Protein kinase CK2 as an ectokinase: The role of the regulatory CK2β subunit

Abstract

Protein kinase CK2 (also known as casein kinase 2) is present in the cytoplasm, nuclei, and several other organelles. In addition, this enzyme has been found bound to the external side of the cell membrane where it acts as an ectokinase phosphorylating several extracellular proteins. Previous experiments with transfection of HEK-293T cells demonstrated that expression of both subunits, CK2␣ (catalytic) and CK2␤ (regulatory), was necessary for the appearance of the ectopic enzyme as an ectokinase. In this work, using deletion and point mutations of CK2␤, it was possible to demonstrate that the region between amino acids 20 and 33 was necessary for the export of the enzyme as an ectokinase. Phenylalanines 21 and 22 and acidic residues in positions 26 -28 are involved in the structural aspects that are required for export. However, the region encompassing amino acids 20 -33 of CK2␤ is not sufficient to make the carboxyl half of this subunit functional in bringing CK2 to the ectokinase locus. In cells transfected with only CK2␤, it was demonstrated that 3-4% of the subunit is exported to the cell medium, but the subunit is not bound to the external membrane.casein kinase 2 ͉ export of proteins ͉ shedding I t has been known for a number of years that protein kinase CK2 (formerly called casein kinase 2) is present in the cytoplasm, nuclei, and several other cell organelles. It has also been found on the external side of the cellular membrane where, acting as an ectokinase, it can phosphorylate extracellular proteins and external domains of proteins. Kubler et al. (1) originally discovered that incubation of cells with CK2 substrates such as casein or phosvitin resulted in the liberation of the enzyme activity into the medium. This substrate-mediated release from the cell membrane was called ''shedding.'' Several extracellular proteins have been identified as substrates of the CK2 ectokinase activity. Vitronectin has been shown to be phosphorylated extracellularly by CK2, and evidence has been presented to demonstrate that its phosphorylation regulates the adhesion of cells to the extracellular matrix (2, 3). More recently, it has been shown that CK2 phosphorylates the C9 complement and it has been suggested that this phosphorylation might control cell lysis caused by this complement protein (4). Another recent example of an extracellular domain phosphorylated by ecto CK2 is the collagen XVII receptor. This phosphorylation may inhibit its degradation catalyzed by metaloproteases (5).In our previous work (6), we explored the conditions and requirements necessary to observe the presence of ectopically expressed CK2 holoenzyme that could be obtained after shedding of human cells in culture that had been transfected with the cDNAs coding for the two CK2 subunits. This work demonstrated that transfection with both catalytic (CK2␣) and regulatory (CK2␤) subunits was necessary to detect ectopically expressed CK2 bound externally to the cellular membrane. It was further shown that the export outside the cell did not require c...

Functional Analysis of Protein Kinase CK2 of the Human Malaria Parasite Plasmodium falciparum

“…PfCK2␤2 possesses several phosphorylatable residues in the N-terminal region that are surrounded by a number of acidic residues, which could therefore be phosphorylated by CK2, and a TESSEE sequence at the C terminus reminiscent of the HsCK2␤ N-terminal phosphorylation site (MSSEE). The stretch of amino acids found to be necessary for the export of CK2 as an ectokinase (CK2␤ amino acids E20 to K33) (44) are largely conserved in the PfCK2␤ sequences, leading to the intriguing possibility that PfCK2 may be exported from the parasite. The acidic stretch responsible for downregulation of CK2 activity and association with the plasma membrane (HsCK2␤ amino acids D55 to D64) (29,32) is present in PfCK2␤1 (D68 to D75) and extended in PfCK2␤2 (D207 to E226).…”

“…It has over 300 cellular substrates catalogued to date (34). Consistent with its multiple substrates, the enzyme plays a crucial role in many cellular processes, including differentiation, proliferation, survival, translation, apoptosis, cell cycle progression, and cellular responses to stress and DNA damage (1,38); mammalian CK2 has even been found to act as an exokinase, phosphorylating several extracellular proteins (44,55). CK2 is essential to viability in yeasts and slime molds (24,37).…”

Cellular function and molecular structure of ecto-nucleotidases

“…ATP can also serve as a co-substrate of ecto-kinases in the phosphorylation of cell surface-located or extracellular proteins [15,16]. In murine tissue, NAD + can (in addition to acting as a P2Y receptor agonist) activate P2X7 receptors as a result of ADP ribosylation [17].…”

The interaction of CK2α and CK2β, the subunits of protein kinase CK2, requires CK2β in a preformed conformation and is enthalpically driven

“…but the association with CK2b significantly changes its enzymological properties, e.g., its substrate specificity (Pinna 2003), its ability to dock to membranes (Sarrouilhe et al 1998), or even to penetrate them (Rodriguez et al 2008). Moreover, CK2a-independent roles for CK2b have been suggested (Bibby and Litchfield 2005;BolanosGarcia et al 2006).…”