Characterization, subcellular localization and nuclear targeting of casein kinase 2 from Zea mays

We have isolated and characterized the genomic clone of maize casein kinase 2 (CK2) alpha subunit using the previously described alpha CK2-1 cDNA clone as a probe. The genomic clone is 7.5 kb long and contains 10 exons, separated by 9 introns of different size, two larger than 1.5 kb and the others around 100-150 bp. The sequence of the exons is 100% homologous to the sequence of the alpha CK2-1 cDNA. Southern hybridization of total genomic DNA from maize embryos with alpha CK2 cDNA indicated that the alpha CK2-1 gene is part of a multigenic family. We also isolated a new embryo cDNA clone coding for an alpha CK2-2 subunit. We studied the regulation of the enzyme in embryos at the mRNA level, at the protein level and by activity testing. By using immunocytochemistry the CK2 protein was localized in several types of cells of mature embryos. Particularly strong signals were visible in the cytoplasm of epidermis and meristematic cells. Decoration of nuclei of root cortex and scutellum cells was also observed suggesting that CK2 can shift from the cytoplasm into nuclei in specific cell types. We examined whether CK2 contained specific protein domains which actively target the protein to the nucleus by using in-frame fusions of the maize CK2 alpha subunit to the reporter gene encoding beta-glucuronidase (GUS) which were assayed in transiently transformed onion epidermal cells. Analysis of chimeric constructs identified one region containing a nuclear localization signal (NLS) that is highly conserved in other alpha CK2 proteins.