An aspartate/insulin receptor chimera mitogenically activates fibroblasts

Evidence that the adaptation region of the aspartate receptor is a dynamic four-helix bundle: cysteine and disulfide scanning studies

The aspartate receptor is one of the ligand-specific, homodimeric chemoreceptors that detects extracellular attractants and triggers the chemotaxis pathway of Escherichia coli and Salmonella typhimurium. This receptor regulates the activity of the histidine kinase CheA, which forms a kinetically stable complex with the receptor cytoplasmic domain. An atomic four-helix bundle model has been constructed for this domain, which is functionally subdivided into the signaling and adaptation subdomains. The proposed four-helix bundle structure of the signaling subdomain, which binds CheA, is fully supported by experimental evidence. Much less evidence is available to test the four-helix bundle model of the adaptation subdomain, which possesses covalent adaptation sites and docking surfaces for adaptation enzymes. The present study focuses on a putative helix near the C terminus of the adaptation subdomain. To probe the structural and functional features of positions G467-A494 in this C-terminal region, a cysteine and disulfide scanning approach has been employed. Measurement of the chemical reactivities of scanned cysteines reveals an alpha-helical periodicity of exposed and buried residues, confirming alpha-helical secondary structure and mapping out a buried packing face. The effects of cysteine substitutions on activity in vivo and in vitro highlight the functional importance of the helix, especially its buried face. A scan for disulfide bond formation between symmetric pairs of engineered cysteines reveals promiscuous collisions between subunits, indicating the presence of significant thermal dynamics. A scan for functional disulfides reveals lock-on and signal-retaining disulfide bonds formed between symmetric pairs of cysteines at buried positions, indicating that the buried face of the helix lies near the subunit interface of the homodimer in the equilibrium structures of both the apo and aspartate-bound states where it plays a critical role in kinase regulation. These results strongly support the existing four-helix bundle model of the adaptation subdomain structure. A mechanistic model is proposed in which a signal is transmitted through the adaptation subdomain by a change in supercoiling of the four-helix bundle.

Site-directed rotational resonance solid-state NMR distance measurements probe structure and mechanism in the transmembrane domain of the serine bacterial chemoreceptor

The serine receptor of bacterial chemotaxis is an ideal system in which to investigate the molecular mechanism of transmembrane signaling. Solid-state nuclear magnetic resonance (NMR) techniques such as rotational resonance provide a means for measuring local structure and ligand-induced structural changes in intact membrane proteins bound to native membrane vesicles. A general site-directed biosynthetic (13)C labeling strategy is used to direct the distance measurements to a specific site; the distance is measured between a unique Cys residue and a non-unique, low-abundance residue (Tyr or Phe). A (13)C-(13)C internuclear distance measurement from (13)CO(i) to (13)C beta(i + 3) at the periplasmic edge of the second membrane-spanning helix (TM2) of 5.1 +/- 0.2 A is consistent with the predicted alpha-helical structure and thus demonstrates an accurate long-distance rotational resonance measurement in the 120 kDa membrane-bound receptor. These measurements require a correction for the rotational resonance exchange between the multiple labels of the non-unique amino acid and the natural-abundance (13)C, which is critical to distance measurements in complex systems. A second (13)C-(13)C distance measurement between the transmembrane helices provides a high-resolution measurement of tertiary structure in the transmembrane region. The measured 5.0-5.3 A distance in the presence and absence of ligand is consistent with structural models for the transmembrane region and a proposed signaling mechanism in which ligand binding induces a 1.6 A translation of TM2. This approach can be used for additional measurements of the structure of the transmembrane region and to determine whether the ligand-induced motion is indeed propagated through the transmembrane helices.

Hydrogen exchange reveals a stable and expandable core within the aspartate receptor cytoplasmic domain

Intensive study of bacterial chemoreceptors has not yet revealed how receptor methylation and ligand binding alter the interactions between the receptor cytoplasmic domain and the CheA kinase to control kinase activity. Both monomeric and dimeric forms of an Asp receptor cytoplasmic fragment have been shown to be highly dynamic, with a small core of slowly exchanging amide hydrogens (Seeley, S. K., Weis, R. M., and Thompson, L. K. (1996) Biochemistry 35, 5199-5206). Hydrogen exchange studies of the wild-type cytoplasmic fragment and an S461L mutant thought to mimic the kinase-inactivating state are used to investigate the relationship between the stable core and dimer dissociation. Our results establish that (i) decreasing pH stabilizes the dimeric state, (ii) the stable core is present also in the transition state for dissociation, and (iii) this core is expanded significantly by small changes in electrostatic and hydrophobic interactions. These kinase-inactivating changes stabilize both the monomeric and the dimeric states of the protein, which has interesting implications for the mechanism of kinase activation. We conclude that the cytoplasmic domain is a flexible region poised for stabilization by small changes in electrostatic and hydrophobic interactions such as those caused by methylation of glutamate residues and by ligand-induced conformational changes during signaling.

Signalling substitutions in the periplasmic domain of chemoreceptor Trg induce or reduce helical sliding in the transmembrane domain

We used in vivo oxidative cross-linking of engineered cysteine pairs to assess conformational changes in the four-helix transmembrane domain of chemoreceptor Trg. Extending previous work, we searched for and found a fourth cross-linking pair that spanned the intrasubunit interface between transmembrane helix 1 (TM1) and its partner TM2. We determined the effects of ligand occupancy on cross-linking rate constants for all four TM1-TM2 diagnostic pairs in conditions that allowed the formation of receptor-kinase complexes for the entire cellular complement of Trg. Occupancy altered all four rates in a pattern that implicated sliding of TM2 relative to TM1 towards the cytoplasm as the transmembrane signalling movement in receptor-kinase complexes. Transmembrane signalling can be reduced or induced by single amino acid substitutions in the ligand-binding region of the periplasmic domain of Trg. We determined the effects of these substitutions on conformation in the transmembrane domain and on ligand-induced changes using the diagnostic TM1-TM2 cysteine pairs. Effects on rates of in vivo cross-linking showed that induced signalling substitutions altered the relative positions of TM1 and TM2 in the same way as ligand binding, and reduced signalling substitutions blocked or attenuated the ligand-induced shift. These results provide strong support for the helical sliding model of transmembrane signalling.

Site-directed solid-state NMR measurement of a ligand-induced conformational change in the serine bacterial chemoreceptor

The challenging nature of studies of membrane proteins has made it difficult to determine the molecular mechanism of transmembrane signaling. For the bacterial chemoreceptor family, there are crystal structures of the internal and external domains, structural models of the transmembrane domain, and evidence for subtle ligand-induced conformational changes, but the signaling mechanism remains controversial. We have used a novel site-directed solid-state NMR distance measurement approach, using (13)C(19)F REDOR, to measure a ligand-induced change of 1.0 +/- 0.3 A in the distance between helices alpha 1 and alpha 4 of the ligand-binding domain in the intact, membrane-bound serine receptor. This distance change is shown not to be due to motion of the side chain and thus is due to motion of either the alpha 1 or the alpha 4 helix. Additional distance measurements can be used to determine the type of backbone motion and to follow it to the cytoplasm, to test and refine current proposals for the mechanism of transmembrane signaling. This is a promising general method for high-resolution measurements of local structure in intact, membrane-bound proteins.

Chemotaxis receptors: a progress report on structure and function

Recent biochemical and structural studies have provided many new insights into the structure and function of bacterial chemoreceptors. Aspects of their ligand binding, conformational changes, and interactions with other members of the signaling pathway are being defined at the structural level. It is anticipated that the combined effort will soon provide a detailed, unified view of an entire response system.

Detection of a conserved alpha-helix in the kinase-docking region of the aspartate receptor by cysteine and disulfide scanning

The transmembrane aspartate receptor of Escherichia coli and Salmonella typhimurium propagates extracellular signals to the cytoplasm, where its cytoplasmic domain regulates the histidine kinase, CheA. Different signaling states of the cytoplasmic domain modulate the kinase autophosphorylation rate over at least a 100-fold range. Biochemical and genetic studies have implicated a specific region of the cytoplasmic domain, termed the signaling subdomain, as the region that transmits regulation from the receptor to the kinase. Here cysteine and disulfide scanning are applied to the N-terminal half of the signaling subdomain to probe its secondary structure, solvent exposure, and protein-protein interactions. The chemical reactivities of the scanned cysteines exhibit the characteristic periodicity of an alpha-helix with distinct solvent-exposed and buried faces. This helix, termed alpha7, ranges approximately from residue 355 through 386. Activity measurements probing the effects of cysteine substitutions in vivo and in vitro reveal that both faces of helix alpha7 are critical for kinase activation, while the buried face is especially critical for kinase down-regulation. Disulfide scanning of the region suggests that helix alpha7 is not in direct contact with its symmetric partner (alpha7') from the other subunit; presently, the structural element that packs against the buried face of the helix remains unidentified. Finally, a novel approach termed "protein interactions by cysteine modification" indicates that the exposed C-terminal face of helix alpha7 provides an essential docking site for the kinase CheA or for the coupling protein CheW.

Cysteine and disulfide scanning reveals two amphiphilic helices in the linker region of the aspartate chemoreceptor

The transmembrane aspartate receptor of E. coli and S. typhimurium mediates cellular chemotaxis toward aspartate by regulating the activity of the cytoplasmic histidine kinase, CheA. Ligand binding results in transduction of a conformational signal through the membrane to the cytoplasmic domain where both kinase regulation and adaptation occur. Of particular interest is the linker region, E213 to Q258, which connects and transduces the conformational signal between the cytoplasmic end of the transmembrane signaling helix (alpha 4/TM2) and the major methylation helix of the cytoplasmic domain (alpha 6). This linker is crucial for stable folding and function of the homodimeric receptor. The present study uses cysteine and disulfide scanning mutagenesis to investigate the secondary structure and packing surfaces within the linker region. Chemical reactivity assays reveal that the linker consists of three distinct subdomains: two alpha-helices termed alpha 4 and alpha 5 and, between them, an ordered region of undetermined secondary structure. When cysteine is scanned through the helices, characteristic repeating patterns of solvent exposure and burial are observed. Activity assays, both in vivo and in vitro, indicate that each helix possesses a buried packing face that is crucial for proper receptor function. The interhelical subdomain is at least partially buried and is also crucial for proper receptor function. Disulfide scanning places helix alpha 4 distal to the central axis of the homodimer, while helix alpha 5 is found to lie at the subunit interface. Finally, sequence alignments suggest that all three linker subdomains are highly conserved among the large subfamily of histidine kinase-coupled sensory receptors that possess methylation sites for use in covalent adaptation.

Cysteine and disulfide scanning reveals a regulatory alpha-helix in the cytoplasmic domain of the aspartate receptor

The transmembrane, homodimeric aspartate receptor of Escherichia coli and Salmonella typhimurium controls the chemotactic response to aspartate, an attractant, by regulating the activity of a cytoplasmic histidine kinase. The cytoplasmic domain of the receptor plays a central role in both kinase regulation and sensory adaptation, although its structure and regulatory mechanisms are unknown. The present study utilizes cysteine and disulfide scanning to probe residues Leu-250 through Gln-309, a region that contains the first of two adaptive methylation segments within the cytoplasmic domain. Following the introduction of consecutive cysteine residues by scanning mutagenesis, the measurement of sulfhydryl chemical reactivities reveals an alpha-helical pattern of exposed and buried positions spanning residues 270-309. This detected helix, termed the "first methylation helix," is strongly amphiphilic; its exposed face is highly anionic and possesses three methylation sites, while its buried face is hydrophobic. In vivo and in vitro assays of receptor function indicate that inhibitory cysteine substitutions are most prevalent on the buried face of the first methylation helix, demonstrating that this face is involved in a critical packing interaction. The buried face is further analyzed by disulfide scanning, which reveals three "lock-on" disulfides that covalently trap the receptor in its kinase-activating state. Each of the lock-on disulfides cross-links the buried faces of the two symmetric first methylation helices of the dimer, placing these helices in direct contact at the subunit interface. Comparative sequence analysis of 56 related receptors suggests that the identified helix is a conserved feature of this large receptor family, wherein it is likely to play a general role in adaptation and kinase regulation. Interestingly, the rapid rates and promiscuous nature of disulfide formation reactions within the scanned region reveal that the cytoplasmic domain of the full-length, membrane-bound receptor has a highly dynamic structure. Overall, the results demonstrate that cysteine and disulfide scanning can identify secondary structure elements and functionally important packing interfaces, even in proteins that are inaccessible to other structural methods.