Article

Role of Plant-Specific N-Terminal Domain of Maize CK2b1 Subunit in CK2B Functions and Holoenzyme Regulation

Department of Molecular Genetics, Centre for Research on Agricultural Genomics CRAG, CSIC-IRTA-UAB, Barcelona, Spain.
PLoS ONE (Impact Factor: 3.23). 07/2011; 6(7):e21909. DOI: 10.1371/journal.pone.0021909
Source: PubMed

ABSTRACT

Protein kinase CK2 is a highly pleiotropic Ser/Thr kinase ubiquituous in eukaryotic organisms. CK2 is organized as a heterotetrameric enzyme composed of two types of subunits: the catalytic (CK2α) and the regulatory (CK2β). The CK2β subunits enhance the stability, activity and specificity of the holoenzyme, but they can also perform functions independently of the CK2 tetramer. CK2β regulatory subunits in plants differ from their animal or yeast counterparts, since they present an additional specific N-terminal extension of about 90 aminoacids that shares no homology with any previously characterized functional domain. Sequence analysis of the N-terminal domain of land plant CK2β subunit sequences reveals its arrangement through short, conserved motifs, some of them including CK2 autophosphorylation sites. By using maize CK2β1 and a deleted version (ΔNCK2β1) lacking the N-terminal domain, we have demonstrated that CK2β1 is autophosphorylated within the N-terminal domain. Moreover, the holoenzyme composed with CK2α1/ΔNCK2β1 is able to phosphorylate different substrates more efficiently than CK2α1/CK2β1 or CK2α alone. Transient overexpression of CK2β1 and ΔNCK2β1 fused to GFP in different plant systems show that the presence of N-terminal domain enhances aggregation in nuclear speckles and stabilizes the protein against proteasome degradation. Finally, bimolecular fluorescence complementation (BiFC) assays show the nuclear and cytoplasmic location of the plant CK2 holoenzyme, in contrast to the individual CK2α/β subunits mainly observed in the nucleus. All together, our results support the hypothesis that the plant-specific N-terminal domain of CK2β subunits is involved in the down-regulation of the CK2 holoenzyme activity and in the stabilization of CK2β1 protein. In summary, the whole amount of data shown in this work suggests that this domain was acquired by plants for regulatory purposes.

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    • "Concerning the CK2β regulatory subunits, CK2β1,Riera et al., 2004). In maize, it has been recently reported that the presence of plant-specific N-terminal domain of CK2β enhances the protein aggregation in nuclear speckles, where it is assumed to be tightly complexed and less accessible to degradation machinery (Riera et al., 2011). Little is known about CK2 holoenzyme localization in plants, but recent work performed in our lab by using bimolecular fluorescence complementation (BiFC) assays shows the nuclear and cytoplasmic location of the maize CK2 holoenzyme composed by α1β1 subunits, in contrast to the individual CK2α/β subunits localization , mainly observed in the nucleus (Figure 9.3). "

    Full-text · Dataset · Jan 2016
    • "Individual CK2a and CK2b are nuclear localized proteins (with the exception of a chloroplastic CK2a4) (Salinas et al., 2006; Riera et al., 2013). The heterotetrameric holoenzyme forms aggregates in both the nucleus (nuclear speckles) and cytoplasm (Riera et al., 2011). Co-transformation of either CK2a or CK2b with ZmOST1 changed the uniform subcellular localization of ZmOST1 to cytosol aggregates and nuclear speckles, respectively. "
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