Differential regulation of PKD isoforms in oxidative stress conditions through phosphorylation of a conserved Tyr in the P+1 loop.

Protein kinases are essential molecules in life and their crucial function requires tight regulation. Many kinases are regulated via phosphorylation within their activation loop. This loop is embedded in the activation segment, which additionally contains the Mg2+ binding loop and a P + 1 loop that is important in substrate binding. In this report, we identify Abl-mediated phosphorylation of a highly conserved Tyr residue in the P + 1 loop of protein kinase D2 (PKD2) during oxidative stress. Remarkably, we observed that the three human PKD isoforms display very different degrees of P + 1 loop Tyr phosphorylation and we identify one of the molecular determinants for this divergence. This is paralleled by a different activation mechanism of PKD1 and PKD2 during oxidative stress. Tyr phosphorylation in the P + 1 loop of PKD2 increases turnover for Syntide-2, while substrate specificity and the role of PKD2 in NF-κB signaling remain unaffected. Importantly, Tyr to Phe substitution renders the kinase inactive, jeopardizing its use as a non-phosphorylatable mutant. Since large-scale proteomics studies identified P + 1 loop Tyr phosphorylation in more than 70 Ser/Thr kinases in multiple conditions, our results do not only demonstrate differential regulation/function of PKD isoforms under oxidative stress, but also have implications for kinase regulation in general.

Trypanosoma cruzi extracellular amastigotes trigger the protein kinase D1-cortactin-actin pathway during cell invasion.

Trypanosoma cruzi extracellular amastigotes (EAs) display unique mechanisms for cell invasion that are highly dependent on host actin filaments. Protein kinase D1 (PKD1) phosphorylates and modulates the activity of cortactin, a key regulator of actin dynamics. We evaluated the role of host cortactin and PKD1 in actin filament dynamics during HeLa cell invasion by EAs. Host cortactin, PKD1 and actin are recruited by EAs based on experiments in fixed and live cells by time lapse confocal microscopy. EAs trigger PKD1 and extracellular signal-regulated kinase 1/2 activation, but not Src family kinases, and selectively phosphorylate cortactin. Heat-killed EAs and non-infective epimastigotes both triggered distinct host responses and did not recruit the molecules studied herein. EA invasion was influenced by depletion or overexpression of host cortactin and PKD1, respectively, suggesting the involvement of both proteins in this event. Collectively, these results show new host cell mechanisms subverted during EA internalization into non-phagocytic cells.

Tissue specific analysis reveals a differential organization and regulation of both ethylene biosynthesis and E8 during climacteric ripening of tomato.

BACKGROUND: Solanum lycopersicum or tomato is extensively studied with respect to the ethylene metabolism during climacteric ripening, focusing almost exclusively on fruit pericarp. In this work the ethylene biosynthesis pathway was examined in all major tomato fruit tissues: pericarp, septa, columella, placenta, locular gel and seeds. The tissue specific ethylene production rate was measured throughout fruit development, climacteric ripening and postharvest storage. All ethylene intermediate metabolites (1-aminocyclopropane-1-carboxylic acid (ACC), malonyl-ACC (MACC) and S-adenosyl-L-methionine (SAM)) and enzyme activities (ACC-oxidase (ACO) and ACC-synthase (ACS)) were assessed. RESULTS: All tissues showed a similar climacteric pattern in ethylene productions, but with a different amplitude. Profound differences were found between tissue types at the metabolic and enzymatic level. The pericarp tissue produced the highest amount of ethylene, but showed only a low ACC content and limited ACS activity, while the locular gel accumulated a lot of ACC, MACC and SAM and showed only limited ACO and ACS activity. Central tissues (septa, columella and placenta) showed a strong accumulation of ACC and MACC. These differences indicate that the ethylene biosynthesis pathway is organized and regulated in a tissue specific way. The possible role of inter- and intra-tissue transport is discussed to explain these discrepancies. Furthermore, the antagonistic relation between ACO and E8, an ethylene biosynthesis inhibiting protein, was shown to be tissue specific and developmentally regulated. In addition, ethylene inhibition by E8 is not achieved by a direct interaction between ACO and E8, as previously suggested in literature. CONCLUSIONS: The Ethylene biosynthesis pathway and E8 show a tissue specific and developmental differentiation throughout tomato fruit development and ripening.

Targeted systems biology profiling of tomato fruit reveals coordination of the Yang cycle and a distinct regulation of ethylene biosynthesis during postclimacteric ripening.

The concept of system 1 and system 2 ethylene biosynthesis during climacteric fruit ripening was initially described four decades ago. Although much is known about fruit development and climacteric ripening, little information is available about how ethylene biosynthesis is regulated during the postclimacteric phase. A targeted systems biology approach revealed a novel regulatory mechanism of ethylene biosynthesis of tomato (Solanum lycopersicum) when fruit have reached their maximal ethylene production level and which is characterized by a decline in ethylene biosynthesis. Ethylene production is shut down at the level of 1-aminocyclopropane-1-carboxylic acid oxidase. At the same time, 1-aminocyclopropane-1-carboxylic acid synthase activity increases. Analysis of the Yang cycle showed that the Yang cycle genes are regulated in a coordinated way and are highly expressed during postclimacteric ripening. Postclimacteric red tomatoes on the plant showed only a moderate regulation of 1-aminocyclopropane-1-carboxylic acid synthase and Yang cycle genes compared with the regulation in detached fruit. Treatment of red fruit with 1-methylcyclopropane and ethephon revealed that the shut-down mechanism in ethylene biosynthesis is developmentally programmed and only moderately ethylene sensitive. We propose that the termination of autocatalytic ethylene biosynthesis of system 2 in ripe fruit delays senescence and preserves the fruit until seed dispersal.

Protein kinase D regulates RhoA activity via rhotekin phosphorylation.

The members of the protein kinase D (PKD) family of serine/threonine kinases are major targets for tumor-promoting phorbol esters, G protein-coupled receptors, and activated protein kinase C isoforms (PKCs). The expanding list of cellular processes in which PKDs exert their function via phosphorylation of various substrates include proliferation, apoptosis, migration, angiogenesis, and vesicle trafficking. Therefore, identification of novel PKD substrates is necessary to understand the profound role of this kinase family in signal transduction. Here, we show that rhotekin, an effector of RhoA GTPase, is a novel substrate of PKD. We identified Ser-435 in rhotekin as the potential site targeted by PKD in vivo. Expression of a phosphomimetic S435E rhotekin mutant resulted in an increase of endogenous active RhoA GTPase levels. Phosphorylation of rhotekin by PKD2 modulates the anchoring of the RhoA in the plasma membrane. Consequently, the S435E rhotekin mutant displayed enhanced stress fiber formation when expressed in serum-starved fibroblasts. Our data thus identify a novel role of PKD as a regulator of RhoA activity and actin stress fiber formation through phosphorylation of rhotekin.

Characterization of EVL-I as a protein kinase D substrate.

EVL-I is a splice variant of EVL (Ena/VASP like protein), whose in vivo function and regulation are still poorly understood. We found that Protein Kinase D (PKD) interacts in vitro and in vivo with EVL-I and phosphorylates EVL-I in a 21 amino acid alternately-included insert in the EVH2 domain. Following knockdown of the capping protein CPbeta and spreading on laminin, phosphorylated EVL-I can support filopodia formation and the phosphorylated EVL-I is localized at filopodial tips. Furthermore, we found that the lamellipodial localization of EVL-I is unaffected by phosphorylation, but that impairment of EVL-I phosphorylation is associated with ruffling of lamellipodia upon PDBu stimulation. Besides the lamellipodial and filopodial localization of phosphorylated EVL-I in fibroblasts, we determined that EVL-I is hyperphosphorylated and localized in the cell-cell contacts of certain breast cancer cells and mouse embryo keratinocytes. Taken together, our results show that phosphorylated EVL-I is present in lamellipodia, filopodia and cell-cell contacts and suggest the existence of signaling pathways that may affect EVL-I via phosphorylation of its EVH2 domain.

Characterization of cortactin as an in vivo protein kinase D substrate: interdependence of sites and potentiation by Src.

Protein Kinase D (PKD) has been implicated in the regulation of actin turnover at the leading edge, invasion and migration. In particular, a complex between cortactin, paxillin and PKD in the invadopodia of invasive breast cancer cells has been described earlier, but so far this complex remained ill defined. Here we have investigated the possible role of PKD as a cortactin kinase. Using a mass spectrometric approach, we found that PKD phosphorylates cortactin on Ser 298 in the 6th cortactin repeat region and on Ser 348, right before the helical-proline rich domain of cortactin. We developed phosphospecific antibodies against these phosphorylated sequences, and used them as tools to follow the in vivo phosphorylation of cortactin by PKD. Examination of cortactin phosphorylation kinetics revealed that Ser 298 serves as a priming site for subsequent phosphorylation of Ser 348. Src, a well-known cortactin kinase, strongly potentiated the in vivo PKD mediated cortactin phosphorylation. This Src effect is neither mediated by pre-phosphorylation of cortactin nor by activation of PKD by Src. Phosphorylation of cortactin by PKD does not affect its subcellular localization, nor does it affect its translocation to podosomes or membrane ruffles. Moreover, there was no effect of PKD mediated cortactin phosphorylation on EGF receptor degradation and LPA induced migration. Taken together, these data establish cortactin as a novel PKD substrate and reveal a novel connection between Src and PKD.

An enzyme-linked immunosorbent assay for protein kinase D activity using phosphorylation site-specific antibodies.

The protein kinase D (PKD) family is a novel group of kinases that are involved in the regulation of cell proliferation and apoptosis, and several other physiological processes. Hence, these enzymes are attractive targets for pharmacological intervention, but no specific PKD inhibitors are known. With this in mind, we have developed a high-throughput, non-radioactive enzyme-linked immunosorbent assay (ELISA) method to monitor the PKD activity with myelin basic protein (MBP) as substrate. We determined that MBP is phosphorylated by PKD on Ser-160 and that this phosphorylation can be quantified in ELISAs, by the use of phosphorylation site-specific antibodies. Antibodies were developed that are highly specific for the MBP peptide sequence surrounding the phosphorylated Ser-160. We show that our high-throughput kinase assay is useful not only for determining the cellular PKD activity but also to screen for PKD-inhibitory compounds. Our ELISA has advantages over the current radioisotope kinase assay in terms of simplicity and environmental safety.

In vitro polarized transport of L-phenylalanine in human nasal epithelium and partial characterization of the amino acid transporters involved.

PURPOSE: The purpose of this study was to provide functional and molecular evidence to support the existence of large neutral amino acid transporters in human nasal epithelium using nasal primary cell culture model. METHODS: L-Phenylalanine was used as a model substrate to characterize carrier-mediated permeation of amino acids across human nasal epithelium. The influence of temperature, concentration, other amino acids, metabolic/transport inhibitors, and polarity/stereo-selectivity on transport of the model compound was investigated. Reverse transcriptase polymerase chain reaction was used for molecular characterization of the existence of the transporters. RESULTS: The transport of L-phenylalanine across the human nasal epithelium was polarized (apical --> basolateral >> basolateral --> apical), saturable (Km = 1.23 mM; Vmax = 805.1 nmol/mg protein/min) and stereo-selective (permeation of L-phenylalanine >> D-Phenylalanine). Its permeation was significantly (< 0.05) reduced by cationic, small and large neutral amino acids, oubain, amiloride, sodium-free medium, and temperature lowering. Reverse transcriptase polymerase chain reaction revealed the presence of the broad-scope cationic-dependent amino acid transporter gene (y+LAT-2) in the human nasal epithelium. CONCLUSIONS: Based on the results of this study, one may postulate that the human nasal epithelium expresses L-amino acid transporters. More studies are necessary for detailed characterization of the transporters.