High-level expression of functional cGMP-dependent protein kinase using the baculovirus system

A Model to Study the Phenotypic Changes of Insect Cell Transfection by Copepod Super Green Fluorescent Protein (cop-GFP) in Baculovirus Expression System

Background:

Baculovirus expression system is one of the most attractive and powerful eukaryotic expression systems for the production of recombinant proteins. The presence of a biomarker is required to monitor transfection efficiency or protein expression levels in insect cells.

Methods:

The aim of this study was to construct a baculovirus expression vector encoding a copepod super green fluorescent protein (copGFP). In this light, the resultant vector was constructed and used for transfection of Spodoptera frugiperda cells.

Results:

Expression of the copGFP protein in insect cells was confirmed by fluorescent microscopy and Western-blot analysis.

Conclusion:

The application of copGFP control bacmid can be considered as an appropriate control for insect cell transfection.

Synthetic Peptides as cGMP-Independent Activators of cGMP-Dependent Protein Kinase Iα

PKG is a multifaceted signaling molecule and potential pharmaceutical target due to its role in smooth muscle function. A helix identified in the structure of the regulatory domain of PKG Iα suggests a novel architecture of the holoenzyme. In this study, a set of synthetic peptides (S-tides), derived from this helix, was found to bind to and activate PKG Iα in a cyclic guanosine monophosphate (cGMP)-independent manner. The most potent S-tide derivative (S1.5) increased the open probability of the potassium channel KCa1.1 to levels equivalent to saturating cGMP. Introduction of S1.5 to smooth muscle cells in isolated, endothelium-denuded cerebral arteries through a modified reversible permeabilization procedure inhibited myogenic constriction. In contrast, in endothelium-intact vessels S1.5 had no effect on myogenic tone. This suggests that PKG Iα activation by S1.5 in vascular smooth muscle would be sufficient to inhibit augmented arterial contractility that frequently occurs following endothelial damage associated with cardiovascular disease.

biGBac enables rapid gene assembly for the expression of large multisubunit protein complexes

Analyses of protein complexes are facilitated by methods that enable the generation of recombinant complexes via coexpression of their subunits from multigene DNA constructs. However, low experimental throughput limits the generation of such constructs in parallel. Here we describe a method that allows up to 25 cDNAs to be assembled into a single baculoviral expression vector in only two steps. This method, called biGBac, uses computationally optimized DNA linker sequences that enable the efficient assembly of linear DNA fragments, using reactions developed by Gibson for the generation of synthetic genomes. The biGBac method uses a flexible and modular "mix and match" approach and enables the generation of baculoviruses from DNA constructs at any assembly stage. Importantly, it is simple, efficient, and fast enough to allow the manual generation of many multigene expression constructs in parallel. We have used this method to generate and characterize recombinant forms of the anaphase-promoting complex/cyclosome, cohesin, and kinetochore complexes.

Recombinant expression and reconstitution of multiprotein complexes by the USER cloning method in the insect cell-baculovirus expression system

The capacity to reconstitute complex biological processes in vitro is a crucial step in providing a quantitative understanding of these systems. It provides material for structural, biochemical and biophysical analyses and allows the testing of biological hypotheses and the introduction of chemical probes and tags for single molecule analysis. Reconstitution of these systems requires access to homogenous components, usually through their over-production in heterologous over-expression systems. Here we describe the application of the USER (Uracil-Specific Excision Reagent) ligation-free cloning method to assemble recombinant MultiBac transfer vectors for the generation of recombinant baculovirus suitable for the expression of multi-protein complexes in insect cells.

Interaction of cCMP with the cGK, cAK and MAPK Kinases in Murine Tissues

cAMP and cGMP are well established second messengers that are essential for numerous (patho)physiological processes. These purine cyclic nucleotides activate cAK and cGK, respectively. Recently, the existence of cCMP was described, and a possible function for this cyclic nucleotide was investigated. It was postulated that cCMP plays a role as a second messenger. However, the functions regulated by cCMP are mostly unknown. To elucidate probable functions, cCMP-binding and -activated proteins were identified using different methods. We investigated the effect of cCMP on purified cyclic nucleotide-dependent protein kinases and lung and jejunum tissues of wild type (WT), cGKI-knockout (cGKI KO) and cGKII-knockout (cGKII KO) mice. The catalytic activity of protein kinases was measured by a (γ-³²P) ATP kinase assay. Cyclic nucleotide-dependent protein kinases (cAK, cGKI and cGKII) in WT tissue lysates were stimulated by cCMP. In contrast, there was no stimulation of phosphorylation in KO tissue lysates. Competitive binding assays identified cAK, cGKI, and cGKII as cCMP-binding proteins. An interaction between cCMP/MAPK and a protein-protein complex of MAPK/cGK were detected via cCMP affinity chromatography and co-immunoprecipitation, respectively. These complexes were abolished or reduced in jejunum tissues from cGKI KO or cGKII KO mice. In contrast, these complexes were observed in the lung tissues from WT, cGKI KO and cGKII KO mice. Moreover, cCMP was also able to stimulate the phosphorylation of MAPK. These results suggest that MAPK signaling is regulated by cGMP-dependent protein kinases upon activation by cCMP. Based on these results, we propose that additional cCMP-dependent protein kinases that are capable of modulating MAPK signaling could exist. Hence, cCMP could potentially act as a second messenger in the cAK/cGK and MAPK signaling pathways and play an important role in physiological processes of the jejunum and lung.

Identification of nitrated tyrosine residues of protein kinase G-Iα by mass spectrometry

The nitration of tyrosine to 3-nitrotyrosine is an oxidative modification of tyrosine by nitric oxide and is associated with many diseases, and targeting of protein kinase G (PKG)-I represents a potential therapeutic strategy for pulmonary hypertension and chronic pain. The direct assignment of tyrosine residues of PKG-I has remained to be made due to the low sensitivity of the current proteomic approach. In order to assign modified tyrosine residues of PKG-I, we nitrated purified PKG-Iα expressed in insect Sf9 cells by use of peroxynitrite in vitro and analyzed the trypsin-digested fragments by matrix-assisted laser desorption/ionization-time of flight mass spectrometry and liquid chromatography-tandem mass spectrometry. Among the 21 tyrosine residues of PKG-Iα, 16 tyrosine residues were assigned in 13 fragments; and six tyrosine residues were nitrated, those at Y71, Y141, Y212, Y336, Y345, and Y567, in the peroxynitrite-treated sample. Single mutation of tyrosine residues at Y71, Y212, and Y336 to phenylalanine significantly reduced the nitration of PKG-Iα; and four mutations at Y71, Y141, Y212, and Y336 (Y4F mutant) reduced it additively. PKG-Iα activity was inhibited by peroxynitrite in a concentration-dependent manner from 30 μM to 1 mM, and this inhibition was attenuated in the Y4F mutant. These results demonstrated that PKG-Iα was nitrated at multiple tyrosine residues and that its activity was reduced by nitration of these residues.

Purification and functional analysis of protein kinase G-1α using a bacterial expression system

3',5' Cyclic guanosine monophosphate (cGMP)-dependent protein kinase G-1α (PKG-1α) is an enzyme that is a target of several anti-hypertensive and erectile dysfunction drugs. Binding of cGMP to PKG-1α produces a conformational change that leads to enzyme activation. Activated PKG-1α performs important roles both in blood vessel vasodilation and in maintaining the smooth muscle cell in a differentiated contractile state. Recombinant PKG-1α has been expressed and purified using Sf9-insect cells. However, attempts at purifying full length protein in a soluble and active form in prokaryotes have thus far been unsuccessful. These attempts have been hampered by the lack of proper eukaryotic protein folding machinery in bacteria. In this study, we report the successful expression and purification of PKG-1α using a genetically engineered Escherichia coli strain, Rosetta-gami 2(DE3), transduced with full-length human PKG-1α cDNA containing a C-terminal histidine tag. PKG-1α was purified to homogeneity using sequential nickel affinity chromatography, gel filtration and ion exchange MonoQ columns. Protein identity was confirmed by immunoblot analysis. N-terminal sequencing using Edman degradation demonstrated that the purified protein was full length. Analysis of enzyme kinetics, using a nonlinear regression curve, identified that, at constant cGMP levels (10μM) and varying ATP concentrations, PKG-1α had a maximal velocity (V(max)) of 5.02±0.25pmol/min/μg and a Michaelis-Menten constant (K(m)) of 11.78±2.68μM ATP. Recent studies have suggested that endothelial function can be attenuated by oxidative and/or nitrosative stress but the role of PKG-1α under these conditions is unclear. We found that PKG-1α enzyme activity was attenuated by exposure to the NO donor, spermine NONOate, hydrogen peroxide, and peroxynitrite but not by superoxide, suggesting that the attenuation of PKG-1α activity may be an under-appreciated mechanism underlying the development of endothelial dysfunction in a number of cardiovascular diseases.

Dual wavelength photoactivation of cAMP- and cGMP-dependent protein kinase signaling pathways

The spatial and temporal organization of biological systems offers a level of complexity that is challenging to probe with conventional reagents. Photoactivatable (caged) compounds represent one strategy by which spatiotemporal organizational complexities can be addressed. However, since the vast majority of caged species are triggered by UV light, it is not feasible to orthogonally control two or more spatiotemporal elements of the phenomenon under investigation. For example, the cGMP- and cAMP-dependent protein kinases are highly homologous enzymes, separated in time and space, which mediate the phosphorylation of both distinct and common protein substrates. However, current technology is unable to discriminate, in a temporally or spatially selective fashion, between these enzymes and/or the pathways they influence. We describe herein the intracellular triggering of a cGMP-mediated pathway with 360 nm light and the corresponding cAMP-mediated pathway with 440 nm light. Dual wavelength photoactivation was assessed in A10 cells by monitoring the phosphorylation of vasodilator-stimulated phosphoprotein (VASP), a known substrate for both the cAMP- and cGMP-dependent protein kinases. Illumination at 440 nm elicits a cAMP-dependent phosphorylation of VASP at Ser157, whereas 360 nm exposure triggers the phosphorylation of both Ser157 and Ser239. This is the first example of wavelength-distinct activation of two separate nodes of a common signaling pathway.

Cyclic cytidine 3',5'-monophosphate (cCMP) signals via cGMP kinase I

We analysed the function and intracellular signalling of the cyclic pyrimidinic nucleotide cCMP. The membrane-permeable cCMP analogue dibutyryl-cCMP mediated mouse aorta relaxation. cCMP activated purified cGMP-dependent protein kinase (cGK) Iα and Iβ and stimulated cGK in aorta lysates. cCMP-induced relaxation was abolished in cGKI-knockout tissue. Additionally, deletion of inositol-trisphosphate receptor associated cGKI substrate (IRAG) suppressed cCMP-mediated relaxation. Signalling of cCMP via cGKI/IRAG appears to be of broader physiological importance because cCMP-mediated inhibition of platelet aggregation was absent in cGKI- and IRAG-deficient platelets. These results demonstrate that cCMP acts as intracellular messenger molecule, most unexpectedly utilizing the cGMP signal transduction pathway.

Improved expression of secreted and membrane-targeted proteins in insect cells

Secretory and membrane-bound proteins are generally produced in lower amounts in insect cells compared with cytoplasmic and nuclear proteins. There may be many reasons for this, including degradation of recombinant proteins by proteases, competition for cellular resources between native and recombinant proteins, and physical blockage of the secretory pathways. In the present study, we describe the construction of a baculovirus in which chiA (chitinase) and cath (cathepsin) genes have been deleted and show improved recombinant protein expression using this vector. We confirmed the complete removal of both genes by PCR, restriction enzyme analysis and enzyme assays, and the modified virus DNA was shown to be stable in bacterial cells over multiple passages. A selection of recombinant genes were inserted into the double-deletion virus and their expression levels compared with recombinant viruses that had single or no gene deletions. In all instances, the double-deletion viruses showed greatly enhanced levels of protein production for both secreted and nuclear/cytoplasmic proteins. In summary, we have conclusively demonstrated the importance of this deletion vector for the high-level production of recombinant proteins.

Surface-plasmon-resonance-based chemical proteomics: efficient specific extraction and semiquantitative identification of cyclic nucleotide-binding proteins from cellular lysates by using a combination of surface plasmon resonance, sequential elution and liquid chromatography-tandem mass spectrometry

Chemical proteomics is a powerful methodology for identifying the cellular targets of small molecules, however, it is biased towards abundant proteins. Therefore, quantitative strategies are needed to distinguish between specific and nonspecific interactions. Here, we explore the potential of the combination of surface plasmon resonance (SPR) coupled to liquid chromatography-tandem mass spectrometry (LC-MS/MS) as an alternative approach in chemical proteomics. We coupled cGMP molecules to the SPR chip, and monitored the binding and dissociation of proteins from a human lysate by using sequential elution steps and SPR. The eluted proteins were subsequently identified by LC-MS/MS. Our approach enabled the efficient and selective extraction of low-abundant cyclic-nucleotide-binding proteins such as cGMP-dependent protein kinase, and a quantitative assessment of the less- and nonspecific competitive binding proteins. The data show that SPR-based chemical proteomics is a promising alternative for the efficient specific extraction and quantitative identification of small-molecule-binding proteins from complex mixtures.

The hinge region operates as a stability switch in cGMP-dependent protein kinase I alpha

The molecular mechanism of cGMP-dependent protein kinase activation by its allosteric regulator cyclic-3',5'-guanosine monophosphate (cGMP) has been intensely studied. However, the structural as well as thermodynamic changes upon binding of cGMP to type I cGMP-dependent protein kinase are not fully understood. Here we report a cGMP-induced shift of Gibbs free enthalpy (DeltaDeltaGD) of 2.5 kJ.mol-1 as determined from changes in tryptophan fluorescence using urea-induced unfolding for bovine PKG Ialpha. However, this apparent increase in overall stability specifically excluded the N-terminal region of the kinase. Analyses of tryptic cleavage patterns using liquid chromatography-coupled ESI-TOF mass spectrometry and SDS/PAGE revealed that cGMP binding destabilizes the N-terminus at the hinge region, centered around residue 77, while the C-terminus was protected from degradation. Furthermore, two recombinantly expressed mutants: the deletion fragment Delta1-77 and the trypsin resistant mutant Arg77Leu (R77L) revealed that the labile nature of the N-terminus is primarily associated with the hinge region. The R77L mutation not only stabilized the N-terminus but extended a stabilizing effect on the remaining domains of the enzyme as well. These findings support the concept that the hinge region of PKG acts as a stability switch.

The role of a parasite-specific allosteric site in the distinctive activation behavior of Eimeria tenella cGMP-dependent protein kinase

A cGMP-dependent protein kinase (PKG) was recently identified as an anticoccidial target for the apicomplexan parasite Eimeria tenella [Gurnett, A., Liberator, P. A., Dulski, P., Salowe, S., Donald, R. G. K., Anderson, J., Wiltsie, J., Diaz, C., Harris, G., Chang, B., Darkin-Rattray, S. J., Nare, B., Crumley, T., Blum, P., Misura, A., Tamas, T., Sardana, M., Yuan, J., Biftu, T., and Schmatz, D. (2002) J. Biol. Chem. (in press)]. Unlike the PKGs of higher organisms that have two cGMP binding sites in their regulatory domain, the PKG from Eimeria tenella (Et-PKG) contains three putative cGMP binding sites and has distinctive activation properties, including a very large stimulation by cGMP ( approximately 1000-fold) with significant cooperativity (Hill coefficient of 1.7). During our investigation of Et-PKG activation, we found that 8-substituted cGMP analogues are weak partial activators. For example, 8-NBD-cGMP provides a maximal stimulation of activity of only 20-fold with little evident cooperativity, although cGMP can synergize with the analogue to provide full activation. The results suggest that partial activation is a consequence of restricted binding of 8-NBD-cGMP to a subset of cGMP sites in the enzyme. Site-directed mutagenesis of conserved arginine and glutamate residues in the parasite-specific third cGMP site confirms that this site is an important functional participant in the allosteric regulation of the kinase and that it exhibits very high selectivity against 8-NBD-cGMP. Since the results are consistent with full activation of Et-PKG requiring cyclic nucleotide binding in all three allosteric sites, one role for the additional cGMP site may be to establish a stricter regulatory mechanism for the kinase activity than is present in the PKGs of higher organisms containing only two allosteric sites.

The large conductance, voltage-dependent, and calcium-sensitive K+ channel, Hslo, is a target of cGMP-dependent protein kinase phosphorylation in vivo

Native large conductance, voltage-dependent, and Ca2+-sensitive K+ channels are activated by cGMP-dependent protein kinase. Two possible mechanisms of kinase action have been proposed: 1) direct phosphorylation of the channel and 2) indirect via PKG-dependent activation of a phosphatase. To scrutinize the first possibility, at the molecular level, we used the human pore-forming alpha-subunit of the Ca2+-sensitive K+ channel, Hslo, and the alpha-isoform of cGMP-dependent protein kinase I. In cell-attached patches of oocytes co-expressing the Hslo channel and the kinase, 8-Br-cGMP significantly increased the macroscopic currents. This increase in current was due to an increase in the channel voltage sensitivity by approximately 20 mV and was reversed by alkaline phosphatase treatment after patch excision. In inside-out patches, however, the effect of purified kinase was negative in 12 of 13 patches. In contrast, and consistent with the intact cell experiments, purified kinase applied to the cytoplasmic side of reconstituted channels increased their open probability. This stimulatory effect was absent when heat-denatured kinase was used. Biochemical experiments show that the purified kinase incorporates gamma-33P into the immunopurified Hslo band of approximately 125 kDa. Furthermore, in vivo phosphorylation largely attenuates this labeling in back-phosphorylation experiments. These results demonstrate that the alpha-subunit of large conductance Ca2+-sensitive K+ channels is substrate for G-Ialpha kinase in vivo and support direct phosphorylation as a mechanism for PKG-Ialpha-induced activation of maxi-K channels.

Identification of the amino acid sequences responsible for high affinity activation of cGMP kinase Ialpha

The cGMP-dependent protein kinases (cGK) Ialpha and Ibeta have identical cGMP binding sites and catalytic domains. However, differences in their first 100 amino acids result in 15-fold different activation constants for cGMP. We constructed chimeras to identify those amino acid sequences that contribute to the high affinity cGK Ialpha and low affinity cGK Ibeta phenotype. The cGK Ialpha/Ibeta chimeras contained permutations of six amino-terminal regions (S1-S6) including the leucine zipper (S2), the autoinhibitory domain (S4), and the hinge domain (S5, S6). The exchange of S2 along with S4 switched the phenotype from cGK Ialpha to cGK Ibeta and vice versa, suggesting that the domains with the highest homology between the two isozymes determine their affinity for cGMP. The high affinity cGK Ialpha phenotype was also obtained by a specific substitution within the hinge domain. Chimeras with the sequence of cGK Ialpha in S5 and cGK Ibeta in S6 were activated at up to 6-fold lower cGMP concentrations than cGK Ialpha. Based on the activation constants of all chimeras constructed, empirical weighting factors have been calculated that quantitatively describe the contribution of the individual amino-terminal domains S1-S6 to the high affinity cGK Ialpha phenotype.

Expression, purification, and characterization of the cGMP-dependent protein kinases I beta and II using the baculovirus system

Detailed studies of differences in distinct cGMP kinase isoforms are highly dependent on expression of large amounts of these enzyme isoforms that are not easily purified by conventional methods. Here cGMP-dependent protein kinases, the type I beta soluble form from human placenta, and the type II membrane-associated form from rat intestine, were each expressed in a baculovirus/Sf9 cell system and purified in milligram amounts by affinity chromatography. The expressed recombinant proteins displayed characteristics like those of their native counterparts. cGK I beta was expressed as a 76 kDa protein predominantly found in the cytosol fraction, whereas cGK II was expressed as an 86 kDa protein predominantly associated with the membrane fraction. The apparent Ka and Vmax of cGMP for activation of cGK I beta were 0.5 microM and 3.4 mumol/min/mg, and for cGK II were 0.04 microM and 1.8 mumol/min/mg.

Expression of the catalytic domain of cyclic GMP-dependent protein kinase in a baculovirus system

The Type I cGMP-dependent protein kinase catalytic domain (residues 336-671 from the I alpha isoform) has been expressed as a cGMP independent kinase in a baculovirus system. Using peptide substrates, the protein retains similar substrate specificity as the native holoenzyme. The recombinant catalytic domain catalyzes the phosphorylation of histone, but does not display the inhibition using non-substrate histones which has been described for the holoenzyme. The catalytic domain is an active kinase in mammalian cells also since vascular smooth muscle cells transfected with the cDNA encoding the catalytic domain display altered morphology. The catalytic domain of G-kinase may be a useful tool for delineating the role of cGMP-mediated protein phosphorylation in cell systems.