Inferences concerning the ATPase properties of DnaK and other HSP70s are affected by the ADP kinase activity of copurifying nucleoside-diphosphate kinase

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins, composed of Gα, Gβ, and Gγ subunits, are a class of signal transduction complexes with broad roles in human health and agriculturally relevant plant physiological and developmental traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. We sought to develop improved methods for heterologous expression and rapid purification of Gα subunits, initially targeting GPA1, the sole canonical Gα subunit of the model plant species, Arabidopsis thaliana. Compared to conventional methods, our expression methodology and rapid StrepII-tag mediated purification facilitates substantially higher yield, and isolation of protein with increased GTP binding and hydrolysis activities. Human GNAI1 purified using our approach displayed the expected binding and hydrolysis activities, indicating our protocol is applicable to mammalian Gα subunits, potentially including those for which purification of enzymatically active protein has been historically problematic. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate that the inherent instability of GPA1 is a function of the interaction between the Ras and helical domains. Additionally, we found that GPA1-GNAO1 domain swaps partially uncouple the instability from the rapid nucleotide binding kinetics displayed by GPA1. In summary, our work provides insights into methods to optimally study heterotrimeric G proteins, and reveals roles of the helical domain in Gα kinetics and stability.

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins are a class of signal transduction complexes with broad roles in human health and agriculturally important plant traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. Using the Arabidopsis thaliana Gα subunit, GPA1, we developed a rapid StrepII-tag mediated purification method that facilitates isolation of protein with increased enzymatic activities as compared to conventional methods, and is demonstrably also applicable to mammalian Gα subunits. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate the instability of recombinant GPA1 is a function of the interaction between the Ras and helical domains, and can be partially uncoupled from the rapid nucleotide binding kinetics displayed by GPA1.

DnaK promotes autophosphorylation of DYRK1A and its family kinases in Escherichia coli-based cell-free protein expression

Dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) is one of the drug target kinases involved in neurological disorders. DYRK1A phosphorylates substrate proteins related to disease progression in an intermolecular manner. Meanwhile, DYRK1A intramolecularly phosphorylates its own residues on key segments during folding process, which is required for its activation and stabilization. To reproduce the autophosphorylation in vitro, DYRK1A was expressed in Escherichia coli-based cell-free protein synthesis system. Although this system was useful for investigating autophosphorylation of serine residue at position 97 (Ser97) in DYRK1A, only a small fraction of the synthesized protein was successfully autophosphorylated. In this study, we found that the addition of DnaK, a bacterial HSP70 chaperone, to cell-free expression of DYRK1A promoted its Ser97 autophosphorylation. Structure prediction with AlphaFold2 indicates that Ser97 forms a hydrogen bond within an α-helix structure, indicating a possibility that DnaK unfolds the α-helix and maintains the structure around Ser97 in a conformation susceptible to phosphorylation. In addition, DnaK promoted phosphorylation of DYRK1B and HIPK2, but not DYRK2 and DYRK4, suggesting a sequence selectivity in the action of DnaK. This study provides a facile method for promoting autophosphorylation of DYRK family kinases in cell-free protein expression.

Regulation of the metastasis suppressor Nm23-H1 by tumor viruses

Metastasis is the most common cause of cancer mortality. To increase the survival of patients, it is necessary to develop more effective methods for treating as well as preventing metastatic diseases. Recent advancement of knowledge in cancer metastasis provides the basis for development of targeted molecular therapeutics aimed at the tumor cell or its interaction with the host microenvironment. Metastasis suppressor genes (MSGs) are promising targets for inhibition of the metastasis process. During the past decade, functional significance of these genes, their regulatory pathways, and related downstream effector molecules have become a major focus of cancer research. Nm23-H1, first in the family of Nm23 human homologues, is a well-characterized, anti-metastatic factor linked with a large number of human malignancies. Mounting evidence to date suggests an important role for Nm23-H1 in reducing virus-induced tumor cell motility and migration. A detailed understanding of the molecular association between oncogenic viral antigens with Nm23-H1 may reveal the underlying mechanisms for tumor virus-associated malignancies. In this review, we will focus on the recent advances to our understanding of the molecular basis of oncogenic virus-induced progression of tumor metastasis by deregulation of Nm23-H1.

Substrate specificity and nucleotides binding properties of NM23H2/nucleoside diphosphate kinase homolog from Plasmodium falciparum

Nucleoside diphosphate kinases (NDKs) play a key role in maintaining the intracellular energy resources as well as the balance of nucleotide pools. Recently, attention has been directed to NDKs owing to its role in activating various chemotherapeutic agents. The binding affinity of different nucleotides with P. falciparum NDK was varied according to the following order ADP ~ GDP > dGDP > dADP > dTDP > CDP > dCDP > UDP. The binding of purines nucleotides was stronger than pyrimidines. Furthermore, PfNDK showed more preferences to ribonucleotides over deoxyribonucleotides. Pyrimidines showed lower negative free energy compared with that of purines. The interaction of all nucleotides showed favorable enthalpic and entropic terms. However, the enthalpic terms were the main deriving forces for purine nucleotides, while the entropic contributions were the predominant forces for pyrimidines. Interestingly, TDP showed marked affinity and more favorable enthalpic and less entropic contributions. In conclusion, the size of nucleotide was the critical factor in PfNDK ligand affinity.

Directed evolution of the DnaK chaperone: mutations in the lid domain result in enhanced chaperone activity

We improved the DnaK molecular chaperone system for increased folding efficiency towards two target proteins, by using a multi-parameter screening procedure. First, we used a folding-deficient C-terminal truncated chloramphenicol acetyl transferase (CAT_Cd9) to obtain tunable selective pressure for enhanced DnaK chaperon function in vivo. Second, we screened selected clones in vitro for CAT_Cd9 activity after growth under selective pressure. We then analyzed how these variants performed as compared to wild type DnaK towards folding assistance of a second target protein; namely, chemically denatured firefly luciferase. A total of 11 single point DnaK mutants and 1 truncated variant were identified using CAT_Cd9 as the protein target, while 4 of the 12 selected variants showed improved luciferase refolding in vitro. This shows that improving the DnaK chaperone by using a certain target substrate protein, does not necessarily result in a loss or reduction in its ability to assist other proteins. Of the 12 identified mutations, half were clustered in the nucleotide binding domain, and half in the lid domain (LD) of DnaK. The truncated variant is characterized by a 35-residue C-terminal truncation (Cd35) and exhibited the highest improvement for luciferase refolding. Cd35 showed a 7-fold increase in initial refolding rate for denatured luciferase and resulted in a 5-fold increase in maximal luminescence as compared to wild type DnaK. Given that the best in vitro performing mutants contained LD substitutions, and that the LD is not involved in ATP binding, ATP hydrolysis or client protein association, but is involved in allosteric regulation of the chaperone cycle, we propose that improved DnaK variants result in changes to allosteric domain communication, ultimately retuning the ATP-dependent chaperone cycle.

ATPase domain of Hsp70 exhibits intrinsic ATP-ADP exchange activity

The chaperone activity of Hsp70 in protein folding and its conformational switching are regulated through the hydrolysis of ATP and the ATP-ADP exchange cycle. It was reported that, in the presence of physiological concentrations of ATP (approximately 5 mM) and ADP (approximately 0.5 mM), Hsp70 catalyzes ATP-ADP exchange through transfer of gamma-phosphate between ATP and ADP, via an autophosphorylated intermediate, whereas it only catalyzes the hydrolysis of ATP in the absence of ADP. To clarify the functional domain of the ATP-ADP exchange activity of Hsp70, we isolated the 44-kD ATPase domain of Hsp70 after limited proteolysis with alpha-chymotrypsin (EC 3.4.21.1). The possibility of ATP-ADP exchange activity of a contaminating nucleoside diphosphate kinase (EC 2.7.4.6) was monitored throughout the experiments. The purified 44-kD ATPase domain exhibited intrinsic ATP-ADP exchange by catalyzing the transfer of gamma-phosphate between ATP and ADP with acid-stable autophosphorylation at Thr204.

Complementation studies of the DnaK-DnaJ-GrpE chaperone machineries from Vibrio harveyi and Escherichia coli, both in vivo and in vitro

The marine bacterium Vibrio harveyi is a potential indicator organism for evaluating marine environmental pollution. The DnaK-DnaJ-GrpE chaperone machinery of V. harveyi has been studied as a model of response to stress conditions and compared to the Escherichia coli DnaK system. The genes encoding DnaK, DnaJ and GrpE of V. harveyi were cloned into expression vectors and grpE was sequenced. It was found that V. harveyi possesses a unique organization of the hsp gene cluster (grpE-gltP-dnaK-dnaJ), which is present exclusively in marine Vibrio species. In vivo experiments showed that suppression of the E. coli dnaK mutation by V. harveyi DnaK protein was weak or absent, while suppression of the dnaJ and grpE mutations by V. harveyi DnaJ and GrpE proteins was efficient. These results suggest higher species-specificity of the DnaK chaperone than the GrpE and DnaJ cochaperones. Proteins of the DnaK chaperone machinery of V. harveyi were purified to homogeneity and their efficient cooperation with the E. coli chaperones in the luciferase refolding reaction and in stimulation of DnaK ATPase activity was demonstrated. Compared to the E. coli system, the purified DnaK-DnaJ-GrpE system of V. harveyi exhibited about 20% lower chaperoning activity in the luciferase reactivation assay. ATPase activity of V. harveyi DnaK protein was at least twofold higher than that of the E. coli model DnaK but its stimulation by the cochaperones DnaJ and GrpE was significantly (10 times) weaker. These results indicate that, despite their high structural identity (approximately 80%) and similar mechanisms of action, the DnaK chaperones of closely related V. harveyi and E.coli bacteria differ functionally.

SwoHp, a nucleoside diphosphate kinase, is essential in Aspergillus nidulans

The temperature-sensitive swoH1 mutant of Aspergillus nidulans was previously identified in a screen for mutants with defects in polar growth. In the present work, we found that the swoH1 mutant swelled, lysed, and did not produce conidia during extended incubation at the restrictive temperature. When shifted from the permissive to the restrictive temperature, swoH1 showed the temperature-sensitive swelling phenotype only after 8 h at the higher temperature. The swoH gene was mapped to chromosome II and cloned by complementation of the temperature-sensitive phenotype. The sequence showed that swoH encodes a homologue of nucleoside diphosphate kinases (NDKs) from other organisms. Deletion experiments showed that the swoH gene is essential. A hemagglutinin-SwoHp fusion complemented the mutant phenotype, and the purified fusion protein possessed phosphate transferase activity in thin-layer chromatography assays. Sequencing of the mutant allele showed a predicted V83F change. Structural modeling suggested that the swoH1 mutation would lead to perturbation of the NDK active site. Crude cell extracts from the swoH1 mutant grown at the permissive temperature had approximately 20% of the NDK activity seen in the wild type and did not show any decrease in activity when assayed at higher temperatures. Though the data are not conclusive, the lack of temperature-sensitive NDK activity in the swoH1 mutant raises the intriguing possibility that the SwoH NDK is required for growth at elevated temperatures rather than for polarity maintenance.

The second metal-binding site of 70 kDa heat-shock protein is essential for ADP binding, ATP hydrolysis and ATP synthesis

The chaperone activity of Hsp70 (70 kDa heat-shock protein) in protein folding and its conformational switch, including oligomeric and monomeric interconversion, are regulated by the hydrolysis of ATP and the ATP-ADP exchange cycle. The crystal structure of human ATPase domain shows two metal-binding sites, the first for ATP binding and a second, in close proximity to the first, whose function remains unknown [Sriram, Osipiuk, Freeman, Morimoto and Joachimiak (1997) Structure 5, 403-414]. In this study, we have characterized the second metal-binding motif by site-directed mutagenesis and the kinetics of ATP and ADP binding, and found that the second metal-binding site, comprising a loop co-ordinated by His-227, Glu-231 and Asp-232, participates both in ATP hydrolysis and ATP-synthetic activities, in co-operation with the first metal-binding site. The first metal-binding site, a catalytic centre, is essential for ATP binding and the second site for ADP binding in the reactions of ATP hydrolysis and ATP synthesis.

Functional similarities and differences of an archaeal Hsp70(DnaK) stress protein compared with its homologue from the bacterium Escherichia coli

Archaea are prokaryotes but some of their chaperoning systems resemble those of eukaryotes. Also, not all archaea possess the stress protein Hsp70(DnaK), in contrast with bacteria and eukaryotes, which possess it without any known exception. Further, the primary structure of the archaeal DnaK resembles more the bacterial than the eukaryotic homologues. The work reported here addresses two questions: Is the archaeal Hsp70 protein a chaperone, like its homologues in the other two phylogenetic domains? And, if so, is the chaperoning mechanism of bacterial or eukaryotic type? The data have shown that the DnaK protein of the archaeon Methanosarcina mazei functions efficiently as a chaperone in luciferase renaturation in vitro, and that it requires DnaJ, and the other bacterial-type chaperone, GrpE, to perform its function. The M. mazei DnaK chaperone activity was enhanced by interaction with the bacterial co-chaperone DnaJ, but not by the eukaryotic homologue HDJ-2. Both the bacterial GrpE and DnaJ stimulated the ATPase activity of the M. mazei DnaK. The M. mazei DnaK-dependent chaperoning pathway in vitro is similar to that of the bacterium Escherichia coli used for comparison. However, in vivo analyses indicate that there are also significant differences. The M. mazei dnaJ and grpE genes rescued E.coli mutants lacking these genes, but E.coli dnaK mutants were not complemented by the M. mazei dnaK gene. Thus, while the data from in vitro tests demonstrate functional similarities between the M. mazei and E.coli DnaK proteins, in vivo results indicate that, intracellularly, the chaperones from the two species differ.

Arabidopsis NDK1 is a Component of ROS Signaling by Interacting with Three Catalases

Plants sense various environmental stimuli and have specific signaling pathways to respond to these cues. We focused on light responsive components and found that NDKs were phosphorylated specifically after red light irradiation in Pisum sativum [Tanaka et al. (1998) J. Photochem. Photobiol. B 45: 113] and after blue light irradiation in Neurospora crassa [Oda and Hasunuma (1997) Mol. Gen. Genet. 256: 593, Ogura et al. (2001) J. Biol. Chem. 276: 21228]. We performed yeast two-hybrid screening using AtNDK1, the counterpart of NDK-P1 (Pisum sativum NDK1) in Arabidopsis, as bait, and isolated catalase3 (AtCat3). Interactions between AtNDK1-AtCAT1 and AtNDK1-AtCAT2 were also detected with the two-hybrid system. Non-denaturing two-dimensional gel electrophoresis of crude extracts from plants revealed that catalase and NDK activities co-migrated in the same area of the gel. Transgenic plants expressing AtNDK1 under control of the CaMV 35S promoter exhibited tolerance to paraquat and high ability to eliminate exogenous H2O2. These results indicate that AtNDK1 has a role in ROS response.

Characterization of a trap mutant of the AAA+ chaperone ClpB

The AAA+ protein ClpB mediates the solubilization of protein aggregates in cooperation with the DnaK chaperone system (KJE). The order of action of ClpB and KJE on aggregated proteins is unknown. We describe a ClpB variant with mutational alterations in the Walker B motif of both AAA domains (E279A/E678A), which binds but does not hydrolyze ATP. This variant associates in vitro and in vivo in a stable manner with protein substrates, demonstrating direct interaction of ClpB with protein aggregates for the first time. Substrate interaction is strictly dependent on ATP binding to both AAA domains of ClpB. The unique substrate binding properties of the double Walker B variant allowed to dissect the order of ClpB and DnaK action during disaggregation reactions. ClpB-E279A/E678A outcompetes the DnaK system for binding to the model substrate TrfA and inhibits the dissociation of small protein aggregates by DnaK only, indicating that ClpB acts prior to DnaK on protein substrates.

Point mutations affecting the oligomeric structure of Nm23-H1 abrogates its inhibitory activity on colonization and invasion of prostate cancer cells

In order to identify Nm23-H1's structural motifs influencing its metastasis-inhibitory activity, we transfected DU 145 human prostate carcinoma cells with the expression vector encoding the Nm23-H1 protein with mutations at the following amino acids: serine-44, a phosphorylation site; proline-96, a site corresponding to the k-pn mutation that causes developmental defects in Drosophila; and serine-120, a site of mutation in human neuroblastoma and phosphorylation. Significant decrease in colonization in soft agar and invasiveness of DU 145 cells was observed in the wild type nm23-H1 transfectants, and also in the serine-44 and serine-120 to alanine mutant nm23-H1-transfected cell lines. However, the k-pn type proline-96 to serine (P96S) and neuroblastoma type serine-120 to glycine (S120G) mutations of Nm23-H1 abrogated its inhibitory activity on colonization and invasion. Meanwhile, all of the recombinant mutant Nm23-H1 proteins produced in Escherichia coli exhibited NDP kinase activity levels at the wild type protein, although the P96S and S120G mutant proteins exhibited decreased histidine protein kinase activity and autophosphorylation level, respectively. Interestingly, only two of the mutant recombinant Nm23-H1 proteins examined, P96S and S120G, exhibited reduced hexameric and increased dimeric oligomerization relative to the wild type. These correlative data suggest that the metastasis-suppressing activity of Nm23-H1 may depend on its oligomeric structure, but not on its NDP kinase activity.

NDP kinase 2 interacts with two oxidative stress-activated MAPKs to regulate cellular redox state and enhances multiple stress tolerance in transgenic plants

NDP kinases (NDPKs) are multifunctional proteins that regulate a variety of eukaryotic cellular activities, including cell proliferation, development, and differentiation. However, much less is known about the functional significance of NDPKs in plants. We show here that NDPK is associated with H(2)O(2)-mediated mitogen-activated protein kinase signaling in plants. H(2)O(2) stress strongly induces the expression of the NDPK2 gene in Arabidopsis thaliana (AtNDPK2). Proteins from transgenic plants overexpressing AtNDPK2 showed high levels of autophosphorylation and NDPK activity, and they have lower levels of reactive oxygen species (ROS) than wild-type plants. Mutants lacking AtNDPK2 had higher levels of ROS than wild type. H(2)O(2) treatment induced the phosphorylation of two endogenous proteins whose molecular weights suggested they are AtMPK3 and AtMPK6, two H(2)O(2)-activated A. thaliana mitogen-activated protein kinases. In the absence of H(2)O(2) treatment, phosphorylation of these proteins was slightly elevated in plants overexpressing AtNDPK2 but markedly decreased in the AtNDPK2 deletion mutant. Yeast two-hybrid and in vitro protein pull-down assays revealed that AtNDPK2 specifically interacts with AtMPK3 and AtMPK6. Furthermore, AtNDPK2 also enhances the myelin basic protein phosphorylation activity of AtMPK3 in vitro. Finally, constitutive overexpression of AtNDPK2 in Arabidopsis plants conferred an enhanced tolerance to multiple environmental stresses that elicit ROS accumulation in situ. Thus, AtNDPK2 appears to play a previously uncharacterized regulatory role in H(2)O(2)-mediated MAPK signaling in plants.

Nm23-H1 metastasis suppressor phosphorylation of kinase suppressor of Ras via a histidine protein kinase pathway

The metastasis-suppressive activity of Nm23-H1 was previously correlated with its in vitro histidine protein kinase activity, but physiological substrates have not been identified. We hypothesized that proteins that interact with histidine kinases throughout evolution may represent partners for Nm23-H1 and focused on the interaction of Arabidopsis "two-component" histidine kinase ERS with CTR1. A mammalian homolog of CTR1 was previously reported to be c-Raf; we now report that CTR1 also exhibits homology to the kinase suppressor of Ras (KSR), a scaffold protein for the mitogen-activated protein kinase (MAPK) cascade. Nm23-H1 co-immunoprecipitated KSR from lysates of transiently transfected 293T cells and at endogenous protein expression levels in MDA-MB-435 breast carcinoma cells. Autophosphorylated recombinant Nm23-H1 phosphorylated KSR in vitro. Phosphoamino acid analysis identified serine as the major target, and two peaks of Nm23-H1 phosphorylation were identified upon high performance liquid chromatography analysis of KSR tryptic peptides. Using site-directed mutagenesis, we found that Nm23-H1 phosphorylated KSR serine 392, a 14-3-3-binding site, as well as serine 434 when serine 392 was mutated. Phosphorylated MAPK but not total MAPK levels were reduced in an nm23-H1 transfectant of MDA-MB-435 cells. The data identify a complex in vitro histidine-to-serine protein kinase pathway, which may contribute to signal transduction and metastasis.

Hsc62, Hsc56, and GrpE, the third Hsp70 chaperone system of Escherichia coli

Hsc62 is the third Hsp70 homolog of Escherichia coli, which we found previously. Hsc62 is structurally and biochemically similar to DnaK, but hscC gene encoding Hsc62 did not compensate for the defects in the dnaK-null mutant of E. coli MC4100 strain. We cloned the ybeV gene and purified the gene product named Hsc56, a 55,687-Da protein with a J-domain like sequence. Hsc56 stimulated the ATPase activity of only Hsc62 but not those of the other Hsp70 homologs, DnaK and Hsc66. Hsc56 contains the -His-Pro-Glu- sequence corresponding to the His-Pro-Asp motif in DnaJ, which is indispensable for DnaJ to interact with DnaK. Conversion of -His-Pro-Glu- to -Ala-Ala-Ala- abolished the ability of Hsc56 to stimulate the ATPase activity of Hsc62. GrpE, a nucleotide exchange factor for DnaK, also stimulated the ATPase activity of Hsc62 in the presence of Hsc56. Hsc62-Hsc56-GrpE is probably a new Hsp70 chaperone system of E. coli.

ATPase-defective derivatives of Escherichia coli DnaK that behave differently with respect to ATP-induced conformational change and peptide release

We have characterized the effects of the T199S, T199A, and K70A mutations on the biochemical activity and in vivo functioning of Escherichia coli DnaK. Threonine-199 is the site of autophosphorylation of DnaK, and the lysine residue of bovine Hsc70 corresponding to K70 of DnaK has been shown to be essential for the hydrolysis of ATP. The dnaK alleles T199A and K70A are completely unable, and the T199S allele is only partially able, to complement the defects of a DeltadnaK mutant. The ATPase activities of the DnaK T199A and DnaK K70A proteins are nearly abolished, while the ATPase activity of the DnaK T199S protein has a steady-state rate similar to that of wild-type DnaK. The DnaK T199S protein also retains approximately 13% of the autophosphorylation activity of wild-type DnaK, while the autophosphorylation activities of the T199A and K70A derivatives are completely abolished. All four DnaK proteins bind a model peptide substrate, and the wild-type, T199A, and T199S DnaK proteins release the peptide with similar kinetics upon the addition of ATP. The DnaK K70A protein, in contrast, does not release the peptide upon the addition of ATP. ATP induces a conformational change in the wild-type, T199A, and T199S DnaK proteins but not in the DnaK K70A protein. The T199A and K70A mutations both disrupt the ATPase activity of DnaK but have profoundly different effects on the ATP-induced conformational change and peptide release activities of DnaK, implying that the two mutations affect different steps in the functional cycle of DnaK. The DnaK T199S protein represents a new class of DnaK mutant, one which has near-normal levels of ATPase activity and undergoes an ATP-induced conformational change that results in the release of peptide but which is not able to fully complement loss of DnaK function in the cell.

Heat stress response in pea involves interaction of mitochondrial nucleoside diphosphate kinase with a novel 86-kilodalton protein

In this work we have further characterized the first mitochondrial nucleoside diphosphate kinase (mtNDPK) isolated from plants. The mitochondrial isoform was found to be especially abundant in reproductive and young tissues. Expression of the pea (Pisum sativum L. cv Oregon sugarpod) mtNDPK was not affected by different stress conditions. However, the pea mtNDPK was found to interact with a novel 86-kD protein, which is de novo synthesized in pea leaves upon exposure to heat. Thus, we have evidence for the involvement of mtNDPK in mitochondrial heat response in pea in vivo. Studies on oligomerization revealed that mtNDPK was found in complexes of various sizes, corresponding to the sizes of e.g. hexamers, tetramers, and dimers, indicating flexibility in oligomerization. This flexibility, also found for other NDPK isoforms, has been correlated with the ability of this enzyme to interact with other proteins. We believe that the mtNDPK is involved in heat stress response in pea, possibly as a modulator of the 86-kD protein.

Secretion of nucleoside diphosphate kinase by mucoid Pseudomonas aeruginosa 8821: involvement of a carboxy-terminal motif in secretion

Nucleoside diphosphate kinase (Ndk) is a ubiquitous enzyme which functions in balancing the nucleotide pool of the cell. We have recently reported that in addition to being intracellular in both mucoid and nonmucoid Pseudomonas aeruginosa, Ndk is also secreted into the extracellular environment by mucoid P. aeruginosa cells. This secreted Ndk has biochemical activity similar to the intracellular Ndk and is 16 kDa in size. To demonstrate that Ndk is indeed secreted and to localize the secretion motif, we constructed an ndk knockout mutant, which lacks both intracellular and extracellular forms of Ndk. In this study, we report the construction of deletion derivatives made from the carboxy-terminal region of Ndk. These deletion derivatives were introduced into the ndk::Cm knockout mutant and were examined for the intracellular and extracellular presence of Ndk. It was observed that the carboxy-terminal 8-amino-acid region is required for the secretion of Ndk into the extracellular region. This region has the sequence DXXX, where X is a predominantly hydrophobic residue. Such sequences represent a conserved motif in proteins secreted by the type I secretory pathway in gram-negative microorganisms. To investigate the significance of this motif in the secretion of Ndk, we constructed a fusion protein of Ndk and the blue fluorescent protein (BFP) as well as a fusion protein of mutated Ndk (whose DTEV motif has been changed to AAAA) and the BFP. The presence of extracellular Ndk was detected only in the ndk::Cm knockout mutant harboring the wild-type BFP-Ndk protein fusion. We could not detect the presence of extracellular Ndk in the ndk::Cm knockout mutant containing the mutated BFP-Ndk protein fusion. In addition, we have also used immunofluorescence microscopy to localize the wild-type and mutated BFP-Ndk proteins in the cell. The significance of these observations is discussed.

NM23/nucleoside diphosphate kinase and signal transduction

NM23s (or NDP kinases) regulate a fascinating variety of cellular activities, including proliferation, development, and differentiation. All these processes are modulated by external stimuli, leading to the idea that this family of proteins modulates transmembrane signaling pathways. This review summarizes the evidence indicating that NM23/NDP kinases participate in transmembrane signaling in eukaryotic cells and discusses the molecular mechanisms proposed to account for these actions.