An extensive survey of CK2 α and β subunits in Arabidopsis: Multiple isoforms exhibit differential subcellular localization

Abstract

Casein kinase 2 (CK2) is a ubiquitous enzyme essential for the viability of eukaryotic cells. In the present work we analyzed the Arabidopsis thaliana genome in a search for the genes coding for all CK2 α and β subunits. We found four α subunit and four β subunit genes. Expression analysis showed that all CK2 subunit genes are expressed in inflorescences, stems, leaves and roots. The level of expression of these genes is very similar, except for the one that codes for an α subunit harboring a putative chloroplastic destination peptide (αcp), which shows a slightly higher expression level in all tissues. Using transgenic plants and agroinfiltration, we have also characterized the subcellular localization of all proteins encoded by CK2 genes. Our results show that all α subunits are localized in the nucleus, with the exception of αcp, which is only found in the chloroplasts. On the other hand, β subunits have a more diverse distribution, with some of them localizing both to the nucleus and to the cytosol, while others are exclusively located in one of these compartments. Remarkably, no CK2β subunit was found in the chloroplasts. Finally, by directly measuring its activity, we have demonstrated that purified Arabidopsis chloroplasts have active CK2 that can be regulated by external addition of CK2β. This study represents a complete survey of the CK2 gene family in Arabidopsis and the first step for future studies on CK2 cellular function in this species. © The Author 2006. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved.