A novel splicing variant encoding putative catalytic α subunit of maize protein kinase CK2

A cDNA highly homologous to the known catalytic alpha subunit of protein kinase CK2 was cloned from maize (Zea mays). It was designated ZmCK2 alpha-4 (accession no. AAF76187). Sequence analysis shows that ZmCK2 alpha-4 and the previously identified ZmCK2 alpha-1 (accession no. X61387) are transcribed from the same gene, ZmPKCK2AL (accession no. Y11649), but at different levels in various maize organs and at different stages of development. The cDNA encoding ZmCK2 alpha-4 has three potential translation initiation sites. The three putative variants of ZmCK2 alpha-4 were expressed in Escherichia coli as GST-fusion proteins and purified from bacterial extracts. In contrast to the previously characterized ZmCK2 alpha s, the obtained GST:ZmCK2 alpha-4 proteins were catalytically inactive as monomers or in the presence of equimolar amounts of the human CK2 beta. However, GST:ZmCK2 alpha-4 did phosphorylate casein in the presence of a large excess of the beta subunit. The activity of ZmCK2 alpha-4 toward casein could also be stimulated by increasing ATP concentration. Modeling studies have shown that there is no interaction between the N-terminal segment of ZmCK2 alpha-4 and the activation loop responsible for constitutive catalytic activity of CK2 alpha. Preliminary results suggest that ZmCK2 alpha-4 may function as a negative regulator of other CK2s, and at certain circumstances as a holoenzyme which catalytic activity is stimulated by specific regulatory subunit(s).