The P22-like bacteriophages have short tails. Their virions bind to their polysaccharide receptors through six trimeric tailspike proteins that surround the tail tip. These short tails also have a trimeric needle protein that extends beyond the tailspikes from the center of the tail tip, in a position that suggests that it should make first contact with the host’s outer membrane during the infection process. The base of the needle serves as a plug that keeps the DNA in the virion, but role of the needle during adsorption and DNA injection is not well understood. Among the P22-like phages are needle types with two completely different C-terminal distal tip domains. In the phage Sf6-type needle, unlike the other P22-type needle, the distal tip folds into a “knob” with a TNF-like fold, similar to the fiber knobs of bacteriophage PRD1 and Adenovirus. The phage HS1 knob is very similar to that of Sf6, and we report here its crystal structure which, like the Sf6 knob, contains three bound L-glutamate molecules. A chimeric P22 phage with a tail needle that contains the HS1 terminal knob efficiently infects the P22 host, Salmonella enterica, suggesting the knob does not confer host specificity. Likewise, mutations that should abrogate the binding of L-glutamate to the needle do not appear to affect virion function, but several different other genetic changes to the tip of the needle slow down potassium release from the host during infection. These findings suggest that the needle plays a role in phage P22 DNA delivery by controlling the kinetics of DNA ejection into the host.
Each chain of the native trimeric P22 tailspike protein has eight cysteines that are reduced and buried in its hydrophobic core. However, disulfide bonds have been observed in the folding pathway and they are believed to play a critical role in the registration of the three chains. Interestingly, in the presence of sodium dodecyl sulfate (SDS) only monomeric chains, rather than disulfide-linked oligomers, have been observed from a mixture of folding intermediates. Here we show that when the oligomeric folding intermediates were separated from the monomer by native gel electrophoresis, the reduction of intermolecular disulfide bonds did not occur in the subsequent second-dimension SDS–gel electrophoresis. This result suggests that when tailspike monomer is present in free solution with SDS, the partially unfolded tailspike monomer can facilitate the reduction of disulfide bonds in the tailspike oligomers.
P22 tailspike protein (TSP); transient disulfide bond; assembly; disulfide shuffling; SDS
The host cell recognition protein of the Escherichia coli bacteriophage HK620 is a large homotrimeric tailspike that cleaves the O18A1 type O antigen. The crystal structure of HK620 tailspike determined in the apo and substrate-bound form is reported by Barbirz et al. in this issue of Molecular Microbiology. Lacking detectable sequence similarity, the fold and overall organization of the HK620 tailspike are similar to those of the tailspikes of the related phages P22 and Sf6. The substrate-binding site is intra-subunit in P22 and HK620 tailspikes, but inter-subunit in Sf6, demonstrating how phages can adapt the same protein fold to recognize different substrates.
tailspike; evolution; polysaccharide; hydrolase; bacteriophage
Newly synthesized proteins must form their native structures in the crowded environment of the cell, while avoiding non-native conformations that can lead to aggregation. Yet remarkably little is known about the progressive folding of polypeptide chains during chain synthesis by the ribosome, or of the influence of this folding environment on productive folding in vivo. P22 tailspike is a homotrimeric protein that is prone to aggregation via misfolding of its central β-helix domain in vitro. We have produced stalled ribosome:tailspike nascent chain complexes of four fixed lengths in vivo, in order to assess co-translational folding of newly synthesized tailspike chains as a function of chain length. Partially synthesized, ribosome-bound nascent tailspike chains populate stable conformations with some native-state structural features even prior to the appearance of the entire β-helix domain, regardless of the presence of the chaperone trigger factor, yet these conformations are distinct from the conformations of released, refolded tailspike truncations. These results suggest that organization of the aggregation-prone β-helix domain occurs co-translationally, prior to chain release, to a conformation that is distinct from the accessible energy minimum conformation for the truncated free chain in solution.
An unexpected aspect of the expression of cloned genes is the frequent failure of newly synthesized polypeptide chains to reach their native state, accumulating instead as insoluble inclusion bodies. Amyloid deposits represent a related state associated with a variety of human diseases. The critical folding intermediates at the juncture of productive folding and the off-pathway aggregation reaction have been identified for the phage P22 tailspike and coat proteins. Though the parallel β coil tailspike is thermostable, an early intracellular folding intermediate is thermolabile. As the temperature of intracellular folding is increased, this species partitions to inclusion bodies, a kinetic trap within the cell. The earliest intermediates along the in vitro aggregation pathway, sequential multimers of the thermolabile folding intermediates, have been directly identified by native gel electrophoresis. Temperature-sensitive folding (tsf) mutations identify sites in the β coil domain, which direct the junctional intermediate down the productive pathway. Global suppressors of tsf mutants inhibit the pathway to inclusion bodies, rescuing the mutant chains. These mutants identify sites important for avoiding aggregation. Coat folding intermediates also partition to inclusion bodies as temperature is increased. Coat tsf mutants are suppressed by overexpression of the GroE chaperonin, indicating that the thermolabile intermediate is a physiological substrate for GroE. We suggest that many proteins are likely to have thermolabile intermediates in their intracellular folding pathways, which will be precursors to inclusion body formation at elevated temperatures and therefore substrates for heat shock chaperonins.
inclusion body; protein folding; chaperones; aggregation
The tailed bacteriophage φ29 has 12 “appendages” (gene product 12, gp12) attached to its neck region that participate in host cell recognition and entry. In the cell, monomeric gp12 undergoes proteolytic processing that releases the C-terminal domain during assembly into trimers. We report here crystal structures of the protein before and after catalytic processing and show that the C-terminal domain of gp12 is an “auto-chaperone” that aids trimerization. We also show that auto-cleavage of the C-terminal domain is a post-trimerization event that is followed by a unique ATP-dependent release. The post-translationally modified N-terminal part has three domains that function to attach the appendages to the phage, digest the cell wall teichoic acids and bind irreversibly to the host, respectively. Structural and sequence comparisons suggest that some eukaryotic and bacterial viruses as well as bacterial adhesins might have similar maturation mechanism as is performed by φ29 gp12 for Bacillus subtilis.
X-ray crystallography; infection; tailspike; receptor
The P22 tailspike protein folds by forming a folding competent monomer species that forms a dimeric, then a non-native trimeric (protrimer) species by addition of folding competent monomers. We have found three residues, R549, R563, and D572, which play a critical role in both the stability of the native tailspike protein and assembly and maturation of the protrimer. King and colleagues reported previously that substitution of R563 to glutamine inhibited protrimer formation. We now show that the R549Q and R563K variants significantly delay the protrimer-to-trimer transition both in vivo and in vitro. Previously, variants that destabilize intermediates have shown wild-type chemical stability. Interestingly, both the R549Q and R563K variants destabilize the tailspike trimer in guanidine denaturation studies, indicating that they represent a new class of tailspike folding variants. R549Q has a midpoint of unfolding at 3.2Mguanidine, compared to 5.6Mfor the wild-type tailspike protein, while R563K has a midpoint of unfolding of 1.8 M. R549Q and R563K also denature over a broader pH range than the wild-type tailspike protein and both proteins have increased sensitivity to pH during refolding, suggesting that both residues are involved in ionic interactions. Our model is that R563 and D572 interact to stabilize the adjacent turn, aiding the assembly of the dimer and protrimer species. We believe that the interaction between R563 and D572 is also critical following assembly of the protrimer to properly orient D572 in order to form a salt bridge with R549 during protrimer maturation.
P22 tailspike; protein folding; stability; mutation; ionic interaction
Mutations in the tailspike gene (gene 9) of Salmonella typhimurium phage P22 have been used to identify amino acid interactions during the folding of a polypeptide chain. Since temperature-sensitive folding (tsf) mutations cause folding defects in the P22 tailspike polypeptide chain, it is likely that mutants derived from these and correcting the original tsf defects (second-site intragenic suppressors) identify interactions during the folding pathway. We report the isolation and identification of second-site revertants to tsf mutants.
Using single particle electron cryomicroscopy that does not impose icosahedral averaging, we determined the structure of the entire infectious Salmonella phage Epsilon151, including both icosahedral and non-icosahedral components. At least three layers of condensed viral DNA were observed to pack in coaxial coils with local 25 Å hexagonal inter-strand spacing. At one of the five-fold vertices, a portal complex with twelve subunits replaces a capsid pentamer. A tail hub with six projecting trimeric tailspikes sits on the external face of the portal. Below the portal is a cylindrical protein core. An extended shaft of density fills the central channel of the protein core and the portal complex and appears to consist of about 90 nucleotides at the terminus of the packaged DNA poised for injection. Using an icosahedral symmetry imposed reconstruction, the fold of the capsid shell protein is seen to resemble the capsid protein fold of other tailed double-stranded DNA phages2–5 and human herpesvirus6. These common structural features suggest a common evolutionary origin among these viruses.
Phages are promising alternatives to antibodies as the biorecognition element in a variety of biosensing applications. In this study, a monolayer of bacteriophage P22 whose tailspike proteins specifically recognize Salmonella serotypes was covalently bound to glass substrates through a bifunctional cross linker 3-aminopropyltrimethoxysilane. The specific binding of Salmonella typhimurium to the phage monolayer was studied by enzyme-linked immunosorbent assay and atomic force microscopy. Escherichia coli and a Gram-positive bacterium Listeria monocytogenes were also studied as control bacteria. The P22 particles show strong binding affinity to Salmonella typhimurium. In addition, the dried P22 monolayer maintained 50% binding capacity to Salmonella typhimurium after a one-week storage time. This is a promising method to prepare phage monolayer coatings on surface plasmon resonance and acoustic biosensor substrates in order to utilize the nascent phage display technology.
bacteriophage; Salmonella typhimurium; lipopolysaccharide membrane
One of the major causes of morbidity and mortality in man and economically important animals is bacterial infections of the gastrointestinal (GI) tract. The emergence of difficult-to-treat infections, primarily caused by antibiotic resistant bacteria, demands for alternatives to antibiotic therapy. Currently, one of the emerging therapeutic alternatives is the use of lytic bacteriophages. In an effort to exploit the target specificity and therapeutic potential of bacteriophages, we examined the utility of bacteriophage tailspike proteins (Tsps). Among the best-characterized Tsps is that from the Podoviridae P22 bacteriophage, which recognizes the lipopolysaccharides of Salmonella enterica serovar Typhimurium. In this study, we utilized a truncated, functionally equivalent version of the P22 tailspike protein, P22sTsp, as a prototype to demonstrate the therapeutic potential of Tsps in the GI tract of chickens. Bacterial agglutination assays showed that P22sTsp was capable of agglutinating S. Typhimurium at levels similar to antibodies and incubating the Tsp with chicken GI fluids showed no proteolytic activity against the Tsp. Testing P22sTsp against the three major GI proteases showed that P22sTsp was resistant to trypsin and partially to chymotrypsin, but sensitive to pepsin. However, in formulated form for oral administration, P22sTsp was resistant to all three proteases. When administered orally to chickens, P22sTsp significantly reduced Salmonella colonization in the gut and its further penetration into internal organs. In in vitro assays, P22sTsp effectively retarded Salmonella motility, a factor implicated in bacterial colonization and invasion, suggesting that the in vivo decolonization ability of P22sTsp may, at least in part, be due to its ability to interfere with motility… Our findings show promise in terms of opening novel Tsp-based oral therapeutic approaches against bacterial infections in production animals and potentially in humans.
Myelination of the central nervous system (CNS) is critical to vertebrate nervous systems for efficient neural signaling. CNS myelination occurs as oligodendrocytes terminally differentiate, a process regulated in part by the myelin regulatory factor, MYRF. Using bioinformatics and extensive biochemical and functional assays, we find that MYRF is generated as an integral membrane protein that must be processed to release its transcription factor domain from the membrane. In contrast to most membrane-bound transcription factors, MYRF proteolysis seems constitutive and independent of cell- and tissue-type, as we demonstrate by reconstitution in E. coli and yeast. The apparent absence of physiological cues raises the question as to how and why MYRF is processed. By using computational methods capable of recognizing extremely divergent sequence homology, we identified a MYRF protein domain distantly related to bacteriophage tailspike proteins. Although occurring in otherwise unrelated proteins, the phage domains are known to chaperone the tailspike proteins' trimerization and auto-cleavage, raising the hypothesis that the MYRF domain might contribute to a novel activation method for a membrane-bound transcription factor. We find that the MYRF domain indeed serves as an intramolecular chaperone that facilitates MYRF trimerization and proteolysis. Functional assays confirm that the chaperone domain-mediated auto-proteolysis is essential both for MYRF's transcriptional activity and its ability to promote oligodendrocyte maturation. This work thus reveals a previously unknown key step in CNS myelination. These data also reconcile conflicting observations of this protein family, different members of which have been identified as transmembrane or nuclear proteins. Finally, our data illustrate a remarkable evolutionary repurposing between bacteriophages and eukaryotes, with a chaperone domain capable of catalyzing trimerization-dependent auto-proteolysis in two entirely distinct protein and cellular contexts, in one case participating in bacteriophage tailspike maturation and in the other activating a key transcription factor for CNS myelination.
Membrane-bound transcription factors are synthesized as integral membrane proteins, but are proteolytically cleaved in response to relevant cues, untethering their transcription factor domains from the membrane to control gene expression in the nucleus. Here, we find that the myelin regulatory factor MYRF, a major transcriptional regulator of oligodendrocyte differentiation and central nervous system myelination, is also a membrane-bound transcription factor. In marked contrast to most well-known membrane-bound transcription factors, cleavage of MYRF appears to be unconditional. Surprisingly, this processing is performed by a protein domain shared with bacteriophages in otherwise unrelated proteins, where the domain is critical to the folding and proteolytic maturation of virus tailspikes. In addition to revealing a previously unknown key step in central nervous system myelination, this work also illustrates a remarkable example of evolutionary repurposing between bacteriophages and eukaryotes, with the same protein domain capable of catalyzing trimerization-dependent auto-proteolysis in two completely distinct protein and cellular contexts.
Biogenesis of a superfamily of surface structures by gram-negative bacteria requires the chaperone/usher pathway, a terminal branch of the general secretory pathway. In this pathway a periplasmic chaperone works together with an outer membrane usher to direct substrate folding, assembly, and secretion to the cell surface. We analyzed the structure and function of the PapC usher required for P pilus biogenesis by uropathogenic Escherichia coli. Structural analysis indicated PapC folds as a β-barrel with short extracellular loops and extensive periplasmic domains. Several periplasmic regions were localized, including two domains containing conserved cysteine pairs. Functional analysis of deletion mutants revealed that the PapC C terminus was not required for insertion of the usher into the outer membrane or for proper folding. The usher C terminus was not necessary for interaction with chaperone-subunit complexes in vitro but was required for pilus biogenesis in vivo. Interestingly, coexpression of PapC C-terminal truncation mutants with the chromosomal fim gene cluster coding for type 1 pili allowed P pilus biogenesis in vivo. These studies suggest that chaperone-subunit complexes target an N-terminal domain of the usher and that subunit assembly into pili depends on a subsequent function provided by the usher C terminus.
The proteasome is the most complex protease known, with a molecular mass of approximately 3 MDa and 33 distinct subunits. Recent studies reported the discovery of four chaperones that promote the assembly of a 19-subunit subcomplex of the proteasome known as the regulatory particle, or RP. These and other findings define a new and highly unusual macromolecular assembly pathway. The RP mediates substrate selection by the proteasome and injects substrates into the core particle (CP) to be degraded. A heterohexameric ring of ATPases, the Rpt proteins, is critical for RP function. These ATPases abut the CP and their C-terminal tails help to stabilize the RP-CP interface. ATPase heterodimers bound to the chaperone proteins are early intermediates in assembly of the ATPase ring. The four chaperones have the common feature of binding the C-domains of Rpt proteins, apparently a remarkable example of convergent evolution; each chaperone binds a specific Rpt subunit. The C-domains are distinct from the C-terminal tails but proximal to them. Some but probably not all of the RP chaperones appear to compete with CP for binding of the Rpt proteins, as a result of the proximity of the tails to the C-domain. This competition may underlie the release mechanism for these chaperones. Genetic studies in yeast point to the importance of the interaction between the CP and the Rpt tails in assembly, and a recent biochemical study in mammals suggests that RP assembly takes place on pre-assembled CP. These results do not exclude a parallel, CP-independent pathway of assembly. Ongoing work should soon clarify the roles of both the CP and the four chaperones in RP assembly.
proteasome; assembly; regulatory particle; AAA protein; chaperone
Mycoplasma pneumoniae is a wall-less human respiratory tract pathogen that colonizes mucosal epithelium via a polar terminal organelle having a central electron-dense core and adhesin-related proteins clustered at a terminal button. A mutant lacking J-domain co-chaperone TopJ is noncytadherent and nonmotile, despite having a core and normal levels of the major cytadherence-associated proteins. J-domain co-chaperones work with DnaK to catalyze polypeptide binding and subsequent protein folding. Here we compared features of the topJ mutant with other cytadherence mutants to elucidate the contribution of TopJ to cytadherence function. The topJ mutant was similar ultrastructurally to a non-cytadherent mutant lacking terminal organelle proteins B/C, including aberrant core positioning and cell morphology in thin sections, but exhibited a hybrid satellite growth pattern with features of mutants both having and lacking a core. Time-lapse images of mycoplasmas expressing a YFP fusion with terminal organelle protein P41 suggested that terminal organelle formation/positioning was delayed or poorly coordinated with cell growth in the absence of TopJ. TopJ required a core for localization, perhaps involving HMW1. P1 trypsin accessibility on other non-cytadherent mutants was significantly enhanced over wild-type but unexpectedly was reduced with topJ mutant cells, suggesting impaired processing, translocation, and / or folding of this adhesin.
J-domain; co-chaperone; cytoskeleton; cytadherence; terminal organelle
The Plasmodium falciparum cysteine proteases falcipain-2 and falcipain-3 are major hemoglobinases and potential antimalarial drug targets. Our previous studies demonstrated that these enzymes are equipped with specific domains for specific functions. Structural and functional analysis of falcipains showed that they have unique domains including a refolding domain and a hemoglobin binding domain. As with many proteases, falcipain-2 and falcipain-3 are synthesized as inactive zymogens. However, it is not known how these enzymes get activated for hemoglobin hydrolysis. In this study, we are presenting the first evidence that salt bridges and hydrophobic interactions are required for the auto activation of cysteine proteases of P.falciparum. To investigate the mechanism of activation of these enzymes, we expressed the wild type protein as well as different mutants in E.coli. Refolding was assessed by circular dichroism. Both CD and trans activation data showed that the wild type enzymes and mutants are rich in secondary structures with similar folds. Our study revealed that prodomain-mature domain of falcipain-2 and falcipain-3 interacts via salt bridges and hydrophobic interactions. We mutated specific residues of falcipain-2 and falcipain-3, and evaluated their ability to undergo auto processing. Mutagenesis result showed that two salt bridges (Arg 185 - Glu 221, Glu 210 - Lys 403) in falcipain-2, and one salt bridge (Arg 202-Glu 238) in falcipain-3, play crucial roles in the activation of these enzymes. Further study revealed that hydrophobic interactions present both in falcipain-2 (Phe214, Trp449 Trp 453) and falcipain-3 (Phe 231 Trp 457 Trp 461) also play important roles in the activation of these enzymes. Our results revealed the interactions involved in auto processing of two major hemoglobinases of malaria parasite.
Although manipulation of the endoplasmic reticulum (ER) folding environment in the yeast Saccharomyces cerevisiae has been shown to increase the secretory productivity of recombinant proteins, the cellular interactions and processes of native enzymes and chaperones such as protein disulfide isomerase (PDI) are still unclear. Previously, we reported that overexpression of the ER chaperone PDI enabled up to a three-fold increase in secretion levels of the Pyrococcus furiosus β-glucosidase in the yeast S. cerevisiae. This result was surprising since β-glucosidase contains only one cysteine per monomer and no disulfide bonds. Two possible mechanisms were proposed: PDI either forms a transient disulfide bond with the lone cysteine residue of the nascent β-glucosidase during the folding and assembly process, or it acts as a chaperone to aid in proper folding. To discern between the two mechanisms, the single cysteine residue was mutated to serine, and the secretion of the two protein variants was determined. The serine mutant still showed increased secretion in vivo when PDI levels were elevated. When the folding bottleneck is removed by increasing expression temperatures to 37°C rather than 30°C, PDI no longer has an improvement on secretion. These results suggest that, unexpectedly, PDI acts in a chaperone-like capacity or possibly cooperates with the cell's folding or degradation mechanisms regardless of whether the protein is redox-active.
Archaea; hyperthermophile; Saccharomyces cerevisiae; protein disulfide isomerase; β-glucosidase; Pyrococcus furiosus; CelB; chaperones
Bacterial CopZ proteins deliver copper to P1B-type Cu+-ATPases that are homologous to the human Wilson and Menkes disease proteins. The genome of the hyperthermophile Archaeoglobus fulgidus encodes a putative CopZ copper chaperone that contains an unusual cysteine-rich N-terminal domain of 130 amino acids in addition to a C-terminal copper binding domain with a conserved CXXC motif. The N-terminal domain (CopZ-NT) is homologous to proteins found only in extremophiles and is the only such protein that is fused to a copper chaperone. Surprisingly, optical, electron paramagnetic resonance, and x-ray absorption spectroscopic data indicate the presence of a [2Fe-2S] cluster in CopZ-NT. The intact CopZ protein binds two copper ions, one in each domain. The 1.8 Å resolution crystal structure of CopZ-NT reveals that the [2Fe-2S] cluster is housed within a novel fold and that the protein also binds a zinc ion at a four-cysteine site. CopZ can deliver Cu+ to the A. fulgidus CopA N-terminal metal binding domain and is capable of reducing Cu2+ to Cu+. This unique fusion of a redox-active domain with a CXXC-containing copper chaperone domain is relevant to the evolution of copper homeostatic mechanisms and suggests new models for copper trafficking.
The endoplasmic reticulum (ER) plays essential roles in protein folding and assembly of secretory proteins. ER-resident molecular chaperones and related enzymes assist in protein maturation by co-operated interactions and modifications. However, the folding/assembly of multimeric proteins is not well understood. Here, we show that the maturation of fibrinogen, a hexameric secretory protein (two trimers from α, β and γ subunits), occurs in a stepwise manner. The αγ complex, a precursor for the trimer, is retained in the ER by lectin-like chaperones, and the β subunit is incorporated into the αγ complex immediately after translation. ERp57, a protein disulfide isomerase homologue, is involved in the hexamer formation from two trimers. Our results indicate that the fibrinogen hexamer is formed sequentially, rather than simultaneously, using kinetic pause by lectin chaperones. This study provides a novel insight into the assembly of most abundant multi-subunit secretory proteins.
Virtually nothing is known about the interaction of co-translationally active chaperones with nascent polypeptides and the resulting effects on peptide conformation and folding. We have explored this issue by NMR analysis of apomyoglobin N-terminal fragments of increasing length, taken as models for different stages of protein biosynthesis, in the absence and presence of the substrate binding domain of Escherichia coli Hsp70, DnaK-β. The incomplete polypeptides misfold and self-associate under refolding conditions. In the presence of DnaK-β, however, formation of the original self-associated species is completely or partially prevented. Chaperone interaction with incomplete protein chains promotes a globally unfolded dynamic DnaK-β-bound state, which becomes folding-competent only upon incorporation of the residues corresponding to the C-terminal H helix. The chaperone does not bind the full-length protein at equilibrium. However, its presence strongly disfavors the kinetic accessibility of misfolding side-routes available to the full-length chain. This work supports the role of DnaK as a “holder” for incomplete N-terminal polypeptides. However, as the chain approaches its full-length status, the tendency to intramolecularly bury non-polar surface efficiently out-competes chaperone binding. Under these conditions, DnaK serves as a “folding enhancer” by supporting folding of a population of otherwise folding-incompetent full-length protein chains.
apomyoglobin; Hsp70; protein folding; DnaK; chaperone
Attachment to host cells via adhesive surface structures is a prerequisite for the pathogenesis of many bacteria. Uropathogenic E. coli assemble P and type 1 pili for attachment to the host urothelium. Assembly of these pili requires the conserved chaperone/usher pathway, in which a periplasmic chaperone controls the folding of pilus subunits and an outer membrane usher provides a platform for pilus assembly and secretion. The usher has differential affinity for pilus subunits, with highest affinity for the tip-localized adhesin. Here, we identify residues F21 and R652 of the P pilus usher PapC as functioning in the differential affinity of the usher. R652 is important for high affinity binding to the adhesin whereas F21 is important for limiting affinity for the PapA major rod subunit. PapC mutants in these residues are specifically defective for pilus assembly in the presence of PapA, demonstrating that differential affinity of the usher is required for assembly of complete pili. Analysis of PapG deletion mutants demonstrated that the adhesin is not required to initiate P pilus biogenesis. Thus, the differential affinity of the usher may be critical to ensure assembly of functional pilus fibers.
bacteria; pili; fimbriae; usher; chaperone; protein secretion
The functional unit of the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins is a trimer composed of three gp120 exterior glycoproteins and three gp41 transmembrane glycoproteins. The lability of intersubunit interactions has hindered the production and characterization of soluble, homogeneous envelope glycoprotein trimers. Here we report three modifications that stabilize soluble forms of HIV-1 envelope glycoprotein trimers: disruption of the proteolytic cleavage site between gp120 and gp41, introduction of cysteines that form intersubunit disulfide bonds, and addition of GCN4 trimeric helices. Characterization of these secreted glycoproteins by immunologic and biophysical methods indicates that these stable trimers retain structural integrity. The efficacy of the GCN4 sequences in stabilizing the trimers, the formation of intersubunit disulfide bonds between appropriately placed cysteines, and the ability of the trimers to interact with a helical, C-terminal gp41 peptide (DP178) support a model in which the N-terminal gp41 coiled coil exists in the envelope glycoprotein precursor and contributes to intersubunit interactions within the trimer. The availability of stable, soluble HIV-1 envelope glycoprotein trimers should expedite progress in understanding the structure and function of the virion envelope glycoprotein spikes.
Protein disulfide isomerase (PDI) is a thiodisulfide oxidoreductase that catalyzes the formation, reduction and rearrangement of disulfide bonds in proteins of eukaryotes. The classical PDI has a signal peptide, two CXXCcontaining thioredoxin catalytic sites (a,a′), two noncatalytic thioredoxin fold domains (b,b′), an acidic domain (c) and a C-terminal endoplasmic reticulum (ER) retention signal. Although PDI resides in the ER where it mediates the folding of nascent polypeptides of the secretory pathway, we recently showed that PDI5 of Arabidopsis thaliana chaperones and inhibits cysteine proteases during trafficking to vacuoles prior to programmed cell death of the endothelium in developing seeds. Here we describe Arabidopsis PDI2, which shares a primary structure similar to that of classical PDI. Recombinant PDI2 is imported into ER-derived microsomes and complements the E. coli protein- folding mutant, dsbA. PDI2 interacted with proteins in both the ER and nucleus, including ER-resident protein folding chaperone, BiP1, and nuclear embryo transcription factor, MEE8. The PDI2-MEE8 interaction was confirmed to occur in vitro and in vivo. Transient expression of PDI2- GFP fusions in mesophyll protoplasts resulted in labeling of the ER, nucleus and vacuole. PDI2 is expressed in multiple tissues, with relatively high expression in seeds and root tips. Immunoelectron microscopy with GFP- and PDI2-specific antisera on transgenic seeds (PDI2-GFP) and wild type roots demonstrated that PDI2 was found in the secretory pathway (ER, Golgi, vacuole, cell wall) and the nuclei. Our results indicate that PDI2 mediates protein folding in the ER and has new functional roles in the nucleus.
chaperone; endoplasmic reticulum; nuclear factor; plant disulfide isomerase; protein folding
P pili are hairlike polymeric structures that mediate binding of uropathogenic Escherichia coli to the surface of the kidney via the PapG adhesin at their tips. PapG is composed of two domains: a lectin domain at the tip of the pilus followed by a pilin domain that comprises the initial polymerizing subunit of the 1,000-plus-subunit heteropolymeric pilus fiber. Prior to assembly, periplasmic pilin domains bind to a chaperone, PapD. PapD mediates donor strand complementation, in which a beta strand of PapD temporarily completes the pilin domain's fold, preventing premature, nonproductive interactions with other pilin subunits and facilitating subunit folding. Chaperone-subunit complexes are delivered to the outer membrane usher where donor strand exchange (DSE) replaces PapD's donated beta strand with an amino-terminal extension on the next incoming pilin subunit. This occurs via a zip-in–zip-out mechanism that initiates at a relatively accessible hydrophobic space termed the P5 pocket on the terminally incorporated pilus subunit. Here, we solve the structure of PapD in complex with the pilin domain of isoform II of PapG (PapGIIp). Our data revealed that PapGIIp adopts an immunoglobulin fold with a missing seventh strand, complemented in parallel by the G1 PapD strand, typical of pilin subunits. Comparisons with other chaperone-pilin complexes indicated that the interactive surfaces are highly conserved. Interestingly, the PapGIIp P5 pocket was in an open conformation, which, as molecular dynamics simulations revealed, switches between an open and a closed conformation due to the flexibility of the surrounding loops. Our study reveals the structural details of the DSE mechanism.
In recent years, structural studies have identified a number of bacterial, viral, and eukaryotic adhesive proteins that have a trimeric architecture. The prototype examples in bacteria are the Haemophilus influenzae Hia adhesin and the Yersinia enterocolitica YadA adhesin. Both Hia and YadA are members of the trimeric-autotransporter subfamily and are characterized by an internal passenger domain that harbors adhesive activity and a short C-terminal translocator domain that inserts into the outer membrane and facilitates delivery of the passenger domain to the bacterial surface. In this study, we examined the relationship between trimerization of the Hia and YadA passenger domains and the capacity for adhesive activity. We found that subunit-subunit interactions and stable trimerization are essential for native folding and stability and ultimately for full-level adhesive activity. These results raise the possibility that disruption of the trimeric architecture of trimeric autotransporters, and possibly other trimeric adhesins, may be an effective strategy to eliminate adhesive activity.