Molecular identification of a calcium-inhibited catalytic subunit of casein kinase type 2 from Paramecium tetraurelia

We have previously described the occurrence in Paramecium of a casein kinase (CK) activity (EC 2.7.1.37) with some unusual properties, including inhibition by Ca(2+) (R. Kissmehl, T. Treptau, K. Hauser, and H. Plattner, FEBS Lett. 402:227-235, 1995). We now have cloned four genes, PtCK2alpha1 to PtCK2alpha4, all of which encode the catalytic alpha subunit of type 2 CK (CK2) with calculated molecular masses ranging from 38.9 to 39.4 kDa and pI values ranging from 8.8 to 9.0. They can be classified into two groups, which differ from each other by 28% on the nucleotide level and by 18% on the derived amino acid level. One of them, PtCK2alpha3, has been expressed in Escherichia coli and characterized in vitro. As we also have observed with the isolated CK, the recombinant protein preferentially phosphorylates casein but also phosphorylates some Paramecium-specific substrates, including the exocytosis-sensitive phosphoprotein pp63/parafusin. Characteristically, Ca(2+) inhibits the phosphorylation at elevated concentrations occurring during stimulation of a cell. Reconstitution with a recombinant form of the regulatory subunit from Xenopus laevis, XlCK2beta, confirms Ca(2+) sensitivity also under conditions of autophosphorylation. This is unusual for CK2 but correlates with the presence of two EF-hand calcium-binding motifs, one of which is located in the N-terminal segment essential for constitutive activity, as well as with an aberrant composition of normally basic domains recognizing acidic substrate domains. Immunogold localization reveals a considerable enrichment in the outermost cell cortex layers, excluding cilia. We discuss a potential role of this Ca(2+)-inhibited PtCK2alpha species in a late step of signal transduction.