Protein tyrosine phosphorylation plays an important role in cell growth, development and oncogenesis. No classical protein tyrosine kinase has hitherto been cloned from plants. Does protein tyrosine kinase exist in plants? To address this, we have performed a genomic survey of protein tyrosine kinase motifs in plants using the delineated tyrosine phosphorylation motifs from the animal system. The Arabidopsis thaliana genome encodes 57 different protein kinases that have tyrosine kinase motifs. Animal non-receptor tyrosine kinases, SRC, ABL, LYN, FES, SEK, KIN and RAS have structural relationship with putative plant tyrosine kinases. In an extended analysis, animal receptor and non-receptor kinases, Raf and Ras kinases, mixed lineage kinases and plant serine/threonine/tyrosine (STY) protein kinases, form a well-supported group sharing a common origin within the superfamily of STY kinases. We report that plants lack bona fide tyrosine kinases, which raise an intriguing possibility that tyrosine phosphorylation is carried out by dual-specificity STY protein kinases in plants. The distribution pattern of STY protein kinase families on Arabidopsis chromosomes indicates that this gene family is partly a consequence of duplication and reshuffling of the Arabidopsis genome and of the generation of tandem repeats. Genome-wide analysis is supported by the functional expression and characterization of At2g24360 and phosphoproteomics of Arabidopsis. Evidence for tyrosine phosphorylated proteins is provided by alkaline hydrolysis, anti-phosphotyrosine immunoblotting, phosphoamino acid analysis and peptide mass fingerprinting. These results report the first comprehensive survey of genome-wide and tyrosine phosphoproteome analysis of plant STY protein kinases.
Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase. We have previously reported that Cdk5 participates in the regulation of nociceptive signaling, and the expression of Cdk5 and its activator, p35, are up-regulated in nociceptive neurons during peripheral inflammation. The aim of our current study was to identify the proinflammatory molecules that regulate Cdk5/p35 activity in response to inflammation. We constructed a vector that contains the mouse p35 promoter driving luciferase expression. We transiently transfected this vector in PC12 cells to test the effect of several cytokines on p35 transcriptional activity and Cdk5 activity. Our results indicate that tumor necrosis factor-␣ (TNF-␣) activates p35 promoter activity in a dose-and time-dependent manner and concomitantly up-regulates Cdk5 activity. Because TNF-␣ is known to activate ERK1/2, p38 MAPK, JNK, and NF-B signaling pathways, we examined their involvement in the activation of p35 promoter activity. MEK inhibitor, which inhibits ERK activation, decreased p35 promoter activity, whereas the inhibitors of p38 MAPK, JNK, and NF-〉 increased p35 promoter activity, indicating that these pathways regulate p35 expression differently. The mRNA and protein levels of Egr-1, a transcription factor, were increased by TNF-␣ treatment, and this increase was dependent on ERK signaling. In a mouse model of inflammation-induced pain in which carrageenan injection into the hind paw causes hypersensitivity to heat stimuli, TNF-␣ mRNA was increased at the site of injection. These findings suggest that TNF-␣-mediated regulation of Cdk5 activity plays an important role in inflammation-induced pain signaling.
Leucine-rich repeat kinase 2 (LRRK2) has been associated with Parkinson's disease (PD) and other disorders. However, its normal physiological functions and pathogenic properties remain elusive. Here we show that LRRK2 regulates the anterograde ER-Golgi transport through anchoring Sec16A at the endoplasmic reticulum exit sites (ERES). LRRK2 interacted and co-localized with Sec16A, a key protein in the formation of ERES. Lrrk2 depletion caused a dispersion of Sec16A from ERES and impaired ER export. In neurons, LRRK2 and Sec16A showed extensive co-localization at the dendritic ERES (dERES) that locally regulate the transport of proteins to the dendritic spines. A loss of Lrrk2 affected the association of Sec16A with dERES and impaired the activity-dependent targeting of glutamate receptors onto the cell/synapse surface. Furthermore, the PD-related LRRK2 R1441C missense mutation in the GTPase domain interfered with the interaction of LRRK2 with Sec16A and also affected ER-Golgi transport, while LRRK2 kinase activity was not required for these functions. Therefore, our findings reveal a new physiological function of LRRK2 in ER-Golgi transport, suggesting ERES dysfunction may contribute to the pathogenesis of PD.
The activity of Cdk5-p35 is tightly regulated in the developing and mature nervous system. Stress-induced cleavage of the activator p35 to p25 and a p10 N-terminal domain induces deregulated Cdk5 hyperactivity and perikaryal aggregations of hyperphosphorylated Tau and neurofilaments, pathogenic hallmarks in neurodegenerative diseases, such as Alzheimer disease and amyotrophic lateral sclerosis, respectively. Previously, we identified a 125-residue truncated fragment of p35 called CIP that effectively and specifically inhibited Cdk5-p25 activity and Tau hyperphosphorylation induced by A peptides in vitro, in HEK293 cells, and in neuronal cells. Although these results offer a possible therapeutic approach to those neurodegenerative diseases assumed to derive from Cdk5-p25 hyperactivity and/or A induced pathology, CIP is too large for successful therapeutic regimens. To identify a smaller, more effective peptide, in this study we prepared a 24-residue peptide, p5, spanning CIP residues Lys 245 -Ala 277 . p5 more effectively inhibited Cdk5-p25 activity than did CIP in vitro. In neuron cells, p5 inhibited deregulated Cdk5-p25 activity but had no effect on the activity of endogenous Cdk5-p35 or on any related endogenous cyclin-dependent kinases in HEK293 cells. Specificity of p5 inhibition in cortical neurons may depend on the p10 domain in p35, which is absent in p25. Furthermore, we have demonstrated that p5 reduced A(1-42)-induced Tau hyperphosphorylation and apoptosis in cortical neurons. These results suggest that p5 peptide may be a unique and useful candidate for therapeutic studies of certain neurodegenerative diseases.The activity of Cdk5, a multifunctional serine/threonine kinase, is critical for neuronal development and synaptic activity; it sustains neurite outgrowth, neuronal migration, cortical lamination, and survival (1-9). Its activity depends on the binding of its neuron-specific, cyclin-related activators, p35 and p39 (10, 11). Cdk5 has also been implicated as a key player in learning and memory (12-15).Normally, Cdk5 activity is tightly regulated, but under conditions of neuronal stress, it is deregulated, leading to hyperactivity, neuronal pathology, and cell death. Accordingly, Cdk5 has been implicated in certain neurodegenerative disorders, such as AD.2 A model of the role of Cdk5 in neurodegeneration suggests that a stress-induced influx of calcium ions into neurons activates calpain, a Ca 2ϩ -activated protease, which cleaves p35 into p25 and a p10 fragment. p25, in turn, forms a more stable Cdk5-p25 hyperactive complex, which hyperphosphorylates Tau and induces cell death (16 -21). Indeed, increased levels of p25 and Cdk5 activity have been reported in AD brains. The finding that p25 may be toxic comes from studies of cortical neurons treated with -amyloid (A), a key marker of AD pathology, where p35 is converted to p25 accompanied by activated Cdk5, Tau hyperphosphorylation, and apoptosis (22,23).Expression of the Cdk5-p25 complex seems to be primarily responsible for the Tau pathology ...
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