Interaction of the Thermoplasma acidophilum A1A0-ATP synthase peripheral stalk with the catalytic domain

The peripheral stalk of the archaeal ATP synthase (A₁A₀)-ATP synthase is formed by the heterodimeric EH complex and is part of the stator domain, which counteracts the torque of rotational catalysis. Here we used nuclear magnetic resonance spectroscopy to probe the interaction of the C-terminal domain of the EH heterodimer (E_CT1H_CT) with the N-terminal 23 residues of the B subunit (B_NT). The data show a specific interaction of B_NT peptide with 26 residues of the E_CT1H_CT domain, thereby providing a molecular picture of how the peripheral stalk is anchored to the A₃B₃ catalytic domain in A₁A₀.

Structured summary

MINT-7260681: Hct (refseq:NP_393485), Ect1 (uniprotkb:Q9HM68) and Bnt (uniprotkb:Q9HM64) physically interact (MI:0915) by nuclear magnetic resonance (MI:0077)     

Chaperone Caf1M Stabilizes Hybrid Proteins Containing Sequences of F1 Antigen Subunit from Yersinia pestis

The Yersinia pestis(causative agent of plague) capsule antigen is a homopolymer of Caf1 protein. Export of the subunits is mediated by the periplasmic chaperone Caf1M. To study the mechanism of Caf1M activity, two hybrid genes including coding sequences for the Caf1 signal peptide, human granulocyte–macrophage colony-stimulating factor (GM-CSF) or interleukin-1 (IL-1) receptor antagonist, and mature Caf1 were constructed and expressed in Escherichia coli.We have shown that in the absence of Caf1M the majority of Caf1 moieties within the hybrid proteins undergo proteolysis in the periplasmic space, presumably by the DegP protease. The coexpression of a gene for chaperone Caf1M significantly increased the amount of full-size hybrid proteins in the periplasm, probably as a result of stabilization of the subunit's spatial structure within the hybrid. This effect was not observed in JCB571 cells, which lack periplasmic disulfide isomerase DsbA, essential for Caf1M activity.     

Analysis of the Open and Closed Conformations of the β Subunits in Thermophilic F1-ATPase by Solution NMR

F₁-ATPase, composed of α, β, γ, δ, and ? subunits, is a unique enzyme in terms of its rotational catalytic activity. The smallest unit showing this function is the α₃β₃γ complex. We have investigated the α₃β₃γ?^ΔC (?^ΔC, truncated ?) complex from thermophilic Bacillus PS3 (TF₁′, 360 kDa) in the solution state by using the combination of extensive deuteration, segmental-labeling, and CRINEPT (cross-correlated relaxation-enhanced polarization transfer) NMR. Well-resolved CRINEPT-HMQC (heteronuclear multiple-quantum correlation) spectra of partially ¹⁵N-labeled TF₁′ were obtained for this huge and asymmetric protein complex. The spectrum of the C-terminal domain of the β subunit revealed that the open form of the β subunit in the TF₁′ complex is similar to that of the free β monomer. The open β subunit in the TF₁′ complex does not exhibit high affinity for nucleotides unlike the monomer, but this is in agreement with the results of single-molecule analysis of TF₁α₃β₃γ. On the other hand, the closed form of the β subunit in the TF₁′ complex was shown to be distinct from that of the nucleotide-bound β monomer. This is consistent with a previous report that the closed form of the TF₁β monomer could be a catalytically activated state. The loop between the N-terminal β-barrel and the central domain is highly flexible in the TF₁′ complex, in contrast to that in the α₃β₃ hexamer, suggesting that it is affected by the presence of the γ subunit in this area.     

Structural basis of chaperone-subunit complex recognition by the type 1 pilus assembly platform FimD

Adhesive type 1 pili from uropathogenic Escherichia coli are filamentous protein complexes that are attached to the assembly platform FimD in the outer membrane. During pilus assembly, FimD binds complexes between the chaperone FimC and type 1 pilus subunits in the periplasm and mediates subunit translocation to the cell surface. Here we report nuclear magnetic resonance and X-ray protein structures of the N-terminal substrate recognition domain of FimD (FimD(N)) before and after binding of a chaperone-subunit complex. FimD(N) consists of a flexible N-terminal segment of 24 residues, a structured core with a novel fold, and a C-terminal hinge segment. In the ternary complex, residues 1-24 of FimD(N) specifically interact with both FimC and the subunit, acting as a sensor for loaded FimC molecules. Together with in vivo complementation studies, we show how this mechanism enables recognition and discrimination of different chaperone-subunit complexes by bacterial pilus assembly platforms.     

Functional characterization of recombinant prefoldin complexes from a hyperthermophilic archaeon, Thermococcus sp. strain KS-1

Prefoldin is a heterohexameric molecular chaperone complex that is found in the eukaryotic cytosol and also in archaea. It captures a nonnative protein and subsequently delivers it to a group II chaperonin for proper folding. Archaeal prefoldin is a heterocomplex containing two α subunits and four β subunits with the structure of a double β-barrel assembly, with six long coiled coils protruding from it like a jellyfish with six tentacles. We have studied the protein folding mechanism of group II chaperonin using those of Thermococcus sp. strain KS-1 (T. KS-1) because they exhibit high protein folding activity in vitro. We have also demonstrated functional cooperation between T. KS-1 chaperonins and prefoldin from Pyrococcus horikoshii OT3. Recent genome analysis has shown that Thermococcus kodakaraensis KOD1 contains two pairs of prefoldin subunit genes, correlating with the existence of two different chaperonin subunits. In this study, we characterized four different recombinant prefoldin complexes composed of two pairs of prefoldin subunits (α1, α2, β1, and β2) from T. KS-1. All of them (α1-β1, α2-β1, α1-β2, and α2-β2) exist as α₂β₄ heterohexamers and can protect several proteins from forming aggregates with different activities. We have also compared the collaborative activity between the prefoldin complexes and the cognate chaperonins. Prefoldin complexes containing the β1 subunit interacted with the chaperonins more strongly than those with the β2 subunit. The results suggest that Thermococcus spp. express different prefoldins for different substrates or conditions as chaperonins.     

Identification and characterization of the chaperone-subunit complex-binding domain from the type 1 pilus assembly platform FimD

The outer membrane protein FimD represents the assembly platform of adhesive type 1 pili from Escherichia coli. FimD forms ring-shaped oligomers of 91.4 kDa subunits that recognize complexes between the pilus chaperone FimC and individual pilus subunits in the periplasm and mediate subunit translocation through the outer membrane. Here, we have identified a periplasmic domain of FimD (FimD(N)) comprising the N-terminal 139 residues of FimD. Purified FimD(N) is a monomeric, soluble protein that specifically recognizes complexes between FimC and individual type 1 pilus subunits, but does not bind the isolated chaperone, or isolated subunits. In addition, FimD(N) retains the ability of FimD to recognize different chaperone-subunit complexes with different affinities, and has the highest affinity towards the FimC-FimH complex. Overexpression of FimD(N) in the periplasm of wild-type E.coli cells diminished incorporation of FimH at the tip of type 1 pili, while pilus assembly itself was not affected. The identification of FimD(N) and its ternary complexes with FimC and individual pilus subunits opens the avenue to structural characterization of critical type 1 pilus assembly intermediates.     

Structural and functional analysis of the coupling subunit F in solution and topological arrangement of the stalk domains of the methanogenic A1AO ATP synthase

The first low-resolution shape of subunit F of the A₁A_O ATP synthase from the archaeon Methanosarcina mazei Gö1 in solution was determined by small angle X-ray scattering. Independent to the concentration used, the protein is monomeric and has an elongated shape, divided in a main globular part with a length of about 4.5 nm, and a hook-like domain of about 3.0 nm in length. The subunit-subunit interaction of subunit F inside the A₁A_O ATP synthase in the presence of 1-ethyl-3-(dimethylaminopropyl)-carbodiimide EDC was studied as a function of nucleotide binding, demonstrating movements of subunits F relative to the nucleotide-binding subunit B. Furthermore, in the intact A₁A_O complex, crosslinking of subunits D-E, A-H and A-B-D was obtained and the peptides, involved, were analyzed by MALDI-TOF mass spectrometry. Based on these data the surface of contact of B-F could be mapped in the high-resolution structure of subunit B of the A₁A_O ATP synthase.     

Autotransporter β-domains have a specific function in protein secretion beyond outer-membrane targeting

Autotransporters (ATs) of Gram-negative bacteria contain an N-proximal passenger domain that is transported to the extracellular milieu and a C-terminal β-domain that inserts into the outer membrane (OM) in a β-barrel conformation. This β-domain facilitates translocation of the passenger domain across the OM and has long been considered to be the translocation pore. However, available crystal structures of β-domains show that the β-barrel pore is too narrow for the observed transport of folded elements within the passenger domains. ATs have recently been shown to interact with the β-barrel assembly machinery. These findings questioned a direct involvement of the β-domain in passenger translocation and suggested that it may only target the passenger to the β-barrel assembly machinery pore. To address the function of the β-domain in more detail, we have replaced the β-domain of the Escherichia coli AT hemoglobin protease by β-domains originating from other OM proteins. Furthermore, we have modified the diameter of the β-domain pore. The mutant proteins were analyzed for their capacity to insert into the OM and for surface display of the passenger. Our results show that efficient passenger secretion requires a specific β-domain that not only functions as a targeting device but also is directly involved in the translocation of the passenger to the cell surface.     

Structure of the type III secretion recognition protein YscU from Yersinia enterocolitica

The inner-membrane protein YscU has an important role during the assembly of the Yersinia enterocolitica type III secretion injectisome. Its cytoplasmic domain (YscU^C) recognizes translocators as individual substrates in the export hierarchy. Activation of YscU entails autocleavage at a conserved NPTH motif. Modification of this motif markedly changes the properties of YscU, including translocator export cessation and production of longer injectisome needles. We determined the crystal structures of the uncleaved variants N263A and N263D of YscU^C at 2.05 Å and 1.55 Å resolution, respectively. The globular domain is found to consist of a central, mixed β-sheet surrounded by α-helices. The NPTH motif forms a type II β-turn connecting two β-strands. NMR analysis of cleaved and uncleaved YscU^C indicates that the global structure of the protein is retained in cleaved YscU^C. The structure of YscU^C variant N263D reveals that wild type YscU^C is poised for cleavage due to an optimal reaction geometry for nucleophilic attack of the scissile bond by the side chain of Asn263. In vivo analysis of N263Q and H266A/R314A YscU variants showed a phenotype that combines the absence of translocator secretion with normal needle-length control. Comparing the structure of YscU to those of related proteins reveals that the linker domain between the N-terminal transmembrane domain and the autocleavage domain can switch from an extended to a largely α-helical conformation, allowing for optimal positioning of the autocleavage domain during injectisome assembly.     

Modulating Effects of the Plug, Helix, and N- and C-terminal Domains on Channel Properties of the PapC Usher

The chaperone/usher system is one of the best characterized pathways for protein secretion and assembly of cell surface appendages in Gram-negative bacteria. In particular, this pathway is used for biogenesis of the P pilus, a key virulence factor used by uropathogenic Escherichia coli to adhere to the host urinary tract. The P pilus individual subunits bound to the periplasmic chaperone PapD are delivered to the outer membrane PapC usher, which serves as an assembly platform for subunit incorporation into the pilus and secretion of the pilus fiber to the cell surface. PapC forms a dimeric, twin pore complex, with each monomer composed of a 24-stranded transmembrane β-barrel channel, an internal plug domain that occludes the channel, and globular N- and C-terminal domains that are located in the periplasm. Here we have used planar lipid bilayer electrophysiology to characterize the pore properties of wild type PapC and domain deletion mutants for the first time. The wild type pore is closed most of the time but displays frequent short-lived transitions to various open states. In comparison, PapC mutants containing deletions of the plug domain, an α-helix that caps the plug domain, or the N- and C-terminal domains form channels with higher open probability but still exhibiting dynamic behavior. Removal of the plug domain results in a channel with extremely large conductance. These observations suggest that the plug gates the usher channel closed and that the periplasmic domains and α-helix function to modulate the gating activity of the PapC twin pore.     

Crystal structure of crotoxin reveals key residues involved in the stability and toxicity of this potent heterodimeric β-neurotoxin

The crystal structure of crotoxin, a potent presynaptic neurotoxin from Crotalus durissusterrificus, was solved at 1.35 Å resolution. It shows the architecture of the three disulfide-linked polypeptide chains (α, β, and γ) of the acidic subunit CA noncovalently complexed with the basic phospholipase A₂ (PLA₂) subunit CB. The unique structural scaffold of the association of the CA and CB subunits indicates that posttranslational cleavage of the pro-CA precursor is a prerequisite for the assembly of the CA-CB complex. These studies provide novel structural insights to explain the role of the CA subunit in the mechanism of action of crotoxin. The crystal structure of the highly toxic and stable CA₂CBb complex crystallized here allows us to identify key amino acid residues responsible for significant differences in the pharmacological activities of the two classes of crotoxin complexes. In particular, we show that critical residues Trp31 and Trp70 of the CBb subunit establish intermolecular polar contacts with Asp99 and Asp89, respectively, of the β-chain of CA₂ and contribute to the stability and toxicity of the CA₂CBb complex. These interactions also lead to decreased PLA₂ activity by partially blocking substrate access to the catalytic dyad and by masking several interfacial binding surface residues important for PLA₂ interaction with phospholipids.Identification of the binding interface between the CA subunits and the CB subunits of crotoxin is important for the structure-based design of antineurotoxic inhibitors. Since crotoxin displays numerous physiological functions, including antitumoral properties, knowledge of its three-dimensional structure will be useful for the understanding of these diverse effects.     

The Outer Membrane Usher Guarantees the Formation of Functional Pili by Selectively Catalyzing Donor-Strand Exchange between Subunits That Are Adjacent in the Mature Pilus

Type 1 pili from uropathogenic Escherichia coli are a prototype of adhesive surface organelles assembled and secreted by the conserved chaperone/usher pathway. They are composed of four different homologous protein subunits that need to be assembled in a defined order. In the periplasm, the pilus chaperone FimC donates a β-strand segment to the subunits to complete their imperfect immunoglobulin-like fold. During subunit assembly, this segment of the chaperone is displaced by an amino-terminal extension of an incoming subunit in a reaction termed donor-strand exchange. To date, the molecular mechanisms underlying the coordinated subunit assembly, in particular the role of the outer membrane usher FimD, are still poorly understood. Here we show that the binding of complexes between FimC and the different pilus subunits to the amino-terminal substrate recognition domain of FimD is an extremely fast process, with association rate constants in the range of 10⁷-10⁸ M^− 1 s^− 1 at 20 °C. Furthermore, we demonstrate that the ordered assembly of pilus subunits is a consequence of the usher's ability to selectively catalyze the assembly of defined subunit-subunit pairs that are adjacent in the mature pilus. The usher therefore coordinates the assembly of pilus subunits at the stage of donor-strand exchange between pairs of subunits and not at the level of the initial binding of chaperone-subunit complexes.     

Crystal Structure of the Electron Carrier Domain of the Reaction Center Cytochrome cz Subunit from Green Photosynthetic Bacterium Chlorobium tepidum

In green sulfur photosynthetic bacteria, the cytochrome c_z (cyt c_z) subunit in the reaction center complex mediates electron transfer mainly from menaquinol/cytochrome c oxidoreductase to the special pair (P840) of the reaction center. The cyt c_z subunit consists of an N-terminal transmembrane domain and a C-terminal soluble domain that binds a single heme group. The periplasmic soluble domain has been proposed to be highly mobile and to fluctuate between oxidoreductase and P840 during photosynthetic electron transfer. We have determined the crystal structure of the oxidized form of the C-terminal functional domain of the cyt c_z subunit (C-cyt c_z) from thermophilic green sulfur bacterium Chlorobium tepidum at 1.3-Å resolution. The overall fold of C-cyt c_z consists of four α-helices and is similar to that of class I cytochrome c proteins despite the low similarity in their amino acid sequences. The N-terminal structure of C-cyt c_z supports the swinging mechanism previously proposed in relation with electron transfer, and the surface properties provide useful information on possible interaction sites with its electron transfer partners. Several characteristic features are observed for the heme environment: These include orientation of the axial ligands with respect to the heme plane, surface-exposed area of the heme, positions of water molecules, and hydrogen-bond network involving heme propionate groups. These structural features are essential for elucidating the mechanism for regulating the redox state of cyt c_z.     

The periplasmic domain of Escherichia coli outer membrane protein A can undergo a localized temperature dependent structural transition

Gram-negative bacteria such as Escherichia coli are surrounded by two membranes with a thin peptidoglycan (PG)-layer located in between them in the periplasmic space. The outer membrane protein A (OmpA) is a 325-residue protein and it is the major protein component of the outer membrane of E. coli. Previous structure determinations have focused on the N-terminal fragment (residues 1–171) of OmpA, which forms an eight stranded transmembrane β-barrel in the outer membrane. Consequently it was suggested that OmpA is composed of two independently folded domains in which the N-terminal β-barrel traverses the outer membrane and the C-terminal domain (residues 180–325) adopts a folded structure in the periplasmic space. However, some reports have proposed that full-length OmpA can instead refold in a temperature dependent manner into a single domain forming a larger transmembrane pore. Here, we have determined the NMR solution structure of the C-terminal periplasmic domain of E. coli OmpA (OmpA^180–325). Our structure reveals that the C-terminal domain folds independently into a stable globular structure that is homologous to the previously reported PG-associated domain of Neisseria meningitides RmpM. Our results lend credence to the two domain structure model and a PG-binding function for OmpA, and we could indeed localize the PG-binding site on the protein through NMR chemical shift perturbation experiments. On the other hand, we found no evidence for binding of OmpA^180–325 with the TonB protein. In addition, we have also expressed and purified full-length OmpA (OmpA^1–325) to study the structure of the full-length protein in micelles and nanodiscs by NMR spectroscopy. In both membrane mimetic environments, the recombinant OmpA maintains its two domain structure that is connected through a flexible linker. A series of temperature-dependent HSQC experiments and relaxation dispersion NMR experiments detected structural destabilization in the bulge region of the periplasmic domain of OmpA above physiological temperatures, which may induce dimerization and play a role in triggering the previously reported larger pore formation.     

Analysis of the requirements for pilus biogenesis at the outer membrane usher and the function of the usher C-terminus

Uropathogenic strains of Escherichia coli assemble type 1 and P pili to colonize the bladder and kidney respectively. These pili are prototype structures assembled by the chaperone/usher secretion pathway. In this pathway, a periplasmic chaperone works together with an outer membrane (OM) usher to control the folding of pilus subunits, their assembly into a pilus fibre and secretion of the fibre to the cell surface. The usher serves as the assembly and secretion platform in the OM. The usher has distinct functional domains, with the N-terminus providing the initial targeting site for chaperone-subunit complexes and the C-terminus required for subsequent stages of pilus biogenesis. In this study, we investigated the molecular interactions occurring at the usher during pilus biogenesis and the function of the usher C-terminus. We provide genetic and biochemical evidence that the usher functions as a complex in the OM and that interaction of the pilus adhesin with the usher is critical to prime the usher for pilus biogenesis. Analysis of C-terminal truncation and substitution mutants of the P pilus usher PapC demonstrated that the C-terminus is required for proper binding of chaperone-subunit complexes to the usher and plays an important role in assembly of complete pili.     

Mutagenesis Studies of the F1F0 ATP Synthase b Subunit Membrane Domain

A homodimer of b subunits constitutes the peripheral stalk linking the F₁ and F₀ sectors of the Escherichia coli ATP synthase. Each b subunit has a single-membrane domain. The constraints on the membrane domain have been studied by systematic mutagenesis. Replacement of a segment proximal to the cytoplasmic side of the membrane had minimal impact on F₁F₀ ATP synthase. However, multiple substitutions on the periplasmic side resulted in defects in assembly of the enzyme complex. These mutants had insufficient oxidative phosphorylation to support growth, and biochemical studies showed little F₁F₀ ATPase and no detectable ATP-driven proton pumping activity. Expression of the b _N2A,T6A,Q10A subunit was also oxidative phosphorylation deficient, but the b _N2A,T6A,Q10A protein was incorporated into an F₁F₀ complex. Single amino acid substitutions had minimal reductions in F₁F₀ ATP synthase function. The evidence suggests that the b subunit membrane domain has several sites of interaction contributing to assembly of F₀, and that these interactions are strongest on the periplasmic side of the bilayer.     

The 'P-usher', a novel protein transporter involved in fimbrial assembly and TpsA secretion

We identified a new bacterial transporter, the Pseudomonas aeruginosa CupB3 protein, which is an outer membrane usher involved in pili assembly. In CupB3, the usher domain has fused during evolution with a POTRA (polypeptide-transport-associated)-like domain found in TpsB transporters of two-partner secretion systems. In TpsBs, the POTRA captures the TpsA passenger, which is then transported across the outer membrane through the TpsB beta-barrel. We named CupB3 a 'P-usher' for POTRA-like domain-containing usher. We showed that CupB3 assembles CupB1 fimbrial subunits into pili and secretes CupB5, a TpsA-like protein. The CupB3 usher domain has the function of a TpsB beta-barrel in CupB5 translocation. We revealed that the POTRA-like domain is neither essential for CupB1 fimbriae assembly nor for cell surface exposition of CupB5, but is crucial to coordinate bona fide transport of CupB1 and CupB5 through the usher domain. The P-usher defines a novel transport pathway involving a molecular machine made with old spare parts.     

Oligomerization of the SPP1 scaffolding protein

Viral scaffolding proteins direct polymerization of major capsid protein subunits into icosahedral procapsid structures. The scaffolding protein of bacteriophage SPP1 was engineered with a C-terminal hexahistidine tag (gp11-His₆) and purified. The protein is an α-helical-rich molecule with a very elongated shape as found for internal scaffolding proteins from other phages. It is a 3.3 S tetramer of 93.6 kDa at micromolar concentrations. Intersubunit cross-linking of these tetramers generated preferentially covalently bound dimers, revealing that gp11-His₆ is structurally a dimer of dimers. Incubation at temperatures above 37 °C correlated with a reduction of its α-helical content and a less effective intersubunit cross-linking. Complete loss of secondary structure was observed at temperatures above 60 °C. Refolding of gp11-His₆ thermally denatured at 65 °C led to reacquisition of the protein native ellipticity spectrum but the resulting population of molecules was heterogeneous. Its hydrodynamic behavior was compatible with a mix of 3.3 S elongated tetramers (∼ 90%) and a smaller fraction of 2.4 S dimers (∼ 10%). This population of gp11-His₆ was competent to direct polymerization of the SPP1 major capsid protein gp13 into procapsid-like structures in a newly developed assembly assay in vitro. Although native tetramers were active in assembly, refolded gp11-His₆ showed enhanced binding to gp13 revealing a more active species for interaction with the major capsid protein than native gp11-His₆.     

The crystal structure of BamB suggests interactions with BamA and its role within the BAM complex

Escherichia coli BamB is the largest of four lipoproteins in the β-barrel assembly machinery (BAM) complex. It interacts with the periplasmic domain of BamA, an integral outer membrane protein (OMP) essential for OMP biogenesis. Although BamB is not essential, it serves an important function in the BAM complex, significantly increasing the folding efficiency of some OMPs in vivo and in vitro. To learn more about the BAM complex, we solved structures of BamB in three different crystal forms. BamB crystallized in space groups P2₁3, I222, and P2₁2₁2₁, with one molecule per asymmetric unit in each case. Crystals from the space group I222 diffracted to 1. 65-Å resolution. BamB forms an eight-bladed β-propeller with a central pore and is shaped like a doughnut. A DALI search revealed that BamB shares structural homology to several eukaryotic proteins containing WD40 repeat domains, which commonly have β-propeller folds and often serve as scaffolding proteins within larger multi-protein complexes that carry out signal transduction, cell division, and chemotaxis. Using mutagenesis data from previous studies, we docked BamB onto a BamA structural model and assessed known and possible interactions between these two proteins. Our data suggest that BamB serves as a scaffolding protein within the BAM complex by optimally orienting the flexible periplasmic domain of BamA for interaction with other BAM components and chaperones. This may facilitate integration of newly synthesized OMPs into the outer membrane.     

The GABAA receptor is an FMRP target with therapeutic potential in fragile X syndrome

Previous research indicates that the GABA_Aergic system is involved in the pathophysiology of the fragile X syndrome, a frequent form of inherited intellectual disability and associated with autism spectrum disorder. However, the molecular mechanism underlying GABA_Aergic deficits has remained largely unknown. Here, we demonstrate reduced mRNA expression of GABA_A receptor subunits in the cortex and cerebellum of young Fmr1 knockout mice. In addition, we show that the previously reported underexpression of specific subunits of the GABA_A receptor can be corrected in YAC transgenic rescue mice, containing the full-length human FMR1 gene in an Fmr1 knockout background. Moreover, we demonstrate that FMRP directly binds several GABA_A receptor mRNAs. Finally, positive allosteric modulation of GABA_A receptors with the neurosteroid ganaxolone can modulate specific behaviors in Fmr1 knockout mice, emphasizing the therapeutic potential of the receptor.