoding CK2β regulatory subunits (CK2β-1, CK2β-2 and CK2β-3) and a cDNA coding for a novel CK2α catalytic subunit, CK2α-3. The pattern of expression of all these α/β subunits has been studied in different organs and developmental stages using specific probes for each isoform, and indicates that while CK2α subunits are constitutive, CK2β subunits are expressed differentially during embryo development. The yeast two-hybrid system and pull-down assays have been used to study specific interactions between the different subunits. While CK2α subunits are unable to self-associate, preferential interactions between α/β isoforms and β/β isoforms can be predicted. Furthermore, we show that maize CK2α/β subunits assemble into a structural tetrameric complex which has very similar properties to those described in other organisms, and that expression of maize CK2β subunits in yeast allows the rescue of the phenotypic defects associated to the lack of CK2 function, thus demonstrating the functionality of maize CK2β regulatory subunits.