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Proc Natl Acad Sci U S A. Sep 26, 2006; 103(39): 14313–14318.
Published online Sep 14, 2006. doi:  10.1073/pnas.0606350103
PMCID: PMC1599961
Biochemistry

Self-assembly of receptor/signaling complexes in bacterial chemotaxis

Abstract

Escherichia coli chemotaxis is mediated by membrane receptor/histidine kinase signaling complexes. Fusing the cytoplasmic domain of the aspartate receptor, Tar, to a leucine zipper dimerization domain produces a hybrid, lzTarC, that forms soluble complexes with CheA and CheW. The three-dimensional reconstruction of these complexes was different from that anticipated based solely on structures of the isolated components. We found that analogous complexes self-assembled with a monomeric cytoplasmic domain fragment of the serine receptor without the leucine zipper dimerization domain. These complexes have essentially the same size, composition, and architecture as those formed from lzTarC. Thus, the organization of these receptor/signaling complexes is determined by conserved interactions between the constituent chemotaxis proteins and may represent the active form in vivo. To understand this structure in its cellular context, we propose a model involving parallel membrane segments in receptor-mediated CheA activation in vivo.

Keywords: CheA, histidine kinase, serine receptor, signal transduction

The Escherichia coli chemotaxis system provides a paradigm for understanding general principles of signal transduction and membrane receptor function. In E. coli, chemotaxis responses are mediated by changes in protein phosphorylation associated with the binding of ligands to membrane receptors at the cell surface. We are interested in defining the key protein–protein interactions that mediate signal transduction between the receptors and the protein kinase CheA, which modulates motor responses within the cytoplasm. Considerable evidence supports a model where CheA activity is regulated within large multimeric complexes formed between CheA, the α-helical cytoplasmic domain of the chemotaxis receptors, and an SH3 (Src homology 3)-like β-barrel protein, CheW. These components and their interactions are highly conserved among virtually all chemotactic bacterial and archaeal species (1, 2).

Atomic resolution structures of the cytoplasmic domain of the serine receptor (TsrC) (3), the cytoplasmic domain of another receptor, MCP1143C (4), CheA (47), and CheW (4, 8) have been determined by x-ray crystallographic and NMR methods. Crystals of TsrC and MCP1143C both contain long, antiparallel α-helical coiled-coil monomers that come together in parallel to form long, dimeric four-helix bundles (3, 4). The cytoplasmic domain of the aspartate receptor (TarC), which is 75% identical to TsrC, is predominantly an α-helical monomer in solution (9, 10).

The periplasmic aspartate-binding domain of the aspartate receptor (Tar) is dimeric both in solution and in crystals (11, 12), and the corresponding serine-binding domain of the serine receptor (Tsr) appears to have a similar structure (13). Cross-linking studies of Tar and Tsr in membranes are consistent with a dimeric organization, although recent results indicate that higher order assemblies are probably involved (1417). Immuno-EM, immunofluorescence, and GFP-tagging studies have established that, within a cell, receptors, CheA, and CheW cluster together into large complexes that may contain thousands of subunits of each protein (18, 19). Formation of these assemblies depends on the presence of all three components. Complexes of Tar or Tsr, CheA, and CheW have also been formed from purified components in vitro (20, 21). The complexes are characterized by >100-fold increases in CheA-kinase activity that are subject to inhibition by addition of aspartate or serine (22, 23).

Soluble fragments of TarC or TsrC together with CheW have been shown to activate CheA (10, 24, 25). In the case of TarC, activation was greatly facilitated by the attachment of a leucine zipper dimerization domain to the TarC N terminus. The resulting lzTarC hybrids formed mixtures of dimers and tetramers in solution (10, 25). Complexes of CheA and CheW with lzTarC have been imaged by using transmission EM (TEM) (2628). The structure of this assembly did not fit preconceived notions of receptor organization. It had generally been assumed that receptor dimers interacted with CheW monomers and CheA dimers to form 2:2:2 complexes (10, 29). The lzTarC complexes are much larger structures, however, with a monomer subunit composition of ≈24 lzTarC:6 CheW:4 CheA. The 12 lzTarC four-helix bundles in the complex exhibited an unexpected barrel-like arrangement, with two sets of 6 parallel four-helix bundles emerging from a central region where CheA and CheW are bound and the leucine zipper domains at either end (27, 28).

The arrangement of lzTarC within these large assemblies (28) is different from the “trimer of dimers” arrangement of TsrC (3) or the “hedgerow of dimers” arrangement of MCP1143C (4) that is seen in crystals in which the signaling domain hairpin regions are closely associated in the absence of CheW and CheA. As seen in these crystals, the unit of lzTarC structure appears to be a four-helix bundle; however, in receptor/signaling complexes, the bundles splay apart from each other in the signaling domain hairpin region where CheW and CheA bind.

To rule out the possibility that the structure of lzTarC/CheW/CheA (lzTarCWA) complexes are a reflection of some special property of the lzTarC hybrid receptor fragment, we have examined complexes formed with a TsrC protein that is essentially the same fragment whose structure has been determined by x-ray crystallography. TsrC does not have any dimerization domain or other elements that do not occur in the full-length Tsr receptor. We show here that although the TsrC protein is predominantly a monomer in solution, it is still capable of self-assembly into complexes with CheW and CheA. Moreover, these TsrC/CheW/CheA (TsrCWA) complexes have a similar size, shape, and organization to those formed with lzTarC. Our results suggest that the structures of these ternary complexes may reflect inherent features of the interactions of these components in functional chemotaxis receptor/signaling complexes in vivo.

Results

The lzTarC and TsrC proteins were compared by using gel filtration chromatography with inline multiangle light scattering to measure the absolute mass of protein eluents (Fig. 1). lzTarC exhibits two elution peaks, one with a molecular mass corresponding to a dimer and the other with the expected mass of a tetramer. In contrast, TsrC eluted as a single peak with a mass corresponding to that of the monomer. Both lzTarC and TsrC eluted with retention times corresponding to globular proteins with about twice the actual molecular mass, consistent with the extended α-helical conformation observed in crystals of TsrC (3) and MCP1143C (4).

Fig. 1.
HPLC and multiangle light-scattering analysis of isolated lzTarC and TsrC proteins. The left axis shows the molar mass of lzTarC tetramers and dimers (○) and TsrC monomers (×). The right axis shows the absorbance at 280 nm of lzTarC (solid ...

Despite its monomeric state in solution, in the presence of CheW, TsrC activates CheA as effectively as lzTarC. Whereas the specific activity of isolated CheA in solution under these conditions is 0.11 s−1, the maximal activity in complexes with TsrC and CheW is 10.3 s−1, which gives an activation factor of 94-fold, nearly identical to the 100-fold activation for CheA in complexes with lzTarC and CheW (27). As reported previously with lzTarC and another fragment of TsrC, this activation is highly dependent on receptor concentration (24, 26). For CheA activation as a function of receptor concentration, we observe a Hill coefficient of 1.7 and a midpoint of 40 μM for lzTarC, as compared with a Hill coefficient of 3.5 and a midpoint of 54 μM for TsrC (data not shown).

Under conditions that are optimal for kinase activation, receptor/signaling complexes formed with lzTarC or TsrC have a very similar hydrodynamic radius and protein composition as measured by gel filtration chromatography (Fig. 2). The active complexes were isolated by HPLC and denatured, and then their components were quantified in the presence of 6 M guanidine-HCl. This analysis indicated that lzTarCWA and TsrCWA complexes had very similar subunit compositions (Fig. 2 Insets and Table 1). These subunit ratios are in good agreement with previous investigations, which used analysis of bands on SDS/PAGE gels and light scattering (26) or a three-dimensional EM reconstruction and fitting of molecular structures (28) to estimate the subunit composition. In both cases, four CheA monomers per complex were found, and we used this number as a known value in our analysis. In Table 1, we present the stoichiometry in terms of the number of CheW and receptor monomers per CheA monomer. We define this composition of one CheA subunit plus the listed number of CheW and receptor subunits as a “stoichiometric unit.” Using these calculated stoichiometries and the absolute molecular mass determined by multiangle light scattering, we find that the TsrCWA complexes each contain ≈4.2 stoichiometric units (Table 1) when the calculations are constrained to give 4.0 stoichiometric units per lzTarCWA complex (data not shown).

Fig. 2.
HPLC two-dimensional gel filtration analysis of soluble receptor/signaling complexes. (A) lzTarCWA. (B) TsrCWA. Complexes were purified by HPLC using a TSK-Gel G5000PWXL column, and fractions were analyzed by using SDS/PAGE gels (not shown) to confirm ...
Table 1.
Stoichiometry of receptor/signaling complexes

The structural similarity between complexes formed with either TsrC or lzTarC was further confirmed by TEM. The TsrCWA and lzTarCWA complexes have the same overall shape and organization (Fig. 3). Both are elongated and have higher density at the middle and ends. The radius of gyration for the lzTarCWA complex is 13.0 nm when calculated from the three-dimensional reconstruction (28, 30). Comparison with the values in Table 1 suggests that the complex may have a more open or extended structure in solution as opposed to when it is fixed in uranyl acetate stain.

Fig. 3.
TEM analysis of receptor/signaling complexes. (A) TEM image of a field of lzTarCWA complexes. (Scale bar, 100 nm.) (B) Two-dimensional average of 4,900 images of individual lzTarCWA complexes (Upper) and the variance among those images (Lower). (Scale ...

At a concentration of 5 μM, lzTarC forms few or no complexes in the presence of 10 μM CheA and CheW (10, 26), and a smaller Tsr cytoplasmic fragment that is similar to TsrC has been reported not to activate CheA at this concentration (24). Using the coupled system assay, we detected no activation of CheA by 5 μM TsrC. To confirm that no complexes form, a 5 μM concentration of either lzTarC or TsrC was mixed with 10 μM CheA and CheW in RBIII buffer (see Materials and Methods for buffer composition) and analyzed by HPLC. As expected, no signal corresponding to the soluble complexes was observed at the elution time of ≈14.5 min. Running the corresponding fractions on the gel, no protein was detectable by Coomassie staining.

To demonstrate that lzTarC and TsrC are functionally interchangeable in the soluble complexes, we attempted to form complexes that contain a mix of the two. For these experiments, we relied on the fact that no complexes formed with a 5 μM concentration of receptor. If 5 μM lzTarC or TsrC was added to a 45 μM concentration of the other and incubated with 10 μM CheA and CheW, complexes formed and CheA was activated. This activation was very similar to complexes formed with a 50 μM concentration of either lzTarC or TsrC. Both types of receptor fragments coelute in the fractions that contain the receptor/signaling complexes and are readily visible on a Coomassie-stained gel of the corresponding fractions (Fig. 4), suggesting that individual complexes contained a mixture of lzTarC and TsrC subunits.

Fig. 4.
HPLC and SDS/PAGE analysis of mixed receptor/signaling complexes. (A) Complexes containing 45 μM lzTarC and 5 μM TsrC. (B) Complexes containing 45 μM TsrC and 5 μM lzTarC. Gray lines indicate the breadth of each HPLC fraction ...

Discussion

We have characterized a fragment of Tsr, TsrC, that corresponds closely to the fragment whose structure was determined by x-ray crystallographic methods (3). The crystallized fragment contained Tsr residues 286–526, whereas our fragment contains residues 290–551. In the crystal structure of TsrC, a trimer of dimers was observed; in solution, however, we see that the TsrC protein is a monomer. The hydrodynamic properties of the TsrC monomers are consistent with the extended, antiparallel, coiled-coil α-helical monomeric unit seen in TsrC crystals and in MCP1143C (4) crystals. These monomers presumably come together during the crystallization process to form the assemblies of dimers seen in the crystal structures.

Immuno-EM labeling and three-dimensional reconstruction of lzTarCWA complexes previously established that CheA and CheW are in the central region of the barrel-like receptor/signaling complexes (27, 28), whereas the N and C termini of the receptors are at the ends. Examination of the corresponding two-dimensional average of the TEM images of the TsrCWA complexes suggests an essentially identical overall organization (Fig. 3). The length of the TsrCWA complex is somewhat shorter, consistent with the lack of density at the ends where the leucine zippers are present in the lzTarCWA complexes. The TsrCWA complexes appear to be better ordered overall, with a more structured average and variance map.

Our finding of 4.2 stoichiometric units versus 4.0 per receptor/signaling complex is probably not a significant difference, given the uncertainties in the calculation. We thus conclude that both types of complexes contain four CheA subunits and four CheW subunits per complex. The difference in the number of receptor subunits (Table 1) is outside of the range of variation observed and suggests that the TsrCWA complexes have, on average, 21 or 22 receptor subunits, rather than the 24 we observe in the lzTarCWA complexes. This result is consistent with the lower stability we observe for the TsrCWA complexes when they are diluted or otherwise perturbed, and it suggests that the TsrC receptor fragments are in equilibrium between the monomer form seen in solution (Fig. 1) and a dimer or multimer in the complex. In contrast, because lzTarC is always dimeric, a full complement of receptor fragments is always observed in the complex.

We have previously shown that, in lzTarCWA complexes, the arrangement of dimeric four-helix bundles differs substantially from what is seen in the crystal structure of TsrC (28). Here, we show that analogous receptor/signaling complexes formed with TsrC have essentially the same structure as those formed with lzTarC. The organization and shape of these complexes therefore appears to be determined by the nature of the conserved interactions among the receptor signaling domains, CheW, and CheA rather than by any particular property of the soluble receptor fragments that are used. This structure is also consistent with a recent analysis of the in vivo FRET data. The analysis suggests that the receptors form isolated, strongly coupled clusters rather than interacting with many receptors in an extended array (31). In addition, the stoichiometries in the soluble receptor/signaling complexes are consistent with the recent crystal structure of the CheA–CheW complex elucidated by Park et al. (4) and are similar to the stoichiometries proposed in their model of the receptor/signaling complexes.

Previously reported in vivo cross-linking studies are consistent with the clusters of dimers that are seen in the lzTarCWA complexes (16, 17), and EM analyses of cell sections have shown evidence of membrane folds at the cell poles that could support the type of receptor signaling domain-to-signaling domain juxtaposition seen in our soluble receptor/signaling complexes (32). Given that the sequences of the receptor cytoplasmic domains, CheA, and CheW are highly conserved in virtually all chemotactic prokaryotes, it seems that this organization of the receptor complexes, a tight complex with two sets of signaling domains oriented toward one another, could reflect the structure of active receptor/CheW/CheA complexes in vivo.

Within the higher order assemblies of membrane receptors, CheW, and CheA that mediate chemotaxis in vivo, only a small fraction of CheA is in the activated state. Estimates of the cellular stoichiometry in vivo are ≈1.2 CheY proteins per CheA monomer (33). Previous studies have estimated that, in wild-type cells under steady-state conditions, ≈30% of CheY is phosphorylated (34). The specific activity of activated CheA in vitro is much higher than the in vitro rate of CheZ-mediated CheY dephosphorylation (21, 35). Although accurate measurements of the latter are difficult, it is almost certain that specific activity of activated CheA is at least 10-fold higher than that of CheZ. Based on these values, at most 4% of the CheA in the cell is fully activated by participation in a receptor/signaling complex. Fig. 5 presents a schematic model with only a small fraction of receptors and CheA participating in a complex that contains the receptor signaling domains from two opposing segments of membrane. In other regions, CheA and CheW may be bound to the receptors, but CheA is not activated. This schematic is similar in appearance to some of the membrane invaginations that have been observed in sections of wild-type cells by TEM (32).

Fig. 5.
A model for membrane receptor/signaling complexes in vivo in the receptor cluster at the pole of an E. coli cell. Receptors transition from a nonactivating (R) to a CheA-activating (T) complex through an invagination of the membrane that permits the formation ...

The response time of E. coli cells to stimuli is generally found to be ≈50–100 ms (36, 37). Although large changes in membrane architecture may not occur on this time scale, local movements could permit additional CheA and receptor proteins near the edge of an active receptor/signaling complex to join into it and thus increase the net CheA activity. Additionally, conformational changes in the receptors seem to be able to inactivate CheA without requiring dissociation (24, 29, 38). The receptors outside the active receptor/signaling complexes may still play an important role in cellular memory and adaptation, because these receptors would be substrates for attractant-mediated methylation and demethylation by CheR and CheB (3942). Over longer time scales, the changing methylation states of the receptors could determine which receptors participate in the active receptor/signaling complexes.

A requirement for interactions between receptors in opposing membranes would imply that a curvature of the membrane is involved in CheA activation (Fig. 5). From this, one would expect that changes in membrane properties would affect CheA activity. For example, CheA is activated in vivo by alcohols that tend to make the membrane more fluid (43), as well as by increases in osmolarity that cause plasmolysis with attendant membrane invaginations (44).

In summary, active, soluble receptor/signaling complexes self-assemble from the monomeric TsrC protein, CheW, and CheA. These complexes have essentially the same size, composition, and architecture as those formed from lzTarC. We conclude that the organization of these receptor/signaling complexes is determined by conserved interactions between the constituent chemotaxis proteins and may genuinely represent the architecture of membrane receptor/signaling complexes in vivo, and we propose a model for the formation and regulation of these complexes.

Materials and Methods

Protein Expression and Purification.

CheA, CheW, CheY, and lzTarC (all Q Tar 257–553 fused to an N-terminal leucine zipper domain) were purified as described in refs. 10, 26, and 27. Calculated extinction coefficients at 278 nm (M−1·cm−1) are 7,000 for lzTarC, 15,527 for CheA, and 5,600 for CheW (45).

Biochemical reagents were from standard suppliers, including Sigma–Aldrich (St. Louis, MO), Fisher (Pittsburgh, PA), and Roche (Indianapolis, IN). All restriction enzymes and DNA purification reagents were from New England Biolabs (Ipswich, MA) and Qiagen (Valencia, CA), respectively.

A cytoplasmic fragment of Tsr (PW029; all Q Tsr 290–551), referred to in this report as TsrC, was purified by using the expression plasmid pPW029. Plasmid pPW029 was constructed in analogy to plasmid pPA90 (24) by using the vector pCJ30 (46), with an insert from plasmid pHSE5.tsrQQQQ (47). pHSE5.tsrQQQQ contains the E. coli tsr gene with all of the methylation sites mutated to glutamine. Plasmid pHSE5.tsrQQQQ was digested with HindIII and partially digested with Sau3AI. A DNA fragment of the desired size was agarose gel-purified and ligated into the BamHI-HindIII sites of pCJ30. The insert was verified by restriction digestion and automated DNA sequencing.

The TsrC protein was purified in a fashion similar to lzTarC (10). Briefly, DH5α cells with pPW029 were grown in Terrific Broth (48) with 50 μg/ml ampicillin at 37°C to OD600 = 0.8, induced with 1 mM isopropyl β-d-thiogalactoside, grown for 5 h more, harvested, washed with 0.1 M NaPO4 (pH 7), and frozen. All further steps were carried out at 4°C or below. Cells were thawed and resuspended in 0.1 M NaPO4, pH 7/0.15 mg/ml lysozyme/1× protease inhibitor mixture (Calbiochem, San Diego, CA)/0.5 mM EDTA. The cells were sonicated three times for 30 s on ice mixed with NaCl. Insoluble material was pelleted at 24,000 rpm in an SW-28 rotor (Beckman Coulter, Fullerton, CA). Protein in the supernatant was precipitated twice with 25% ammonium sulfate, collected by centrifugation at 14,000 rpm in an SLC-1500 rotor (Sorvall, Asheville, NC), and then desalted by using a G-25 column (Amersham Biosciences, Piscataway, NJ). Using a FPLC system (Pharmacia, Uppsala, Sweden), protein was loaded onto a Source-30Q column (Amersham Biosciences) in 25 mM Tris/25 mM NaCl/0.5 mM EDTA, pH 8, and eluted with a linear gradient of 0–400 mM NaCl in the same buffer. The TsrC protein eluted as a broad peak. The TsrC-containing fractions were precipitated with 40% ammonium sulfate, resuspended, and run on a Sephacryl S-300 column (Pharmacia) in 25 mM Tris/50 mM NaCl/0.5 mM EDTA, pH 7.5. Fractions containing pure TsrC protein were concentrated by using a Centriprep YM-10 concentrator (Millipore, Bedford, MA) to a final concentration of 582 μM, based on a calculated ε at 280 nm of 5,690 M−1·cm−1 (45). At 278 nm, the ε is 5,600. Concentrated protein was 0.22-μm filtered by using Spin-X filters (Corning, Corning, NY), snap-frozen in liquid nitrogen, and stored at −80°C. Mass spectrometry of the purified protein gave a single peak at a mass of 27,925 Da, which agrees with the predicted mass of 27,928 Da to within the estimated accuracy of the instrument. Mass spectra were acquired at the mass spectrometry facility of the Princeton University Department of Chemistry by using a model 1100 HPLC (Hewlett Packard, Palo Alto, CA) equipped with a Luna C-8 reverse-phase column (Phenomenex, Torrance, CA) and a mass-selective detector electrospray mass spectrometer (Hewlett Packard).

HPLC Characterization of lzTarC and TsrC.

All HPLC analyses were performed with a model 1100 HPLC (Agilent Technologies, Palo Alto, CA). Samples of lzTarC and TsrC were analyzed by using a Superose-12 HR 10/300 column (Pharmacia) in 50 mM KPO4/200 mM NaCl/0.01% NaN3, pH 7.8, connected to a UV detector inline with a miniDawn multiangle light-scattering detector (Wyatt Technology, Santa Barbara, CA). Data were collected and analyzed by using ASTRA 4.73 software (Wyatt Technology) to estimate the absolute mass of each protein by using the extinction coefficients at 278 nm, a value of 0.165 for the differential refractive index (dn/dc), and 1.330 (water) for the solvent refractive index. Gel filtration standards from BioRad (Hercules, CA) and purified CheW were used to determine the relationship between retention time and log(molecular mass).

Formation of Soluble Receptor/Signaling Complexes.

Ternary complexes of lzTarC or TsrC with CheW and CheA were generated as described in refs. 26 and 27. Proteins were mixed in RBIII buffer (25 mM Tris·HCl/50 mM potassium glutamate/25 mM NaCl/5 mM MgCl2/5% DMSO/10% glycerol, pH 7.5), the final concentration of potassium glutamate was adjusted to 50 mM, and DTT was added to a concentration of 10 mM. A range of concentrations of lzTarC and TsrC were tested in the presence of 10 μM CheA and 10 μM CheW. Mixtures were incubated at 30°C overnight. After formation, complexes were stored at room temperature (≈25°C). For TEM analysis, TsrCWA complexes were assembled instead by using a final concentration of 300 mM KPO4 (pH 7.5), 5 mM MgCl2, and 10 mM DTT or 5 mM neutralized Tris(2-carboxyethyl)phosphine (TCEP). In this buffer, complexes formed as efficiently as in RBIII, but the absence of DMSO and glycerol resulted in improved TEM sample preparation.

HPLC Analysis of Receptor/Signaling Complexes.

lzTarCWA and TsrCWA complexes were analyzed as described in refs. 26 and 27 by using a TSK-Gel G5000PWXL column (30 × 0.78 cm) with a TSK-Gel G4000PW guard column (Tosoh Bioscience, Tokyo, Japan) using a flow rate of 0.5 ml/min of RBIII (with additional MgCl2 up to 10 mM). Receptor/signaling complexes with final receptor concentrations of 60 μM lzTarC and 80 μM TsrC were used for the determination of the complex stoichiometry and absolute molecular mass. Fractions were collected across the elution peak corresponding to receptor/signaling complexes and were analyzed by using 15% SDS/PAGE gels and Coomassie staining.

For determination of the stoichiometry of proteins in the receptor/signaling complexes, two-dimensional HPLC was used. Using the TSK-Gel G5000PWXL column, two 0.4-min fractions beginning at elution times of 14.1 and 14.5 min were collected for each sample, and a final concentration of 2.5 mM neutralized Tris(2-carboxyethyl)phosphine (TCEP) was added to each. The resulting protein mixture was precipitated by using a final concentration of 10% trichloroacetic acid at 0°C, centrifuging at 12,400 × g for 15 min, washing twice with −20°C acetone, and allowing to air dry at 25°C. Pellets were resuspended in 6 M guanidine-HCl/20 mM NaPO4/1.5 mM TCEP, pH 6.5, and loaded onto a Zorbax GF-250 (4.6 × 250 mm) column (Agilent Technologies) running at 0.25 ml/min in the same buffer. Absorbance was monitored at 278 nm, and protein concentrations were determined by using extinction coefficients at 278 nm (45). The area under each peak of the chromatogram was divided by the extinction coefficient of the corresponding protein to determine the number of moles of protein in the peak, and then the numbers for CheW and lzTarC or TsrC were divided by the number for CheA to find the stoichiometry.

The molecular masses and rms radii of the lzTarCWA and TsrCWA complexes were determined by HPLC by using the inline miniDawn multiangle light-scattering detector and the TSK-Gel G5000PWXL column with the ASTRA software. Analyses were performed on data centered on each elution peak, from 14.1 to 14.9 min for the lzTarCWA complex and from 14.2 to 15.0 min for the TsrCWA complex. The concentrations of the lzTarCWA and TsrCWA complexes were estimated from UV absorption by using the extinction coefficients for the receptor/signaling complexes, determined as follows. The extinction coefficient of each complex was calculated from the extinction coefficients of the component proteins by using the stoichiometry that was determined as described above (Table 1). Similar to previous analyses, the calculated coefficient for the complex was corrected to account for the difference between the actual UV absorbance of the proteins in the lzTarCWA complex in the elution buffer and the predicted UV absorbance of the individual protein components in 6 M guanidine-HCl (26). The correction factor was adjusted so that the analysis returned the known value of 4.0 CheA subunits per complex. This same correction was applied to the calculated extinction coefficient of the TsrCWA complex. We used a value of 0.165 for dn/dc, and the solvent refractive index of the RBIII elution buffer was measured to be 1.358 by using a model 33-45-58 refractometer (Bausch & Lomb, Rochester, NY).

Receptor/signaling complexes formed with mixtures of lzTarC and TsrC were also analyzed by using the TSK-Gel G5000PWXL column in RBIII (with additional MgCl2 to a concentration of 10 mM). Fractions were collected every 0.4 min from 13.5 to 21.5 min and analyzed by using 15% SDS/PAGE gels with Coomassie staining. Complexes were formed at 30°C with a 5 μM concentration of either lzTarC or TsrC, a 0 or 45 μM concentration of the other receptor fragment type, and a 10 μM concentration each of CheW and CheA in RBIII. Gels were scanned with a Scanmaker 4900 flat-bed scanner (Microtek, Carson, CA).

Kinase Activity Measurements.

Kinase activity of CheA alone and in complexes was determined by using a spectrophotometric coupled ATPase assay in RBIII buffer, as described in ref. 26. Data were collected at 20°C by using a DU-530 spectrophotometer (Beckman Coulter) with a temperature-controlled cuvette holder.

EM.

Microscopy of the lzTarCWA complexes is described in ref. 28. TsrCWA complexes were diluted up to 20-fold in 50 mM Tris/250 mM KCl/5 mM MgCl2, pH 7.5, as necessary to give a suitable density of particles on the grids. Five-microliter aliquots of the diluted complexes were immediately applied to 400 mesh copper microscope grids covered by continuous carbon foils, stained with 2% uranyl acetate, washed with deionized water, stained again with 2% uranyl acetate, blotted, and allowed to dry. TEM images at ×60,000 magnification were recorded with doses of <20 electrons per Å2 on SO163 film (Eastman Kodak, Rochester, NY) by using a CM12 microscope (Philips Electronic Instruments, Mahwah, NJ) equipped with a model 626 cryostage and a model 651 anticontaminator (Gatan, Pleasanton, CA), both cooled to ≈−170°C. Micrographs were digitized by using an SCAI scanner (Carl Zeiss, Oberkochen, Germany) at 7 μm/pixel. Pixels were averaged 2 by 2 to give 14-μm pixels corresponding to 2.33 Å/pixel in the specimen.

Image Analysis.

The defocus of the individual micrographs was determined by using CTFFIND2 (49). Images with appreciable astigmatism were not used. The micrographs were phase-corrected for the contrast transfer function (CTF) by reversing the phase flip imposed by the CTF. Images of single particles were manually selected by using Ximdisp (MRC package) (50) and were windowed from the phase-corrected TEM micrographs by using SPIDER (51). Images were treated to remove any ramp of density and then were normalized, setting the average to zero and the standard deviation to a constant. The particles were iteratively aligned in two dimensions by using SPIDER. Particles were aligned first to an average of the lzTarCWA complex (28) and then were aligned in subsequent rounds to the averages of the aligned TsrCWA images.

The radius of gyration for the lzTarCWA complex was calculated from the surface map corresponding to the previously reported three-dimensional reconstruction by using the program HYDROMIC (30).

Acknowledgments

We thank J. Z. Chen for molecular cloning, D. M. Little for liquid chromatography/MS analysis, Prof. J. S. Parkinson (University of Utah, Salt Lake City, UT) for plasmid pCJ30, and Prof. R. M. Weis (University of Massachusetts, Amherst, MA) for plasmid pHSE5.tsrQQQQ. This work was supported by National Institutes of Health Grants R01 GM057773 (to J.B.S.) and P01 GM062580 and R01 GM035433 (to D.J.D.) and a United Negro College Fund–Merck predoctoral fellowship (to M.D.B.).

Abbreviations

Tsr
serine receptor
TsrC
cytoplasmic domain of Tsr
Tar
aspartate receptor
TarC
cytoplasmic domain of Tar
TEM
transmission EM
lzTarCWA
lzTarC/CheW/CheA
TsrCWA
TsrC/CheW/CheA

Footnotes

The authors declare no conflict of interest.

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