Channel characteristics of VDAC-3 from Arabidopsis thaliana.

Four different isoforms of the Voltage-Dependent Anion Channel (VDAC) have been identified in Arabidopsis plant cells. The electrophysiological characteristics of several VDAC channels from animal as well as plant cells are well documented, but those of this model plant are unknown. One isoform, AtVDAC-3 was obtained either directly by cell-free synthesis or produced in Escherichia coli, as inclusion bodies, and re-natured. An electrophysiological study of the purified proteins in planar lipid bilayers showed that both methods yielded proteins with similar channel activity. The characteristics of AtVDAC-3 are that of a bona fide VDAC-like channel.

Creating a bifunctional protein by insertion of beta-lactamase into the maltodextrin-binding protein.

Hybrid proteins were generated by inserting the penicillin-hydrolyzing enzyme, TEM beta-lactamase (Bla), into the maltodextrin-binding protein (MalE). The inserted Bla was functionally accommodated by MalE when it was placed within permissive sites. The maltose binding and penicillinase activities of purified hybrids were indistinguishable from those of the wild-type MalE and Bla proteins. Moreover, these hybrids displayed an additional unexpected property: maltose stabilized the active site of inserted Bla.

In vitro folding and thermodynamic stability of an antibody fragment selected in vivo for high expression levels in Escherichia coli cytoplasm.

We recently isolated a mutant of a human anti-beta-galactosidase single chain antibody fragment (scFv) able to fold at high levels in Escherichia coli cytoplasm. When targeted to the periplasm, this mutant and the wild-type scFv are both expressed at comparable levels in a soluble, active and oxidized form. If a reducing agent is added to the growth medium, only the mutant scFv is still able to fold, showing that in vivo aggregation is a direct consequence of the lack of disulphide bond formation and not of the cellular localization. In vitro denaturation/renaturation experiments show that the mutant protein is more stable than the wild-type scFv. Furthermore, refolding kinetics under reducing conditions show that the mutant folds faster than the wild-type protein. Aggregation does not proceed from the native or unfolded conformation of the protein, but from a species only present during the unfolding/refolding transition. In conclusion, the in vivo properties of the mutant scFv can be explained by, first, an increase in the stability of the protein in order to tolerate the removal of the two disulphide bonds and, second, a modification of its folding properties that reduces the kinetic competition between folding and aggregation of a reduced folding intermediate.

Probing the structural role of an alpha beta loop of maltose-binding protein by mutagenesis: heat-shock induction by loop variants of the maltose-binding protein that form periplasmic inclusion bodies.

The maltose-binding protein (MBP) of Escherichia coli is the periplasmic receptor of the maltose transport system. Previous studies have identified amino acid substitutions in an alpha/beta loop of the structure of MBP that are critical for the in vivo folding. To probe genetically the structural role of this surface loop, we generated a library in which the corresponding codons 32 and 33 of malE were mutagenized. The maltose phenotype, which correlates with a biologically active structure of MBP in the periplasm, indicated a considerable variability in the loop residues compatible with a correct in vivo folding pathway of the protein. By the same genetic screens, we characterized loop-variant MBPs associated with a defective periplasmic folding pathway and aggregated into inclusion bodies. Heat-shock induction with production of misfolded loop variants was examined using both lon-lacZ and htrA-lacZ fusions. We found that the extent of formation of inclusion bodies in the periplasm of E. coli, from misfolded loop variant MBPs, correlated with the level of heat-shock response regulated by the alternate heat-shock sigma factor, sigma 24.

Structural and functional studies of FkpA from Escherichia coli, a cis/trans peptidyl-prolyl isomerase with chaperone activity.

The protein FkpA from the periplasm of Escherichia coli exhibits both cis/trans peptidyl-prolyl isomerase (PPIase) and chaperone activities. The crystal structure of the protein has been determined in three different forms: as the full-length native molecule, as a truncated form lacking the last 21 residues, and as the same truncated form in complex with the immunosuppressant ligand, FK506. FkpA is a dimeric molecule in which the 245-residue subunit is divided into two domains. The N-terminal domain includes three helices that are interlaced with those of the other subunit to provide all inter-subunit contacts maintaining the dimeric species. The C-terminal domain, which belongs to the FK506-binding protein (FKBP) family, binds the FK506 ligand. The overall form of the dimer is V-shaped, and the different crystal structures reveal a flexibility in the relative orientation of the two C-terminal domains located at the extremities of the V. The deletion mutant FkpNL, comprising the N-terminal domain only, exists in solution as a mixture of monomeric and dimeric species, and exhibits chaperone activity. By contrast, a deletion mutant comprising the C-terminal domain only is monomeric, and although it shows PPIase activity, it is devoid of chaperone function. These results suggest that the chaperone and catalytic activities reside in the N and C-terminal domains, respectively. Accordingly, the observed mobility of the C-terminal domains of the dimeric molecule could effectively adapt these two independent folding functions of FkpA to polypeptide substrates.

Tertiary structure-dependence of misfolding substitutions in loops of the maltose-binding protein.

We previously identified and characterized amino acid substitutions in a loop connecting helix I to strand B, the alphaI/betaB loop, of the N-domain that are critical for in vivo folding of the maltose-binding protein (MalE31). The tertiary context-dependence of this mutation in MalE folding was assessed by probing the tolerance of an equivalent alphabeta loop of the C-domain to the same amino acid substitutions (MalE219). Moving the loop mutation from the N- to the C-domain eliminated the in vivo misfolding step that led to the formation of inclusion bodies. In vitro, both loop variants exhibited an important decrease of stability, but their intrinsic tendency to aggregate was well correlated with their periplasmic fates in Escherichia coli. Furthermore, the noncoincidence of the unfolding and refolding transition curves and increase of light scattering during the refolding of MalE31 indicate that a competing off-pathway reaction could occurs on the folding pathway of this variant. These results strongly support the notion that the formation of super-secondary structures of the N-domain is a rate-limiting step in the folding pathway of MalE.

Kinetic studies of the refolding of yeast phosphoglycerate kinase: comparison with the isolated engineered domains.

Unfolding and refolding kinetics of yeast phosphoglycerate kinase were studied by following the time-dependent changes of two signals: the ellipticity at 218 nm and 222 nm, and the fluorescence emission at 330 nm (following excitation at 295 nm). The protein is composed of two similar-sized structural domains. Each domain has been produced by recombinant DNA techniques. It has been previously demonstrated that the engineered isolated domains are able to fold into a quasinative structure (Minard, P., et al., 1989b, Protein Eng. 3, 55-60; Missiakas, D., Betton, J.M., Minard, P., & Yon, J.M., 1990, Biochemistry 29, 8683-8689). The behavior of the isolated domains was studied using the same two conformational probes as for the whole enzyme. We found that the refolding kinetics of each domain are multiphasic. In the whole protein, domain folding and pairing appeared to be simultaneous events. However, it was found that some refolding steps occurring during the refolding of the isolated C-domain are masked during the refolding of yeast phosphoglycerate kinase. The N-domain was also found to refold faster when it was isolated than when integrated.

Rapid translation system (RTS): a promising alternative for recombinant protein production.

Rapid Translation System (RTS) is a cell-free protein production system employing an enhanced Escherichia coli lysate to perform coupled in vitro transcription-translation reactions. A continuous supply of energy substrates, nucleotides and amino acids combined with the removal of by-products guarantees a high yield of protein production. The gene to express is either cloned into a plasmid vector or introduced as a PCR product amenable to automation. The main property of this alternative system to cellular expression systems is its open design allowing direct manipulation of the reaction conditions and applications that are impossible or difficult in cell-based systems. RTS offers new promising possibilities in the postgenomic era.

Location of tolerated insertions/deletions in the structure of the maltose binding protein.

In a previous study [(1987) J. Mol. Biol. 194, 663-673], we isolated ten insertion/deletion mutants (indels) of the maltose binding protein for which the maltose binding constant was only a little or not at all affected. In this paper, we have localized these mutations in the recently solved three-dimensional structure. Contrary to the general expectation, most of the insertion/deletion modifications occurred within elements of secondary structure. An analysis of the inserted residues for three indels found within alpha helices allowed an interpretation regarding protein structure accommodation to such modifications.

The effect of phosphate on the unfolding-refolding of phosphoglycerate kinase induced by guanidine hydrochloride.

Phosphate ions were found to stabilize the native structure of phosphoglycerate kinase without modifying the folding pathway. The transition curves obtained from different signals: enzyme activity, ellipticity at 220 nm and fluorescence intensity at 336 nm (excitation at 292 nm) are shifted to smaller guanidine hydrochloride cm values in the absence of phosphate. The kinetic characteristics are qualitatively similar, unfolding rate constants being slightly smaller in the presence of phosphate. The mechanism by which the native structure of phosphoglycerate kinase is stabilized by phosphate probably occurs upon specific phosphate binding to the nucleotide beta- or gamma-phosphate binding site of nucleotides.

Flexibility and folding of phosphoglycerate kinase.

Flexibility and folding of phosphoglycerate kinase, a two-domain monomeric enzyme, have been studied using a wide variety of methods including theoretical approaches. Mutants of yeast phosphoglycerate kinase have been prepared in order to introduce cysteinyl residues as local probes throughout the molecule without perturbating significantly the structural or the functional properties of the enzyme. The apparent reactivity of a unique cysteine in each mutant has been used to study the flexibility of PGK. The regions of larger mobility have been found around residue 183 on segment beta F in the N-domain and residue 376 on helix XII in the C-domain. These regions are also parts of the molecule which unfold first. Ligand binding induces conformational motions in the molecule, especially in the regions located in the cleft. Moreover, the results obtained by introducing a fluorescent probe covalently linked to a cysteine are in agreement with the helix scissor motion of helices 7 and 14 assumed by Blake to direct the hinge bending motion of the domains during the catalytic cycle. The folding process of both horse muscle and yeast phosphoglycerate kinases involves intermediates. These intermediates are more stable in the horse muscle than in the yeast enzyme. In both enzymes, domains behave as structural modules capable of folding and stabilizing independently, but in the horse muscle enzyme the C-domain is more stable and refolds prior to the N-domain, contrary to that which has been observed in the yeast enzyme. A direct demonstration of the independence of domains in yeast phosphoglycerate kinase has been provided following the obtention of separated domains by site-directed mutagenesis. These domains have a native-like structure and refold spontaneously after denaturation by guanidine hydrochloride.

Unfolding of proteins and long transient conformations detected by single nanopore recording.

We study the electrophoretic blockades due to entries of partially unfolded proteins into a nanopore as a function of the concentration of the denaturing agent. Short and long pore blockades are observed by electrical detection. Short blockades are due to the passage of completely unfolded proteins, their frequency increases as the concentration of the denaturing agent increases, following a sigmoidal denaturation curve. Long blockades reveal partially folded conformations. Their duration increases as the proteins are more folded. The observation of a Vogel-Fulcher law suggests a glassy behavior.

Application of the use of high-throughput technologies to the determination of protein structures of bacterial and viral pathogens.

The Structural Proteomics In Europe (SPINE) programme is aimed at the development and implementation of high-throughput technologies for the efficient structure determination of proteins of biomedical importance, such as those of bacterial and viral pathogens linked to human health. Despite the challenging nature of some of these targets, 175 novel pathogen protein structures (approximately 220 including complexes) have been determined to date. Here the impact of several technologies on the structural determination of proteins from human pathogens is illustrated with selected examples, including the parallel expression of multiple constructs, the use of standardized refolding protocols and optimized crystallization screens.

Protein folding in vitro and in the cellular environment.

The main concepts concerning protein folding have been developed from in vitro refolding studies. They state that the folding of a polypeptide chain is a spontaneous process depending only on the amino-acid sequence in a given environment. It is thermodynamically controlled and driven by the hydrophobic effect. Consequently, it has been accepted that the in vitro refolding process is a valuable model to understand the mechanisms involved during the folding of a nascent polypeptide chain in the cell. Although it does not invalidate the main rules deduced from the in vitro studies, the discovery of molecular chaperones has led to a re-evaluation of this last point. Indeed, in cells molecular chaperones are able to mediate the folding of polypeptide chains and the assembly of subunits in oligomeric proteins. The possible mechanisms by which these folding helpers act are discussed in the light of the data available in the literature. The folding process is assisted in the cell in different ways, preventing premature folding of the polypeptide chain and suppressing the incorrectly folded species and aggregates. Molecular chaperones bind to incompletely folded proteins in a conformation which suggests that the latter are in the "molten globule" state. However, very little is known about the recognition process.

In vivo assembly of active maltose binding protein from independently exported protein fragments.

The maltose binding protein (MBP or MalE) of Escherichia coli is the periplasmic component of the transport system for malto-oligosaccharides. It is synthesized in the cytoplasm with an N-terminal signal peptide that is cleaved upon export. We examined whether active MBP could assemble into an active protein in bacteria, from N- and COOH-terminal complementary protein fragments encoded by distinct, engineered segments of its structural gene. We found export and functional periplasmic assembly of MBP fragments, despite the complex polypeptide chain topology of this protein, if two conditions were satisfied. First, each of the two fragments must carry a signal peptide. Second, the boundaries between the two fragments must correspond to a permissive site within the protein. Functional assembly of active MBP occurred in five cases where these conditions were met: sites after residues 133, 161, 206, 285 and 303; but not in three other cases where the break junction corresponded to a non-permissive site: after residues 31, 120 and 339. Thus, permissive sites which were initially characterized because they could accept extensive genetic insertion/deletion modifications without loss of most biological properties provide a means of defining complementing protein fragments. This observation opens a way to study genetically the relationships between protein export and folding into the periplasm.

Folding of a mutant maltose-binding protein of Escherichia coli which forms inclusion bodies.

The maltose-binding protein (MalE) of Escherichia coli is the periplasmic component of the transport system for malto-oligosaccharides. We have examined the characteristics of a Mal- mutant of malE corresponding to the double substitution Gly32 --> Asp/Ile33 --> Pro, MalE31, previously obtained by random mutagenesis. In vivo, the MalE31 precursor is efficiently processed, but the mature protein forms inclusion bodies in the periplasm. Furthermore, the accumulation of insoluble MalE31 is independent of its cellular localization; MalE31 lacking its signal sequence forms inclusion bodies in the cytoplasm. The native MalE31 protein can be purified by affinity chromatography from inclusion bodies after denaturation by 8 M urea. The renatured protein exhibits full maltose binding affinity (Kd= 9 x 10(-7) M), suggesting that its folded structure is similar to that of the wild-type protein. Unfolding/refolding experiments show that MalE31 is less stable (-5. 5 kcal/mol) than the wild-type protein (-9.5 kcal/mol) and that folding intermediates have a high tendency to form aggregates. In conclusion, the observed phenotype of cells expressing malE31 can be explained by a defective folding pathway of the protein.

The slow-refolding step of phosphoglycerate kinase as monitored by pulse proteolysis.

The kinetics of refolding of yeast phosphoglycerate kinase were studied by following the variation in circular dichroism at 218 nm, the recovery of enzyme activity, and the susceptibility to proteolysis by trypsin and V8-protease. A very rapid phase followed by a slower one was detected by circular dichroism, which revealed the formation of secondary structures. The slower phase, with a macroscopic rate constant of 0.35 min-1, was also detected by the susceptibility of the enzyme to both proteases. It was shown that cleavage sites located in the hinge region, in a part of the C-domain and, to a lesser extent, in a region of the N-domain, which are accessible in the intermediate state, became inaccessible during the slow-refolding step of the molecule. These results demonstrate, on the one hand, the role of domains as folding intermediates, and, on the other hand, the locking of the domain structure and the domain pairing that occurs during the slow-refolding step with a rate constant of 0.35 min-1. The return of the enzyme activity occurred in a slower last step upon conformational readjustments induced by domain interactions.

Chaperone function of FkpA, a heat shock prolyl isomerase, in the periplasm of Escherichia coli.

The nature of molecular chaperones in the periplasm of Escherichia coli that assist newly translocated proteins to reach their native state has remained poorly defined. Here, we show that FkpA, a heat shock periplasmic peptidyl-prolyl cis/trans isomerase (PPIase), suppresses the formation of inclusion bodies from a defective-folding variant of the maltose-binding protein, MalE31. This chaperone-like activity of FkpA, which is independent of its PPIase activity, requires a full-length structure of the protein. In vitro, FkpA does not catalyse a slow rate-limiting step in the refolding of MalE31, but prevents its aggregation at stoichiometric amounts and promotes the reactivation of denaturated citrate synthase. We propose that FkpA functions as a chaperone for envelope proteins in the bacterial periplasm.

Detection of intermediates in the unfolding transition of phosphoglycerate kinase using limited proteolysis.

The accessibility of peptide bonds to cleavage by Staphylococcus aureus V8 protease bound on a Sepharose matrix was used as a conformational probe in the study of the unfolding-folding transition of phosphoglycerate kinase induced by guanidine hydrochloride. It was shown that the protein is resistant to proteolysis below a denaturant concentration of 0.4 M. The transition curve, determined by susceptibility toward proteolysis, was similar to that obtained following the enzyme activity [Betton et al. (1984) Biochemistry 23, 6654-6661]. Proteolysis under conditions where the folding intermediates are more populated, i.e., 0.7 M Gdn.HCl, gave two major fragments of Mr 25K and 11K, respectively. The 25K polypeptide fragment was identified as the carboxy-terminal domain. Its conformation was similar to that of a folding intermediate trapped at a critical concentration of denaturant, and in this form, it was not able to bind nucleotide substrates [Mitraki et al. (1987) Eur. J. Biochem. 163, 29-34]. From the present data and those previously reported, we concluded that the intermediate detected on the folding pathway of phosphoglycerate kinase has a partially folded carboxy-terminal domain and an unfolded amino-terminal domain.

New components of protein folding in extracytoplasmic compartments of Escherichia coli SurA, FkpA and Skp/OmpH.

A global search for extracytoplasmic folding catalysts in Escherichia coli was undertaken using different genetic systems that produce unstable or misfolded proteins in the periplasm. The extent of misfolding was monitored by the increased activity of the sigma E regulon that is specifically induced by misfolded proteins in the periplasm. Using multicopy libraries, we cloned two genes, surA and fkpA, that decreased the sigma E-dependent response constitutively induced by misfolded proteins. According to their sequences and their biochemical activities, SurA and FkpA belong to two different peptidyl prolyl isomerase (PPI) families. Interestingly, surA was also selected as a multicopy suppressor of a defined htrM (rfaD) null mutation. Such mutants produce a defective lipopolysaccharide that is unable to protect outer membrane proteins from degradation during folding. The SurA multicopy suppression effect in htrM (rfaD) mutant bacteria was directly associated with its ability to catalyse the folding of outer membrane proteins immediately after export. Finally, Tn10 insertions were isolated, which led to an increased activity of the sigma E regulon. Such insertions were mapped to the dsb genes encoding catalysts of the protein disulphide isomerase (PDI) family, as well as to the surA, fkpA and ompH/skp genes. We propose that these three proteins (SurA, FkpA and OmpH/Skp) play an active role either as folding catalysts or as chaperones in extracytoplasmic compartments.