Calcium-dependent protein kinases (CDPKs) are involved in plant tolerance mechanisms to abiotic stresses. Although CDPKs are recognized as key messengers in signal transduction, the specific role of most members of this family remains unknown. Here we test the hypothesis that OsCPK17 plays a role in rice cold stress response by analyzing OsCPK17 knockout, silencing, and overexpressing rice lines under low temperature. Altered OsCPK17 gene expression compromises cold tolerance performance, without affecting the expression of key cold stress-inducible genes. A comparative phosphoproteomic approach led to the identification of six potential in vivo OsCPK17 targets, which are associated with sugar and nitrogen metabolism, and with osmotic regulation. To test direct interaction, in vitro kinase assays were performed, showing that the sucrose phosphate synthase OsSPS4, and the aquaporin OsPIP2;1/OsPIP2;6 are phosphorylated by OsCPK17 in a calcium-dependent manner. Altogether, our data indicates that OsCPK17 is required for a proper cold stress response in rice, likely affecting the activity of membrane channels and sugar metabolism.Rice production is severely affected by different abiotic stresses, including cold. Cold perception is mediated by calcium signals that activate kinases to elicit the adequate cellular response. In this work, we show the involvement of the rice calciumdependent protein kinase 17 (OsCPK17) in such a process. We show that altered OsCPK17 gene expression in transgenic lines affects cold tolerance performance, This article is protected by copyright. All rights reserved.and that OsCPK17 targets proteins are associated with osmotic regulation, and sugar and nitrogen metabolism.

Galectin-3 binding protein (LGALS3BP or 90 K) is a secreted glycoprotein found in human body fluids. Deregulated levels were observed in cancer and infection and its study in neurological diseases is more recent. Here, we have investigated 90 K from human cerebrospinal fluid (CSF) of patients with amyotrophic lateral sclerosis (ALS, n = 35) and other neurological diseases (n = 23). CSF was fractionated by ultrafiltration/size-exclusion chromatography (SEC) and eluted fractions were analysed by complementary techniques including immunoblotting, electron microscopy and nano-liquid chromatography-tandem mass spectrometry. A fraction of 90 K appeared as nanoparticles of irregular shape with heterogeneous dimensions of 15-60 nm that co-eluted with extracellular vesicles in SEC. Median levels of 90 K quantified by ELISA were not different between ALS patients (215.8 ng/ml) and controls (213.3 ng/ml) in contrast with the benchmark biomarker for ALS phosphoneurofilament heavy chain (1750 and 345 pg/ml, respectively). A multiregression model supported age is the only independent predictor of 90 K level in both groups (p < 0.05). Significant correlation was found between 90 K levels and age for the ALS group (r = 0.366, p = 0.031) and for all subjects (r = 0.392, p = 0.003). In conclusion, this study unveils the presence of 90 K-containing nanoparticles in human CSF and opens novel perspectives to further investigate 90 K as potential aging marker. Galectin-3-binding protein (LGALS3BP, also known as Mac-2BP or tumor-associated antigen 90 K) is a secretory protein with 567 amino acid residues 1. It is heavily N-glycosylated with seven glycosylation sites 2. In early studies 90 K was purified from supernatants of tumor cells where it appeared as large molecular mass complexes 3. Purified full-length recombinant glycoprotein from human embryonic kidney cells was found to self-associate into non-covalent oligomers of 1000-1500 kDa that appeared as ring-like structures and other shapes by electron microscopy. Recombinant 90 K also associated with collagens IV, V and VI, fibronectin and nidogen, it mediated cell adhesion and it was present in the extracellular matrix of mouse tissues 4. 90 K was visualized by electron microscopy in nanoparticles of irregular shape found in extracellular vesicle (EV) fractions from tumor cells in culture 5. More recently, asymmetric flow field-flow fractionation revealed novel nanoparticles from supernatants of tumor cells in culture termed exomeres, which contained 90 K 6. Cells from the human body as well as cells in culture release EV to the surroundings. EV include two major groups, exosomes of endosomal origin and microvesicles that derive from the plasma membrane of cells. EV can be found in body fluids, such as blood 7 or CSF 8-11 and constitute promising tools in disease diagnosis 12. In view of EV heterogeneity several isolation techniques have been used including ultracentrifugation and size-exclusion chromatography (SEC) 12 .

Plant calcium-dependent protein kinases (CDPKs) are key proteins implicated in calcium-mediated signaling pathways of a wide range of biological events in the organism. The action of each particular CDPK is strictly regulated by many mechanisms in order to ensure an accurate signal translation and the activation of the adequate response processes. In this work, we investigated the regulation of a CDPK involved in rice cold stress response, OsCPK17, to better understand its mode of action. We identified two new alternative splicing (AS) mRNA forms of OsCPK17 encoding truncated versions of the protein, missing the CDPK activation domain. We analyzed the expression patterns of all AS variants in rice tissues and examined their subcellular localization in onion epidermal cells. The results indicate that the AS of OsCPK17 putatively originates truncated forms of the protein with distinct functions, and different subcellular and tissue distributions. Additionally, we addressed the regulation of OsCPK17 by post-translational modifications in several in vitro experiments. Our analysis indicated that OsCPK17 activity depends on its structural rearrangement induced by calcium binding, and that the protein can be autophosphorylated. The identified phosphorylation sites mostly populate the OsCPK17 N-terminal domain. Exceptions are phosphosites T107 and S136 in the kinase domain and S558 in the C-terminal domain. These phosphosites seem conserved in CDPKs and may reflect a common regulatory mechanism for this protein family.

Evolution of the C 4 photosynthetic pathway involved in some cases recruitment of housekeeping proteins through gene duplication and their further neofunctionalization. NADP-malic enzyme (ME), the most widespread C 4 decarboxylase, has increased its catalytic efficiency and acquired regulatory properties that allowed it to participate in the C 4 pathway. Here, we show that regulation of maize (Zea mays) C 4-NADP-ME activity is much more elaborate than previously thought. Using mass spectrometry, we identified phosphorylation of the Ser419 residue of C 4-NADP-ME in protein extracts of maize leaves. The phosphorylation event increases in the light, with a peak at Zeitgeber time 2. Phosphorylation of ZmC 4-NADP-ME drastically decreases its activity as shown by the low residual activity of the recombinant phosphomimetic mutant. Analysis of the crystal structure of C 4-NADP-ME indicated that Ser419 is involved in the binding of NADP at the active site. Molecular dynamics simulations and effective binding energy computations indicate a less favorable binding of the cofactor NADP in the phosphomimetic and the phosphorylated variants. We propose that phosphorylation of ZmC 4-NADP-ME at Ser419 during the first hours in the light is a cellular mechanism that fine tunes the enzymatic activity to coordinate the carbon concentration mechanism with the CO 2 fixation rate, probably to avoid CO 2 leakiness from bundle sheath cells.