Folding and assembly of phage P22 tailspike endorhamnosidase lacking the N-terminal, head-binding domain

Pantoea stewartii WceF is a glycan biofilm-modifying enzyme with a bacteriophage tailspike-like fold

Pathogenic microorganisms often reside in glycan-based biofilms. Concentration and chain length distribution of these mostly anionic exopolysaccharides (EPS) determine the overall biophysical properties of a biofilm and result in a highly viscous environment. Bacterial communities regulate this biofilm state via intracellular small-molecule signaling to initiate EPS synthesis. Reorganization or degradation of this glycan matrix, however, requires the action of extracellular glycosidases. So far, these were mainly described for bacteriophages that must degrade biofilms for gaining access to host bacteria. The plant pathogen Pantoea stewartii (P. stewartii) encodes the protein WceF within its EPS synthesis cluster. WceF has homologs in various biofilm forming plant pathogens of the Erwinia family. In this work, we show that WceF is a glycosidase active on stewartan, the main P. stewartii EPS biofilm component. WceF has remarkable structural similarity with bacteriophage tailspike proteins (TSPs). Crystal structure analysis showed a native trimer of right-handed parallel β-helices. Despite its similar fold, WceF lacks the high stability found in bacteriophage TSPs. WceF is a stewartan hydrolase and produces oligosaccharides, corresponding to single stewartan repeat units. However, compared with a stewartan-specific glycan hydrolase of bacteriophage origin, WceF showed lectin-like autoagglutination with stewartan, resulting in notably slower EPS cleavage velocities. This emphasizes that the bacterial enzyme WceF has a role in P. stewartii biofilm glycan matrix reorganization clearly different from that of a bacteriophage exopolysaccharide depolymerase.

Self-Competitive Inhibition of the Bacteriophage P22 Tailspike Endorhamnosidase by O-Antigen Oligosaccharides

The P22 tailspike endorhamnosidase confers the high specificity of bacteriophage P22 for some serogroups of Salmonella differing only slightly in their O-antigen polysaccharide. We used several biophysical methods to study the binding and hydrolysis of O-antigen fragments of different lengths by P22 tailspike protein. O-Antigen saccharides of defined length labeled with fluorophors could be purified with higher resolution than previously possible. Small amounts of naturally occurring variations of O-antigen fragments missing the nonreducing terminal galactose could be used to determine the contribution of this part to the free energy of binding to be ∼7 kJ/mol. We were able to show via several independent lines of evidence that an unproductive binding mode is highly favored in binding over all other possible binding modes leading to hydrolysis. This is true even under circumstances under which the O-antigen fragment is long enough to be cleaved efficiently by the enzyme. The high-affinity unproductive binding mode results in a strong self-competitive inhibition in addition to product inhibition observed for this system. Self-competitive inhibition is observed for all substrates that have a free reducing end rhamnose. Naturally occurring O-antigen, while still attached to the bacterial outer membrane, does not have a free reducing end and therefore does not perform self-competitive inhibition.

Yersinia Phages and Food Safety

One of the human- and animal-pathogenic species in genus Yersinia is Yersinia enterocolitica, a food-borne zoonotic pathogen that causes enteric infections, mesenteric lymphadenitis, and sometimes sequelae such as reactive arthritis and erythema nodosum. Y. enterocolitica is able to proliferate at 4 C, making it dangerous if contaminated food products are stored under refrigeration. The most common source of Y. enterocolitica is raw pork meat. Microbiological detection of the bacteria from food products is hampered by its slow growth rate as other bacteria overgrow it. Bacteriophages can be exploited in several ways to increase food safety with regards to contamination by Y. enterocolitica. For example, Yersinia phages could be useful in keeping the contamination of food products under control, or, alternatively, the specificity of the phages could be exploited in developing rapid and sensitive diagnostic tools for the identification of the bacteria in food products. In this review, we will discuss the present state of the research on these topics.

Structure of the Receptor-Binding Carboxy-Terminal Domain of the Bacteriophage T5 L-Shaped Tail Fibre with and without Its Intra-Molecular Chaperone

Bacteriophage T5, a Siphovirus belonging to the order Caudovirales, has a flexible, three-fold symmetric tail, to which three L-shaped fibres are attached. These fibres recognize oligo-mannose units on the bacterial cell surface prior to infection and are composed of homotrimers of the pb1 protein. Pb1 has 1396 amino acids, of which the carboxy-terminal 133 residues form a trimeric intra-molecular chaperone that is auto-proteolyzed after correct folding. The structure of a trimer of residues 970–1263 was determined by single anomalous dispersion phasing using incorporated selenomethionine residues and refined at 2.3 Å resolution using crystals grown from native, methionine-containing, protein. The protein inhibits phage infection by competition. The phage-distal receptor-binding domain resembles a bullet, with the walls formed by partially intertwined beta-sheets, conferring stability to the structure. The fold of the domain is novel and the topology unique to the pb1 structure. A site-directed mutant (Ser1264 to Ala), in which auto-proteolysis is impeded, was also produced, crystallized and its 2.5 Å structure solved by molecular replacement. The additional chaperone domain (residues 1263–1396) consists of a central trimeric alpha-helical coiled-coil flanked by a mixed alpha-beta domain. Three long beta-hairpin tentacles, one from each chaperone monomer, extend into long curved grooves of the bullet-shaped domain. The chaperone-containing mutant did not inhibit infection by competition.

Bacteriophage P22 tailspike: structure of the complete protein and function of the interdomain linker

Attachment of phages to host cells, followed by phage DNA ejection, represents the first stage of viral infection of bacteria. Salmonella phage P22 has been extensively studied, serving as an experimental model for bacterial infection by phages. P22 engages bacteria by binding to the sugar moiety of lipopolysaccharides using the viral tailspike protein for attachment. While the structures of the N-terminal particle-binding domain and the major receptor-binding domain of the tailspike have been analyzed individually, the three-dimensional organization of the intact protein, including the highly conserved linker region between the two domains, remained unknown. A single amino-acid exchange in the linker sequence made it possible to crystallize the full-length protein. Two crystal structures of the linker region are presented: one attached to the N-terminal domain and the other present within the complete tailspike protein. Both retain their biological function, but the mutated full-length tailspike displays a retarded folding pathway. Fitting of the full-length tailspike into a published cryo-electron microscopy map of the P22 virion requires an elastic distortion of the crystal structure. The conservation of the linker suggests a role in signal transmission from the distal tip of the molecule to the phage head, eventually leading to DNA ejection.

The C-terminal cysteine annulus participates in auto-chaperone function for Salmonella phage P22 tailspike folding and assembly

Elongated trimeric adhesins are a distinct class of proteins employed by phages and viruses to recognize and bind to their host cells, and by bacteria to bind to their target cells and tissues. The tailspikes of E. coli phage K1F and Bacillus phage Ø29 exhibit auto-chaperone activity in their trimeric C-terminal domains. The P22 tailspike is structurally homologous to those adhesins. Though there are no disulfide bonds or reactive cysteines in the native P22 tailspikes, a set of C-terminal cysteines are very reactive in partially folded intermediates, implying an unusual local conformation in the domain. This is likely to be involved in the auto-chaperone function. We examined the unusual reactivity of C-terminal tailspike cysteines during folding and assembly as a potential reporter of auto-chaperone function. Reaction with IAA blocked productive refolding in vitro, but not off-pathway aggregation. Two-dimensional PAGE revealed that the predominant intermediate exhibiting reactive cysteine side chains was a partially folded monomer. Treatment with reducing reagent promoted native trimer formation from these species, consistent with transient disulfide bonds in the auto-chaperone domain. Limited enzymatic digestion and mass spectrometry of folding and assembly intermediates indicated that the C-terminal domain was compact in the protrimer species. These results indicate that the C-terminal domain of the P22 tailspike folds itself and associates prior to formation of the protrimer intermediate, and not after, as previously proposed. The C-terminal cysteines and triple β-helix domains apparently provide the staging for the correct auto-chaperone domain formation, needed for alignment of P22 tailspike native trimer.

Orally administered P22 phage tailspike protein reduces salmonella colonization in chickens: prospects of a novel therapy against bacterial infections

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.

Phage tailspike proteins with beta-solenoid fold as thermostable carbohydrate binding materials

We have investigated the stability of three tailspike proteins (TSPs) from bacteriophages Sf6, P22, and HK620. Tailspikes are rod-like homotrimers with comparable beta-solenoid folds and similarly high kinetic stability in spite of different amino acid sequences. As tailspikes bind polysaccharides to recognize the bacterial host cell, their stability is required for maintenance of bacteriophage infectivity under harsh extracellular conditions. They resist denaturation by SDS at ambient temperature and their unfolding is slow even in 6 M guanidinium hydrochloride (GdmHCl). This makes them interesting candidates for very stable carbohydrate binding protein materials.

The C-terminus of the P22 tailspike protein acts as an independent oligomerization domain for monomeric proteins

TSP (P22 tailspike protein) is a well-established model system for studying the folding and assembly of oligomeric proteins, and previous studies have documented both in vivo and in vitro folding intermediates using this protein. Especially important is the C-terminus of TSP, which plays a critical role in the assembly and maturation of the protrimer intermediate to its final trimeric form. In the present study, we show that by grafting the C-terminus of TSP on to the monomeric MBP (maltose-binding protein), the resulting chimaera (MBP-537) is a trimeric protein. Moreover, Western blot studies (using an anti-TSP antibody) indicate that the TSP C-terminus in the MBP-537 chimaera has the same conformation as the native TSP. The oligomerization of the MBP-537 chimaera appears to involve hydrophobic interactions and a refolding sequence, both of which are analogous to the native TSP. These results underscore the importance of the TSP C-terminus in the assembly of the mature trimer and demonstrate its potential utility as a model to study the folding and assembly of the TSP C-terminus in isolation.

Cotranslational folding promotes beta-helix formation and avoids aggregation in vivo

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 beta-helix domain in vitro. We have produced stalled ribosome:tailspike nascent chain complexes of four fixed lengths in vivo, in order to assess cotranslational 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 beta-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 beta-helix domain occurs cotranslationally, prior to chain release, to a conformation that is distinct from the accessible energy minimum conformation for the truncated free chain in solution.

Structure of the receptor-binding protein of bacteriophage det7: a podoviral tail spike in a myovirus

A new Salmonella enterica phage, Det7, was isolated from sewage and shown by electron microscopy to belong to the Myoviridae morphogroup of bacteriophages. Det7 contains a 75-kDa protein with 50% overall sequence identity to the tail spike endorhamnosidase of podovirus P22. Adsorption of myoviruses to their bacterial hosts is normally mediated by long and short tail fibers attached to a contractile tail, whereas podoviruses do not contain fibers but attach to host cells through stubby tail spikes attached to a very short, noncontractile tail. The amino-terminal 150 residues of the Det7 protein lack homology to the P22 tail spike and are probably responsible for binding to the base plate of the myoviral tail. Det7 tail spike lacking this putative particle-binding domain was purified from Escherichia coli, and well-diffracting crystals of the protein were obtained. The structure, determined by molecular replacement and refined at a 1.6-A resolution, is very similar to that of bacteriophage P22 tail spike. Fluorescence titrations with an octasaccharide suggest Det7 tail spike to bind its receptor lipopolysaccharide somewhat less tightly than the P22 tail spike. The Det7 tail spike is even more resistant to thermal unfolding than the already exceptionally stable homologue from P22. Folding and assembly of both trimeric proteins are equally temperature sensitive and equally slow. Despite the close structural, biochemical, and sequence similarities between both proteins, the Det7 tail spike lacks both carboxy-terminal cysteines previously proposed to form a transient disulfide during P22 tail spike assembly. Our data suggest receptor-binding module exchange between podoviruses and myoviruses in the course of bacteriophage evolution.

Chemical chaperone-mediated protein folding: stabilization of P22 tailspike folding intermediates by glycerol

Polyol co-solvents such as glycerol increase the thermal stability of proteins. This has been explained by preferential hydration favoring the more compact native over the denatured state. Although polyols are also expected to favor aggregation by the same mechanism, they have been found to increase the folding yields of some large, aggregation-prone proteins. We have used the homotrimeric phage P22 tailspike protein to investigate the origin of this effect. The folding of this protein is temperature-sensitive and limited by the stability of monomeric folding intermediates. At non-permissive temperature (≥35°C), tailspike refolding yields were increased significantly in the presence of 1–4 mglycerol. At low temperature, tailspike refolding is prevented when folding intermediates are destabilized by the addition of urea. Glycerol could offset the urea effect, suggesting that the polyol acts by stabilizing crucial folding intermediates and not by increasing solvent viscosity. The stabilization effect of glycerol on tailspike folding intermediates was confirmed in experiments using a temperature-sensitive folding mutant protein, by fluorescence measurements of subunit folding kinetics, and by temperature up-shift experiments. Our results suggest that the chemical chaperone effect of polyols observed in the folding of large proteins is due to preferential hydration favoring structure formation in folding intermediates.

Trimeric autotransporters require trimerization of the passenger domain for stability and adhesive activity

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.

An elongated spine of buried core residues necessary for in vivo folding of the parallel beta-helix of P22 tailspike adhesin

The parallel beta-helix is an elongated beta-sheet protein domain associated with microbial virulence factors, toxins, viral adhesins, and allergens. Long stacks of similar, buried residues are a prominent feature of this fold, as well as the polypeptide chain fold of an amyloid structure. The 13-rung, right-handed, parallel beta-helix of the homotrimeric P22 tailspike adhesin exhibits predominantly hydrophobic stacks. The role of these stacked residues in the folding and stabilization of the protein is unclear. Through scanning alanine mutagenesis we have identified a folding spine of stacked residues in continuous contact along the length of P22 tailspike's beta-helix domain that is necessary for folding within cells. Nearly all chains carrying alanine substitutions of the 103 buried nonalanines were defective in folding in vivo at 37 degrees C. However, the majority of these chains successfully reached a native state, stable to >80 degrees C, when folded inside cells at low temperatures. Thus, nearly the entire buried core was critical for in vivo beta-helix folding but negligible for stability. Folding at 18 degrees C revealed the minimal folding spine of 29 nonglycine stack positions that were intolerant to alanine substitution. These results indicate that a processive folding mechanism, dependent on stacking contacts, controls beta-helix formation. Such a stepwise folding pathway offers a new target for drug design against this class of microbial virulence factors.

Stalled folding mutants in the triple beta-helix domain of the phage P22 tailspike adhesin

The trimeric bacteriophage P22 tailspike adhesin exhibits a domain in which three extended strands intertwine, forming a single turn of a triple beta-helix. This domain contains a single hydrophobic core composed of residues contributed by each of the three sister polypeptide chains. The triple beta-helix functions as a molecular clamp, increasing the stability of this elongated structural protein. During folding of the tailspike protein, the last precursor before the native state is a partially folded trimeric intermediate called the protrimer. The transition from the protrimer to the native state results in a structure that is resistant to denaturation by heat, chemical denaturants, and proteases. Random mutations were made in the region encoding residues 540-548, where the sister chains begin to wrap around each other. From a set of 26 unique single amino acid substitutions, we characterized mutations at G546, N547, and I548 that retarded or blocked the protrimer to native trimer transition. In contrast, many non-conservative substitutions were tolerated at residues 540-544. Sucrose gradient analysis showed that protrimer-like mutants had reduced sedimentation, 8.0 S to 8.3 S versus 9.3 S for the native trimer. Mutants affected in the protrimer to native trimer transition were also destabilized in their native state. These data suggest that the folding of the triple beta-helix domain drives transition of the protrimer to the native state and is accompanied by a major rearrangement of polypeptide chains.

Three amino acids that are critical to formation and stability of the P22 tailspike trimer

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 3.2M guanidine, compared to 5.6M for 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.

Trimeric autotransporters: a distinct subfamily of autotransporter proteins

Autotransporter proteins are a large family of gram-negative bacterial extracellular proteins. These proteins have a characteristic arrangement of functional domains, including an N-terminal signal peptide, an internal passenger domain, and a C-terminal translocator domain. Recent studies have identified a novel subfamily of autotransporters, defined by a short trimeric C-terminal translocator domain and known as trimeric autotransporters. In this article, we review our current knowledge of the structural and functional characteristics of trimeric autotransporters, highlighting the distinctions between this subfamily and conventional autotransporters. We speculate that trimeric autotransporters evolved to enable high-affinity multivalent adhesive interactions with host surfaces and circulating host molecules to take place.

Homology between two different Salmonella phages: Salmonella enterica serovar Typhimurium phage P22 and Salmonella enterica serovar Anatum var. 15 + phageepsilon34

A distinguishing feature of many microorganisms, belonging to the Gram negative group of bacteria, is the presence of the lipopolysaccharide on their cell surface. Salmonella is a prominent member of this group of bacteria. Many Salmonella phages use the LPS as the initial receptor in the infection process and they can distinguish subtle changes in the LPS molecules. The phage protein that is responsible for recognition of these cells is the tail or tailspike protein (TSP). Those TSPs, which use LPS as a receptor, are prokaryotic LPS-binding proteins. As an initial step in using phage TSPs as model systems for a detailed molecular genetic analysis of protein-LPS interactions, a comparison of two phages and their TSPs from two different Salmonella bacterial viruses (phages), Salmonella enterica serovar Typhimurium phage P22 and Salmonella enterica serovar Anatum var. 15 + phage epsilon34, is being carried out. This present study shows significant viral protein homology between many viral structural proteins from these two phages including their TSPs. Significantly this report suggests a general structural motif for part of the TSP of phages and suggests that a more detailed comparative analysis of these TSPs is warranted.

C-terminal hydrophobic interactions play a critical role in oligomeric assembly of the P22 tailspike trimer

The tailspike protein from the bacteriophage P22 is a well characterized model system for folding and assembly of multimeric proteins. Folding intermediates from both the in vivo and in vitro pathways have been identified, and both the initial folding steps and the protrimer-to-trimer transition have been well studied. In contrast, there has been little experimental evidence to describe the assembly of the protrimer. Previous results indicated that the C terminus plays a critical role in the overall stability of the P22 tailspike protein. Here, we present evidence that the C terminus is also the critical assembly point for trimer assembly. Three truncations of the full-length tailspike protein, TSPDeltaN, TSPDeltaC, and TSPDeltaNC, were generated and tested for their ability to form mixed trimer species. TSPDeltaN forms mixed trimers with full-length P22 tailspike, but TSPDeltaC and TSPDeltaNC are incapable of forming similar mixed trimer species. In addition, mutations in the hydrophobic core of the C terminus were unable to form trimer in vivo. Finally, the hydrophobic-binding dye ANS inhibits the formation of trimer by inhibiting progression through the folding pathway. Taken together, these results suggest that hydrophobic interactions between C-terminal regions of P22 tailspike monomers play a critical role in the assembly of the P22 tailspike trimer.

The tailspike protein of Shigella phage Sf6. A structural homolog of Salmonella phage P22 tailspike protein without sequence similarity in the beta-helix domain

Bacteriophage Sf6 tailspike protein is functionally equivalent to the well characterized tailspike of Salmonella phage P22, mediating attachment of the viral particle to host cell-surface polysaccharide. However, there is significant sequence similarity between the two 70-kDa polypeptides only in the N-terminal putative capsid-binding domains. The major, central part of P22 tailspike protein, which forms a parallel beta-helix and is responsible for saccharide binding and hydrolysis, lacks detectable sequence homology to the Sf6 protein. After recombinant expression in Escherichia coli as a soluble protein, the Sf6 protein was purified to homogeneity. As shown by circular dichroism and Fourier transform infrared spectroscopy, the secondary structure contents of Sf6 and P22 tailspike proteins are very similar. Both tailspikes are thermostable homotrimers and resist denaturation by SDS at room temperature. The specific endorhamnosidase activities of Sf6 tailspike protein toward fluorescence-labeled dodeca-, deca-, and octasaccharide fragments of Shigella O-antigen suggest a similar active site topology of both proteins. Upon deletion of the N-terminal putative capsid-binding domain, the protein still forms a thermostable, SDS-resistant trimer that has been crystallized. The observations strongly suggest that the tailspike of phage Sf6 is a trimeric parallel beta-helix protein with high structural similarity to its functional homolog from phage P22.

Biochemical characterization of beta2-adrenergic receptor dimers and oligomers

G Protein-coupled receptor dimerization/oligomerization has been well established during the last several years. Studies have demonstrated the existence of dimers/digomers both in vitro and in living cells. However, a thorough characterization of the biochemical nature of receptor dimers and oligomers as well as their occurrence at the cell surface has not been properly addressed. In this study, we show that both beta2-adrenergic receptor (beta2AR) dimers and oligomers exist at the plasma membrane and that the detection of such species, following receptor solubilization and resolution by denaturing polyacrylamide gel electrophoresis (SDS-PAGE), does not result from the formation of spurious disulfide bonds during cell lysis. Moreover, our results indicate that the biochemical nature of beta2AR dimers is different from that of the oligomers. Although both complexes are partially resistant to SDS denaturation, disulfide bonding is absolutely required for the stability of beta2AR oligomers but not dimers in SDS-PAGE. Indeed, dimeric species can be detected even in the presence of high concentrations of reducing and alkylating agents. Although the different biochemical nature of the dimers and oligomers may be indicative of distinct biological roles in cells, additional studies will be required to further elucidate the biosynthesis and function of these receptor forms.

Characterization of the protrimer intermediate in the folding pathway of the interdigitated beta-helix tailspike protein

P22 tailspike is a homotrimeric, thermostable adhesin that recognizes the O-antigen lipopolysaccharide of Salmonella typhimurium. The 70 kDa subunits include long beta-helix domains. After residue 540, the polypeptide chains change their path and wrap around one another, with extensive interchain contacts. Formation of this interdigitated domain intimately couples the chain folding and assembly mechanisms. The earliest detectable trimeric intermediate in the tailspike folding and assembly pathway is the protrimer, suspected to be a precursor of the native trimer structure. We have directly analyzed the kinetics of in vitro protrimer formation and disappearance for wild type and mutant tailspike proteins. The results confirm that the protrimer intermediate is an on-pathway intermediate for tailspike folding. Protrimer was originally resolved during tailspike folding because its migration through nondenaturing polyacrylamide gels was significantly retarded with respect to the migration of the native tailspike trimer. By comparing protein mobility versus acrylamide concentration, we find that the retarded mobility of the protrimer is due exclusively to a larger overall size than the native trimer, rather than an altered net surface charge. Experiments with mutant tailspike proteins indicate that the conformation difference between protrimer and native tailspike trimer is localized toward the C-termini of the tailspike polypeptide chains. These results suggest that the transformation of the protrimer to the native tailspike trimer represents the C-terminal interdigitation of the three polypeptide chains. This late step may confer the detergent-resistance, protease-resistance, and thermostability of the native trimer.

The architecture of parallel beta-helices and related folds

Three-dimensional structures have been determined of a large number of proteins characterized by a repetitive fold where each of the repeats (coils) supplies a strand to one or more parallel beta-sheets. Some of these proteins form superfamilies of proteins, which have probably arisen by divergent evolution from a common ancestor. The classical example is the family including four families of pectinases without obviously related primary sequences, the phage P22 tailspike endorhamnosidase, chrondroitinase B and possibly pertactin from Bordetella pertusis. These show extensive stacking of similar residues to give aliphatic, aromatic and polar stacks such as the asparagine ladder. This suggests that coils can be added or removed by duplication or deletion of the DNA corresponding to one or more coils and explains how homologous proteins can have different numbers of coils. This process can also account for the evolution of other families of proteins such as the beta-rolls, the leucine-rich repeat proteins, the hexapeptide repeat family, two separate families of beta-helical antifreeze proteins and the spiral folds. These families need not be related to each other but will share features such as relative untwisted beta-sheets, stacking of similar residues and turns between beta-strands of approximately 90 degrees often stabilized by hydrogen bonding along the direction of the parallel beta-helix.Repetitive folds present special problems in the comparison of structures but offer attractive targets for structure prediction. The stacking of similar residues on a flat parallel beta-sheet may account for the formation of amyloid with beta-strands at right-angles to the fibril axis from many unrelated peptides.

A newly synthesized, ribosome-bound polypeptide chain adopts conformations dissimilar from early in vitro refolding intermediates

Little is known about the conformations of newly synthesized polypeptide chains as they emerge from the large ribosomal subunit, or how these conformations compare with those populated immediately after dilution of polypeptide chains out of denaturant in vitro. Both in vivo and in vitro, partially folded intermediates of the tailspike protein from Salmonella typhimurium phage P22 can be trapped in the cold. A subset of monoclonal antibodies raised against tailspike recognize partially folded intermediates, whereas other antibodies recognize only later intermediates and/or the native state. We have used a pair of monoclonal antibodies to probe the conformational features of full-length, newly synthesized tailspike chains recovered on ribosomes from phage-infected cells. The antibody that recognizes early intermediates in vitro also recognizes the ribosome-bound intermediates. Surprisingly, the antibody that did not recognize early in vitro intermediates did recognize ribosome-bound tailspike chains translated in vivo. Thus, the newly synthesized, ribosome-bound tailspike chains display structured epitopes not detected upon dilution of tailspike chains from denaturant. As opposed to the random ensemble first populated when polypeptide chains are diluted out of denaturant, folding in vivo from the ribosome may begin with polypeptide conformations already directed toward the productive folding and assembly pathway.

Identification and crystallisation of a heat- and protease-stable fragment of the bacteriophage T4 short tail fibre

Irreversible binding of T-even bacteriophages to Escherichia coli is mediated by the short tail fibres, which serve as inextensible stays during DNA injection. Short tail fibres are exceptionally stable elongated trimers of gene product 12 (gp12), a 56 kDa protein. The N-terminal region of gp12 is important for phage attachment, the central region forms a long shaft, while a C-terminal globular region is implicated in binding to the bacterial lipopolysaccharide core. When gp12 was treated with stoichiometric amounts of trypsin or chymotrypsin at 37 degrees C, an N-terminally shortened fragment of 52 kDa resulted. If the protein was incubated at 56 degrees C before trypsin treatment at 37 degrees C, we obtained a stable trimeric fragment of 3 x 33 kDa lacking residues from both the N- and C-termini. Apparently, the protein unfolds partially at 56 degrees C, thereby exposing protease-sensitive sites in the C-terminal region and extra sites in the N-terminal region. Well-diffracting crystals of this fragment could be grown. Our results indicate that gp12 carries a stable central region, consisting of the C-terminal part of the shaft and the attached N-terminal half of the globular region. Implications for structure determination of the gp12 protein and its folding are discussed.

Enthalpic barriers to the hydrophobic binding of oligosaccharides to phage P22 tailspike protein

The structural thermodynamics of the recognition of complex carbohydrates by proteins are not well understood. The recognition of O-antigen polysaccharide by phage P22 tailspike protein is a highly suitable model for advancing knowledge in this field. The binding to octa- and dodecasaccharides derived from Salmonella enteritidis O-antigen was studied by isothermal titration calorimetry and stopped-flow spectrofluorimetry. At room temperature, the binding reaction is enthalpically driven with an unfavorable change in entropy. A large change of -1.8 +/- 0.2 kJ mol(-1) K(-1) in heat capacity suggests that the hydrophobic effect and water reorganization contribute substantially to complex formation. As expected from the large heat-capacity change, we found enthalpy-entropy compensation. The calorimetrically measured binding enthalpies were identical within error to van't Hoff enthalpies determined from fluorescence titrations. Binding kinetics were determined at temperatures ranging from 10 to 30 degrees C. The second-order association rate constant varied from 1 x 10(5) M(-1) s(-1) for dodecasaccharide at 10 degrees C to 7 x 10(5) M(-1) s(-1) for octasaccharide at 30 degrees C. The first-order dissociation rate constants ranged from 0.2 to 3.8 s(-1). The Arrhenius activation energies were close to 50 and 100 kJ mol(-1) for the association and dissociation reactions, respectively, indicating mainly enthalpic barriers. Despite the fact that this system is quite complex due to the flexibility of the saccharide, both the thermodynamic and kinetic data are compatible with a simple one-step binding model.

Functional significance of oligomerization of G-protein-coupled receptors

In contrast to other families of cell surface receptors for which dimerization is an integral part of the activation process, G-protein-coupled receptors (GPCRs) were thought, until recently, to function as monomeric units. However, a growing body of evidence indicates that GPCRs could exist and be active as oligomeric complexes. Because they are major pharmacological targets, their existence as homo- or heterodimers could have important implications for the development and screening of new drugs.

Folding defects caused by single amino acid substitutions in a subunit are not alleviated by assembly

Significant stabilization of a protein often occurs when it is assembled into an oligomer. Bacteriophage P22 contains 420 monomers of coat protein that are stabilized by the assembly and maturation processes. The effects of eight single amino acid substitutions in coat protein that each cause a temperature-sensitive-folding defect were investigated to determine if the conformational differences previously observed in the monomers could be alleviated by assembly or maturation. Several techniques including differential scanning calorimetry, heat-induced expansion, urea denaturation, and sensitivity to protease digestion were used to explore the effects of the amino acid substitutions on the conformation of coat protein, once assembled. Each of the amino acid substitutions caused a change in the conformation as compared to wild-type coat protein, observed by at least one of the probes used. Thus, neither assembly nor expansion entirely corrected the conformational defects in the monomeric subunits of the folding mutants.

Mutations improving the folding of phage P22 tailspike protein affect its receptor binding activity

Four previously isolated mutations in Salmonella phage P22 tailspike protein were used to study the relationship between protein stability, folding, and function. Tailspike protein binds and hydrolyzes the repetitive O-antigen structure in Salmonella lipopolysaccharide. Four mutations (V331G, V331A, A334V, A334I) are known to increase the folding efficiency, and two of them (at position 331) also increase the thermal stability of the protein. Octasaccharides comprising two repeating units of the O-antigens from two different Salmonella strains were employed to analyze the receptor binding function of the mutant proteins. Their endorhamnosidase enzymatic activity was assayed with the aid of a fluorescence-labeled dodecasaccharide. Both V331A and V331G were found to strongly affect O-antigen binding. Octasaccharide binding affinities of the mutant proteins are reduced tenfold and 200-fold, corresponding to a loss of 17% and 36% of the standard free energy of binding, respectively. Both mutations at position 334 affected O-antigen binding only slightly (DeltaDeltaG(0)B approximately 1 kJ/mol), but these mutations reduce the thermal stability of the protein. The observed effects on the endoglycosidase activity are fully explained by the changes in substrate binding, suggesting that neither of the mutations affect the catalytic rate. Crystal structures of all four mutants were determined to a resolution of 2.0 A. Except for the partly or completely missing side-chain, no significant changes compared to the wild-type protein structure were found for the mutants at position 331, whereas a small but significant backbone displacement around the mutation site in A334V and A334I may explain the observed thermal destabilization.

Formation of fibrous aggregates from a non-native intermediate: the isolated P22 tailspike beta-helix domain

In the assembly pathway of the trimeric P22 tailspike protein, the protein conformation critical for the partitioning between productive folding and off-pathway aggregation is a monomeric folding intermediate. The central domain of tailspike, a large right-handed parallel beta-helix, is essentially structured in this species. We used the isolated beta-helix domain (Bhx), expressed with a hexahistidine tag, to investigate the mechanism of aggregation without the two terminal domains present in the complete protein. Although Bhx has been shown to fold reversibly at low ionic strength conditions, increased ionic strength induced aggregation with a maximum at urea concentrations corresponding to the midpoint of urea-induced folding transitions. According to size exclusion chromatography, aggregation appeared to proceed via a linear polymerization mechanism. Circular dichroism indicated a secondary structure content of the aggregates similar to that of the native state, but at the same time their tryptophan fluorescence was largely quenched. Microscopic analysis of the aggregates revealed a variety of morphologies; among others, fibrils with fine structure were observed that exhibited bright green birefringence if viewed under cross-polarized light after staining with Congo red. These observations, together with the effects of folding mutations on the aggregation process, indicate the involvement of a partially structured intermediate distinct from both unfolded and native Bhx.

There's a right way and a wrong way: in vivo and in vitro folding, misfolding and subunit assembly of the P22 tailspike

The in vivo and in vitro folding, assembly and misfolding of an elongated protein, the thermostable tailspike adhesin of phage P22, reveals important aspects of the sequence control of chain folding as well as its failure mode, inclusion body formation.

Phage P22 tailspike protein: removal of head-binding domain unmasks effects of folding mutations on native-state thermal stability

A shortened, recombinant protein comprising residues 109-666 of the tailspike endorhamnosidase of Salmonella phage P22 was purified from Escherichia coli and crystallized. Like the full-length tailspike, the protein lacking the amino-terminal head-binding domain is an SDS-resistant, thermostable trimer. Its fluorescence and circular dichroism spectra indicate native structure. Oligosaccharide binding and endoglycosidase activities of both proteins are identical. A number of tailspike folding mutants have been obtained previously in a genetic approach to protein folding. Two temperature-sensitive-folding (tsf) mutations and the four known global second-site suppressor (su) mutations were introduced into the shortened protein and found to reduce or increase folding yields at high temperature. The mutational effects on folding yields and subunit folding kinetics parallel those observed with the full-length protein. They mirror the in vivo phenotypes and are consistent with the substitutions altering the stability of thermolabile folding intermediates. Because full-length and shortened tailspikes aggregate upon thermal denaturation, and their denaturant-induced unfolding displays hysteresis, kinetics of thermal unfolding were measured to assess the stability of the native proteins. Unfolding of the shortened wild-type protein in the presence of 2% SDS at 71 degrees C occurs at a rate of 9.2 x 10(-4) s(-1). It reflects the second kinetic phase of unfolding of the full-length protein. All six mutations were found to affect the thermal stability of the native protein. Both tsf mutations accelerate thermal unfolding about 10-fold. Two of the su mutations retard thermal unfolding up to 5-fold, while the remaining two mutations accelerate unfolding up to 5-fold. The mutational effects can be rationalized on the background of the recently determined crystal structure of the protein.

Folding and function of repetitive structure in the homotrimeric phage P22 tailspike protein

The Salmonella bacteriophage P22 recognizes its host cell receptor, lipopolysaccharide, by means of six tailspikes, thermostable homotrimers of 72-kDa polypeptides. Biophysical results on the binding reaction, together with high-resolution structural information from X-ray crystallography, have shed light on the interactions determining the viral host range. Folding and assembly of the tailspike protein in vitro have been analyzed in detail, and the data have been compared with observations on the in vivo assembly pathway. Repetitive structural elements in the tailspike protein, like a side-by-side trimer of parallel beta-helices, a parallel alpha-helical bundle, a triangular prism made up from antiparallel beta-sheets, and a short segment of a triple beta-helix can be considered building blocks for larger structural proteins, and thus, the results on P22 tailspike may have implications for fibrous protein structure and folding.

P22 tailspike folding mutants revisited: effects on the thermodynamic stability of the isolated beta-helix domain

The folding of the trimeric phage P22 tailspike protein is influenced by amino acid substitutions of two types, virtually all of which affect residues in the central domain, a large parallel beta-helix. Temperature sensitive folding (tsf) mutations lead to drastically decreased folding yields at elevated temperature. Their phenotype can be alleviated by global suppressor (su) mutations. Both types of mutations appeared to have no influence on the stability of the native protein at the time of their first isolation and were thus suggested to carry information needed for the folding pathway exclusively. The monomeric beta-helix of tailspike, expressed as an isolated domain, exhibits freely reversible unfolding and refolding transitions, allowing us to analyse the effects of two well-characterised tsf and all four known su mutations on its thermodynamic stability. We find a marked decrease in stability for the tsf mutants and a striking increase in stability for all su mutants. This leads to the conception that the isolated beta-helix domain, although active in receptor-binding and native-like in its spectroscopic properties, is close in conformation to a crucial monomeric folding intermediate whose thermolability is responsible for the kinetic partitioning between productive folding and irreversible aggregation during the maturation process of P22 tailspike protein.

A reversibly unfolding fragment of P22 tailspike protein with native structure: the isolated beta-helix domain

The homotrimeric tailspike endorhamnosidase of phage P22 has been used to compare in vivo and in vitro folding pathways and the influence of single amino acid substitutions thereon. Its main structural motif, which contains the known folding mutation sites, consists of three large right-handed parallel beta-helices. A thermodynamic analysis of the stability of tailspike is prevented by the irreversibility of unfolding at high temperatures or high concentrations of denaturant, probably due to interdigitation of the domains neighboring the beta-helix. We therefore expressed and isolated a tailspike fragment comprising only its central beta-helix domain (residues 109-544). As shown by equilibrium ultracentrifugation, the isolated beta-helix is a monomer at concentrations below 1 microM and trimerizes reversibly at higher protein concentrations. Both the similarity of fluorescence and CD spectra, compared to the complete protein, and the specific binding and hydrolysis of substrate suggest a nativelike structure. Moreover, urea denaturation transitions of the beta-helix domain are freely reversible, providing the basis for a future quantitative analysis of the effects of the folding mutations on the thermodynamic stability of the domain and of structural features responsible for folding and stability of the parallel beta-helix motif in general.

Phage P22 tailspike protein: crystal structure of the head-binding domain at 2.3 A, fully refined structure of the endorhamnosidase at 1.56 A resolution, and the molecular basis of O-antigen recognition and cleavage

The tailspike protein of Salmonella phage P22 is a viral adhesion protein with both receptor binding and destroying activities. It recognises the O-antigenic repeating units of cell surface lipopolysaccharide of serogroup A, B and D1 as receptor, but also inactivates its receptor by endoglycosidase (endorhamnosidase) activity. In the final step of bacteriophage P22 assembly six homotrimeric tailspike molecules are non-covalently attached to the DNA injection apparatus, mediated by their N-terminal, head-binding domains. We report the crystal structure of the head-binding domain of P22 tailspike protein at 2.3 A resolution, solved with a recombinant telluromethionine derivative and non-crystallographic symmetry averaging. The trimeric dome-like structure is formed by two perpendicular beta-sheets of five and three strands, respectively in each subunit and caps a three-helix bundle observed in the structure of the C-terminal receptor binding and cleaving fragment, reported here after full refinement at 1.56 A resolution. In the central part of the receptor binding fragment, three parallel beta-helices of 13 complete turns are associated side-by-side, while the three polypeptide strands merge into a single domain towards their C termini, with close interdigitation at the junction to the beta-helix part. Complex structures with receptor fragments from S. typhimurium, S. enteritidis and S. typhi253Ty determined at 1.8 A resolution are described in detail. Insertions into the beta-helix form the O-antigen binding groove, which also harbours the active site residues Asp392, Asp395 and Glu359. In the intact structure of the tailspike protein, head-binding and receptor-binding parts are probably linked by a flexible hinge whose function may be either to deal with shearing forces on the exposed, 150 A long tailspikes or to allow them to bend during the infection process.

Prevalence of temperature sensitive folding mutations in the parallel beta coil domain of the phage P22 tailspike endorhamnosidase

Temperature sensitive mutations fall into two general classes: tl mutations, which render the mature protein thermolabile, and tsf (temperature sensitive folding) mutations, which destabilize an intermediate in the folding pathway without altering the functions of the folded state. The molecular defects caused by tsf mutations have been intensively studied for the elongated tailspike endorhamnosidase of Salmonella phage P22. The tailspike, responsible for host cell recognition and attachment, contains a 13 strand parallel beta coil domain. A set of tsf mutants located in the beta coil domain have been shown to cause folding defects in the in vivo folding pathway for the tailspike. We report here additional data on 17 other temperature sensitive mutants which are in the beta coil domain. Using mutant proteins formed at low temperature, the essential functions of assembling on the phage head, and binding to the O-antigen lipopolysaccharide (LPS) receptor of Salmonella were examined at high temperatures. All of the mutant proteins once folded at permissive temperature, were functional at restrictive temperatures. When synthesized at restrictive temperature the mutant chains formed an early folding intermediate, but failed to reach the mature conformation, accumulating instead in the aggregated inclusion body state. Thus this set of mutants all have the temperature sensitive folding phenotype. The prevalence of tsf mutants in the parallel beta coil domain presumably reflects properties of its folding intermediates. The key property may be the tendency of the intermediate to associate off pathway to the kinetically trapped inclusion body state.

Equilibrium intermediates in the reversible unfolding of firefly (Photinus pyralis) luciferase

Firefly luciferase has been used as a model protein to study cotranslational and chaperone-assisted protein folding. We found conditions for reversible unfolding of luciferase in the absence of cellular factors, and we characterized denaturant-induced equilibrium unfolding transitions and refolding kinetics of the enzyme. Luciferase unfolding induced by guanidinium chloride at 10 degrees C can be described as a four-state equilibrium with two inactive intermediates highly populated around 1 and 3 M denaturant. The transitions occur around 0.3, 1.7, and 3.8 M denaturant. The free energy of denaturation to the first inactive intermediate (DeltaG0N <==> I1 = 15 +/- 3 kJ.mol-1) is small for a protein of 60 kDa. Fluorescence and circular dichroism spectra of the intermediates indicate that I1 has a compact conformation, whereas aromatic side chains are highly exposed in the second intermediate, I2, despite its high content of secondary structure. In the presence of a hydrophilic detergent, significant reactivation of luciferase is observed up to temperatures at which the native protein is unstable. Reactivation kinetics of luciferase are exceedingly slow and probably not limited by proline isomerization, as suggested by their independence from the time spent in the unfolded state.

Interaction of Salmonella phage P22 with its O-antigen receptor studied by X-ray crystallography

The O-antigenic repeating units of the Salmonella cell surface lipopolysaccharides (serotypes A, B and D1) serve as receptors for phage P22. This initial binding step is mediated by the tailspike protein (TSP), which is present in six copies on the base plate of the phage. In addition to the binding activity, TSP also displays a low endoglycolytic activity, cleaving the alpha(1,3)-O-glycosidic bond between rhamnose and galactose of the O-antigenic repeats. The crystal structure of TSP in complex with receptor fragments allowed to identify the receptor binding site for the octasaccharide product of the enzymatic action of TSP on delipidated LPS and the active site consisting of Asp392, Asp395 and Glu359. The structure comprises a large right-handed parallel beta-helix of 13 turns. These fold independently in the trimer, whereas the N-terminus forms a cap-like structure and the C-terminal parts of the three polypeptide strands merge to a single common domain. In addition, TSP has served as model system for the folding of large, multisubunit proteins. Its folding pathway is influenced by a large number of point mutations, classified as lethal, temperature sensitive or general suppressor mutations, which influence the partitioning between aggregation and the productive folding pathway.

Conformation of P22 tailspike folding and aggregation intermediates probed by monoclonal antibodies

The partitioning of partially folded polypeptide chains between correctly folded native states and off-pathway inclusion bodies is a critical reaction in biotechnology. Multimeric partially folded intermediates, representing early stages of the aggregation pathway for the P22 tailspike protein, have been trapped in the cold and isolated by nondenaturing polyacrylamide gel electrophoresis (PAGE) (speed MA, Wang DIC, King J. 1995. Protein Sci 4:900-908). Monoclonal antibodies against tailspike chains discriminate between folding intermediates and native states (Friguet B, Djavadi-Ohaniance L, King J, Goldberg ME. 1994. J Biol Chem 269:15945-15949). Here we describe a nondenaturing Western blot procedure to probe the conformation of productive folding intermediates and off-pathway aggregation intermediates. The aggregation intermediates displayed epitopes in common with productive folding intermediates but were not recognized by antibodies against native epitopes. The nonnative epitope on the folding and aggregation intermediates was located on the partially folded N-terminus, indicating that the N-terminus remained accessible and nonnative in the aggregated state. Antibodies against native epitopes blocked folding, but the monoclonal directed against the N-terminal epitope did not, indicating that the conformation of the N-terminus is not a key determinant of the productive folding and chain association pathway.

Interactions of phage P22 tails with their cellular receptor, Salmonella O-antigen polysaccharide

Bacteriophage P22 binds to its cell surface receptor, the repetitive O-antigen structure in Salmonella lipopolysaccharide, by its six homotrimeric tailspikes. Receptor binding by soluble tailspikes and the receptor-inactivating endorhamnosidase activity of the tailspike protein were studied using octa- and dodecasaccharides comprising two and three O-antigen repeats of Salmonella enteritidis and Salmonella typhimurium lipopolysaccharides. Wild-type tailspike protein and three mutants (D392N, D395N, and E359Q) with defective endorhamnosidase activity were used. Oligosaccharide binding to all three subunits, measured by a tryptophan fluorescence quench or by fluorescence depolarization of a coumarin label attached to the reducing end of the dodecasaccharide, occurs independently. At 10 degrees C, the binding affinities of all four proteins to oligosaccharides from both bacterial strains are identical within experimental error, and the binding constants for octa- and dodecasaccharides are 1 x 10(6) M(-1) and 2 x 10(6) M(-1), proving that two O-antigen repeats are sufficient for lipopolysaccharide recognition by the tailspike. Equilibration with the oligosaccharides occurs rapidly, but the endorhamnosidase produces only one cleavage every 100 s at 10 degrees C or about 2 min(-1) at the bacterial growth temperature. Thus, movement of virions in the lipopolysaccharide layer before DNA injection may involve the release and rebinding of individual tailspikes rather than hydrolysis of the O-antigen.

A peptide derived from a beta2-adrenergic receptor transmembrane domain inhibits both receptor dimerization and activation

One of the assumptions of the mobile receptor hypothesis as it relates to G protein-coupled receptors is that the stoichiometry of receptor, G protein, and effector is 1:1:1 (Bourne, H. R., Sanders, D. A., and McCormick, F.(1990) Nature 348, 125-132). Many studies on the cooperativity of agonist binding are incompatible with this notion and have suggested that both G proteins and their associated receptors can be oligomeric. However, a clear physical demonstration that G protein-coupled receptors can indeed interact as dimers and that such interactions may have functional consequences was lacking. Here, using differential epitope tagging we demonstrate that beta2-adrenergic receptors do form SDS-resistant homodimers and that transmembrane domain VI of the receptor may represent part of an interface for receptor dimerization. The functional importance of dimerization is supported by the observation that a peptide derived from this domain that inhibits dimerization also inhibits beta-adrenergic agonist-promoted stimulation of adenylyl cyclase activity. Moreover, agonist stimulation was found to stabilize the dimeric state of the receptor, while inverse agonists favored the monomeric species, which suggests that interconversion between monomeric and dimeric forms may be important for biological activity.