Protein kinase CK2 modulates developmental functions of the abscisic acid responsive protein Rab17 from maize

Abstract

The maize abscisic acid responsive protein Rab17 is a highly phosphorylated late embryogenesis abundant protein involved in plant responses to stress. In this study, we provide evidence of the importance of Rab17 phosphorylation by protein kinase CK2 in growth-related processes under stress conditions. We show the specific interaction of Rab17 with the CK2 regulatory subunits CK2␤-1 and CK2␤-3, and that these interactions do not depend on the phosphorylation state of Rab17. Live-cell fluorescence imaging of both CK2 and Rab17 indicates that the intracellular dynamics of Rab17 are regulated by CK2 phosphorylation. We found both CK2␤ subunits and Rab17 distributed over the cytoplasm and nucleus. By contrast, catalytic CK2␣ subunits and a Rab17 mutant protein (mRab17) that is not a substrate for CK2 phosphorylation remain accumulated in the nucleoli. A dual-color image shows that the CK2 holoenzyme accumulates mainly in the nucleus. The importance of Rab17 phosphorylation in vivo was assessed in transgenic plants. The overexpression of Rab17, but not mRab17, arrests the process of seed germination under osmotic stress conditions. Thus, the role of Rab17 in growth processes is mediated through its phosphorylation by protein kinase CK2. T he plant hormone abscisic acid (ABA) plays a major role in adaptation to osmotic stress and induces a number of genes that encode proteins generally assumed to be involved in protecting the cell and promoting recovery from stress. Proteins responsive to ABA accumulate during seed maturation; they naturally disappear during seed germination and can be induced to reappear by ABA treatment or osmotic stress in vegetative tissues (1, 2).Late embryogenesis abundant proteins (Lea) from group 2, responsive to ABA (Rab), or dehydrins are among the most common plant proteins involved in adaptation to water or osmotic stress. Several hypothetical roles have been proposed for Rab͞dehydrin proteins based on different experimental evidence, including binding to phosphate or sulfate ions (3), nuclear localization signal (NLS) peptides (4), calcium (5), and lipid vesicles containing acidic phospholipids (6), among others. All are aimed toward a protective role as chaperones to stabilize molecules or structures under stress conditions (4,7,8).Many proteins included in this Lea family contain the S domain (8), consisting of a tract of serines with several phosphorylation sites (9). Maize Rab17 protein is one of the most heavily phosphorylated proteins in mature embryos and is found both in nucleus and cytoplasm (4). The S domain of Rab17 is followed by a protein kinase CK2 phosphorylation consensus site (9). We previously established that Rab17 is phosphorylated by CK2 in serine residues of the S domain (10). Phosphorylation͞ dephosphorylation is an important mechanism that may regulate the function of Rab17 in the cell; however, the actual physiological function for Rab͞dehydrin proteins is still unknown, and a precise understanding of the function of Rab17 and the importance of its phosphorylation...

Plant dehydrins and stress tolerance

“…In the case of DHN-5, the regulatory role may be attributed to its potential capacity to shuttle between the cytoplasm and the nucleus, 47 perhaps according to its phosphorylation status as was previously reported on the maize counterpart RAB17. 45 Finally, these findings provide for the first time that a DHN might contribute to understanding the mechanism that regulates the plant defense responses. Future work should broadly examine other DHNs to learn whether DHN-dependent regulatory mechanisms modulate pathogen responses.…”

“…[43][44][45][46][47] This phosphorylation occurring on the conformation similar to class A2 amphipathic α-helices found in apolipoproteins and α-synucleins. 21 When α-helix is formed within a K-segment, negatively charged aa (with acidic pI, e.g., D and E) lie on one side of the helix, hydrophobic aa (nonpolar, e.g., I and L) lie on the opposite side of the helix, and positively charged aa (with basic pI, e.g., K and R) lie on the polar-non polar interface.…”

Tunable Membrane Binding of the Intrinsically Disordered Dehydrin Lti30, a Cold-Induced Plant Stress Protein

“…Comparison of the different phosphorylation sites among the divergent proteins in Table 1 points again at a functional division within the dehydrin family. As demonstrated earlier, the Arabidopsis dehydrins Cor47 and Lti29 (both SK n dehydrins), but not Lti30, become phosphorylated in vitro by CKII (Riera et al, 2004;Alsheikh et al, 2005;Mouillon et al, 2008). The main reason for this different kinase selectivity is the lack of conserved multi-S-segments, which constitute the prime target for CKII activity, in the K n dehydrins, such as Lti30.…”

Phosphorylation by CK2 Enhances the Rapid Light-induced Degradation of Phytochrome Interacting Factor 1 in Arabidopsis

“…eIF2␣, eIF2␤, eIF3c, eIF4B, and eIF5) (40,41), a chromatin remodeling enzyme (histone deacetylase 2B) (40), circadian clock components (e.g. CCA1 and LHY) (33,36,37,46), HMBG proteins from maize and Arabidopsis (47), abscisic acid responsive protein Rab17 in maize (48), and positively acting transcription factors (e.g. HY5 and HFR1) involved in light signaling pathways (34,35).…”