Lambda repressor N-terminal DNA-binding domain as an assay for protein transmembrane segment interactions in vivo

Bacterial One- and Two-Hybrid Assays to Monitor Transmembrane Helix Interactions

In transenvelope multiprotein machines such as bacterial secretion systems, protein-protein interactions not only occur between soluble domains but might also be mediated by helix-helix contacts in the inner membrane. Several assays have been therefore developed to test homotypic and heterotypic interactions between transmembrane α-helices in their native membrane environment. Here, we provide detailed protocols for two genetic assays, TOXCAT and GALLEX, which are based on the reconstitution of dimeric regulators allowing the control of expression of reporter genes.

SUE4, a novel PIN1-interacting membrane protein, regulates acropetal auxin transport in response to sulfur deficiency

Sulfur (S) is an essential macronutrient for plants and a signaling molecule in abiotic stress responses. It is known that S availability modulates root system architecture; however, the underlying molecular mechanisms are largely unknown. We previously reported an Arabidopsis gain-of-function mutant sulfate utilization efficiency4 (sue4) that could tolerate S deficiency during germination and early seedling growth with faster primary root elongation. Here, we report that SUE4, a novel plasma membrane-localized protein, interacts with the polar auxin transporter PIN1, resulting in reduced PIN1 protein levels and thus decreasing auxin transport to the root tips, which promotes primary root elongation. Moreover, SUE4 is induced by sulfate deficiency, consistent with its role in root elongation. Further analyses showed that the SUE4-PIN1 interaction decreased PIN1 levels, possibly through 26 S proteasome-mediated degradation. Taken together, our finding of SUE4-mediated root elongation is consistent with root adaptation to highly mobile sulfate in soil, thus revealing a novel component in the adaptive response of roots to S deficiency.

Functional determinants of a small protein controlling a broadly conserved bacterial sensor kinase

The PhoQ/PhoP two-component system plays a vital role in the regulation of Mg2+ homeostasis, resistance to acid and hyperosmotic stress, cationic antimicrobial peptides, and virulence in Escherichia coli, Salmonella and related bacteria. Previous studies have shown that MgrB, a 47 amino acid membrane protein that is part of the PhoQ/PhoP regulon, inhibits the histidine kinase PhoQ. MgrB is part of a negative feedback loop modulating this two-component system that prevents hyperactivation of PhoQ and may also provide an entry point for additional input signals for the PhoQ/PhoP pathway. To explore the mechanism of action of MgrB, we have analyzed the effects of point mutations, C-terminal truncations and transmembrane region swaps on MgrB activity. In contrast with two other known membrane protein regulators of histidine kinases in E. coli, we find that the MgrB TM region is necessary for PhoQ inhibition. Our results indicate that the TM region mediates interactions with PhoQ and that W20 is a key residue for PhoQ/MgrB complex formation. Additionally, mutations of the MgrB cytosolic region suggest that the two N-terminal lysines play an important role in regulating PhoQ activity. Alanine scanning mutagenesis of the periplasmic region of MgrB further indicates that, with the exception of a few highly conserved residues, most residues are not essential for MgrB's function as a PhoQ inhibitor. Our results indicate that the regulatory function of the small protein MgrB depends on distinct contributions from multiple residues spread across the protein. Interestingly, the TM region also appears to interact with other non-cognate histidine kinases in a bacterial two-hybrid assay, suggesting a potential route for evolving new small protein modulators of histidine kinases.

Fusion Reporter Approaches to Monitoring Transmembrane Helix Interactions in Bacterial Membranes

In transenvelope multiprotein machines such as bacterial secretion systems, protein-protein interactions not only occur between soluble domains but might also be mediated by helix-helix contacts in the inner membrane. Here we describe genetic assays commonly used to test interactions between transmembrane α-helices in their native membrane environment. These assays are based on the reconstitution of dimeric regulators allowing the control of expression of reporter genes. We provide detailed protocols for the TOXCAT and GALLEX assays used to monitor homotypic and heterotypic transmembrane helix-helix interactions.

A genetic bistable switch utilizing nonlinear protein degradation

Background

Bistability is a fundamental property in engineered and natural systems, conferring the ability to switch and retain states. Synthetic bistable switches in prokaryotes have mainly utilized transcriptional components in their construction. Using both transcriptional and enzymatic components, creating a hybrid system, allows for wider bistable parameter ranges in a circuit.

Results

In this paper, we demonstrate a tunable family of hybrid bistable switches in E. coli using both transcriptional components and an enzymatic component. The design contains two linked positive feedback loops. The first loop utilizes the lambda repressor, CI, and the second positive feedback loop incorporates the Lon protease found in Mesoplasma florum (mf-Lon). We experimentally tested for bistable behavior in exponential growth phase, and found that our hybrid bistable switch was able to retain its state in the absence of an input signal throughout 40 cycles of cell division. We also tested the transient behavior of our switch and found that switching speeds can be tuned by changing the expression rate of mf-Lon.

Conclusions

To our knowledge, this work demonstrates the first use of dynamic expression of an orthogonal and heterologous protease to tune a nonlinear protein degradation circuit. The hybrid switch is potentially a more robust and tunable topology for use in prokaryotic systems.

Expression, purification and characterization of recombinant phospholipase B from Moraxella bovis with anomalous electrophoretic behavior

Moraxella bovis is the causative agent of infectious bovine keratoconjunctivitis (IBK) also known as pinkeye, a highly contagious and painful eye disease that is common in cattle throughout the world. Vaccination appears to be a reasonable and cost-effective means of control of pinkeye. Identification of genes encoding novel secreted antigens have been reported, and these antigens are being assessed for use in a vaccine. One of the genes encodes phospholipase B, which can be expressed with high purity and yield in recombinant Escherichia coli as a secreted, soluble, non-tagged, mature construct (less signal peptide with predicted mass 63 kDa). The recombinant phospholipase B exhibited anomalous electrophoretic mobility that was dependent on the temperature of the denaturing process, with bands observed at either 52 or 63 kDa. Analysis by in-gel digestion and liquid chromatography-mass spectrometry revealed these two distinct forms most likely had identical sequences. Phospholipase B is a compact, globular protein with a predicted structure typical of a conventional autotransporter. It is suggested that high temperature is required to unfold the protein (to denature the beta-barrel-rich transporter domain) and to ensure accessibility of the reducing agent. Interestingly, the two forms of the enzyme, differing in size and isoelectric points, were also detected in cell-free supernatants of M. bovis cultures, indicating that native phospholipase B may exist in two differentially folded states possibly also differing in oxidation status of cysteine residues.

A eukaryotic-type serine/threonine protein kinase StkP of Streptococcus pneumoniae acts as a dimer in vivo

Streptococcus pneumoniae carries a single Ser/Thr protein kinase gene stkP in its genome. Biochemical studies performed with recombinant StkP have revealed that this protein is a functional membrane-linked eukaryotic-type Ser/Thr protein kinase. Here, we demonstrate that the deletion of its extracellular domain negatively affects the stability of a core kinase domain. In contrast, the membrane anchored kinase domain and the full-length form of StkP were stable and capable of autophosphorylation. Furthermore, evidence is presented that StkP forms dimers through its transmembrane and extracellular domains.

Sequence dependence of BNIP3 transmembrane domain dimerization implicates side-chain hydrogen bonding and a tandem GxxxG motif in specific helix-helix interactions

The transmembrane domain of the pro-apoptotic protein BNIP3 self-associates strongly in membranes and in detergents. We have used site-directed mutagenesis to analyze the sequence dependence of BNIP3 transmembrane domain dimerization, from which we infer the physical basis for strong and specific helix-helix interactions in this system. Hydrophobic substitutions identify six residues as critical to dimerization, and the pattern of sensitive residues suggests that the BNIP3 helices interact at a right-handed crossing angle. Based on the dimerization propensities of single point mutants, we propose that: polar residues His173 and Ser172 make inter-monomer hydrogen bonds to one another through their side-chains; Ala176, Gly180, and Gly184 form a tandem GxxxG motif that allows close approach of the helices; and Ile183 makes inter-monomer van der Waals contacts. Since neither the tandem GxxxG motif nor the hydrogen bonding pair is sufficient to drive dimerization, our results demonstrate the importance of sequence context for either hydrogen bonding or GxxxG motif involvement in BNIP3 transmembrane helix-helix interactions. In this study, hydrophobic substitutions away from the six interfacial positions have almost no effect on dimerization, confirming the expectation that hydrophobic replacements affect helix-helix interactions only if they interfere with packing or hydrogen bonding by interfacial residues. However, changes to slightly polar residues are somewhat disruptive even when located away from the interface, and the degree of disruption correlates with the decrease in hydrophobicity. Changing the hydrophobicity of the BNIP3 transmembrane domain alters its helicity and protection of its backbone amides. We suggest that polar substitutions decrease the fraction of dimer by stabilizing an unfolded monomeric state of the transmembrane span, rather than by affecting helix-helix interactions. This result has broad implications for interpreting the sequence dependence of membrane protein stability in detergents.

RcsF is an outer membrane lipoprotein involved in the RcsCDB phosphorelay signaling pathway in Escherichia coli

The RcsCDB signal transduction system is an atypical His-Asp phosphorelay conserved in gamma-proteobacteria. Besides the three proteins directly involved in the phosphorelay, two proteins modulate the activity of the system. One is RcsA, which can stimulate the activity of the response regulator RcsB independently of the phosphorelay to regulate a subset of RcsB targets. The other is RcsF, a putative outer membrane lipoprotein mediating the signaling to the sensor RcsC. How RcsF transduces the signal to RcsC is unknown. Although the molecular and physiological signals remain to be identified, the common feature among the reported Rcs-activating conditions is perturbation of the envelope. As an initial step to explore the RcsF-RcsC functional relationship, we demonstrate that RcsF is an outer membrane lipoprotein oriented towards the periplasm. We also report that a null mutation in surA, a gene required for correct folding of periplasmic proteins, activates the Rcs pathway through RcsF. In contrast, activation of this pathway by overproduction of the membrane chaperone-like protein DjlA does not require RcsF. Conversely, activation of the pathway by RcsF overproduction does not require DjlA either, indicating the existence of two independent signaling pathways toward RcsC.

Activity of Lac repressor anchored to the Escherichia coli inner membrane

The transient inactivation of gene regulatory proteins by their sequestration to the cytoplasmic membrane in response to cognate signals is an increasingly recognized mechanism of gene regulation in bacteria. It remained to be shown, however, whether tethering to the membrane per se could be responsible for inactivation, i.e. whether such relocation leads to a spatial separation from the chromosome that results in inactivity or whether other mechanisms are involved. We, therefore, investigated the activity of Lac repressor artificially attached to the Escherichia coli cytoplasmic membrane. We demonstrate that this chimeric protein perfectly represses transcription initiated at the tac operator–promoter present on a plasmid and even in the chromosome. Moreover, this repression is inducible as normal. The data suggest that proteins localized to the inner face of the cytoplasmic membrane in principle have unrestricted access to the chromosome. Thus sequestration to the membrane in terms of physical separation from the chromosome cannot account alone for the inactivation of regulatory proteins. Other mechanisms, like induction of a conformational change or masking of binding domains are required additionally.

A complex of the Escherichia coli cell division proteins FtsL, FtsB and FtsQ forms independently of its localization to the septal region

Three membrane proteins required for cell division in Escherichia coli, FtsQ, FtsL and FtsB, localize to the cell septum. FtsL and FtsB, which each contain a leucine zipper-like sequence, are dependent on each other for this localization, and each of them is dependent on FtsQ. However, FtsQ is found at the cell division site in the absence of FtsL and FtsB. FtsQ, in turn, requires FtsK for its localization. Here, we show that FtsL, FtsB and FtsQ form a complex in vivo. Strikingly, this complex forms in the absence of FtsK, which is required for the localization of all three proteins to the mid-cell. These findings indicate that the FtsL, FtsB, FtsQ interactions can take place in cells before movement to the mid-cell and that migration to this position might occur only after the formation of the complex. Evidence indicating the regions of the three proteins involved in complex formation is presented. These findings provide the first example of preassembly of a subcomplex of cell division proteins before their localization to the septal region.

Interactions of glutaredoxins, ribonucleotide reductase, and components of the DNA replication system of Escherichia coli

A strain of Escherichia coli missing three members of the thioredoxin superfamily, thioredoxins 1 and 2 and glutaredoxin 1, is unable to grow, a phenotype presumed to be due to the inability of cells to reduce the essential enzyme ribonucleotide reductase. Two classes of mutations can restore growth to such a strain. First, we have isolated a collection of mutations in the gene for the protein glutaredoxin 3 that suppress the growth defect. Remarkably, all eight independent mutations alter the same amino acid, methionine-43, changing it to valine, isoleucine, or leucine. From the position of the amino acid changes and their effects, we propose that these alterations change the protein so that its properties are closer to those of glutaredoxin 1. The second means of suppressing the growth defects of the multiply mutant strain was by mutations in the DNA replication genes, dnaA and dnaN. These mutations substantially increase the expression of ribonucleotide reductase, most likely by altering the interaction of the regulatory protein DnaA with the ribonucleotide reductase promoter. Our results suggest that this increase in the concentration of ribonucleotide reductase in the cell allows more effective interaction with glutaredoxin 3, thus restoring an effective pool of deoxyribonucleotides. Our studies present direct evidence that ribonucleotide reductase is the only essential enzyme that requires the three reductive proteins missing in our strains. Our results also suggest an unexpected regulatory interaction between the DnaA and DnaN proteins.

The transmembrane domain of the DnaJ-like protein DjlA is a dimerisation domain

DjlA is a bitopic inner membrane protein, which belongs to the DnaJ co-chaperone family in Escherichia coli. Overproduction of DjlA leads to the synthesis of colanic acid, resulting in mucoidy, via the activation of the two-component regulatory system RcsC/B that controls the cps (capsular polysaccharide) operon. This induction requires both the co-chaperone activity of DjlA, in cooperation with DnaK and GrpE, and its unique transmembrane (TM) domain. Here, we show that the TM segment of DjlA acts as a dimerisation domain: when fused to the N-terminal DNA-binding domain of the lambda cI repressor protein, it can substitute for the native C-terminal dimerisation domain of cI, thus generating an active cI repressor. Replacing the TM domain of DjlA by other TM domains, with or without dimerising capacity, revealed that dimerisation is not sufficient for the induction of cps expression, indicating an additional sequence- or structurally specific role for the TM domain. Finally, the conserved glycines present in the TM domain of DjlA are essential for the induction of mucoidy, but not for dimerisation.

A novel cytology-based, two-hybrid screen for bacteria applied to protein-protein interaction studies of a type IV secretion system

DivIVA of Bacillus subtilis and FtsZ of Escherichia coli were used to target heterologous protein complexes to cell division sites of E. coli and Agrobacterium tumefaciens. DivIVA and FtsZ that were fused to the dimerizing leucine zipper (LZ) domain of the yeast transcription activator GCN4 directed the green fluorescent protein (GFP) that was fused to an LZ domain to E. coli division sites, resulting in fluorescence patterns identical to those observed with DivIVA::GFP and FtsZ::GFP. These cell division proteins also targeted the VirE1 chaperone and VirE2 secretion substrate complex to division sites of E. coli and A. tumefaciens. Coproduction of the native VirE1 or VirE2 proteins inhibited the dihybrid interaction in both species, as judged by loss of GFP targeting to division sites. The VirE1 chaperone bound independently to N- and C-terminal regions of VirE2, with a requirement for residues 84 to 147 and 331 to 405 for these interactions, as shown by dihybrid studies with VirE1::GFP and DivIVA fused to N- and C-terminal VirE2 fragments. DivIVA also targeted homo- and heterotypic complexes of VirB8 and VirB10, two bitopic inner membrane subunits of the A. tumefaciens T-DNA transfer system, in E. coli and homotypic complexes of VirB10 in A. tumefaciens. VirB10 self-association in bacteria was mediated by the C-terminal periplasmic domain, as shown by dihybrid studies with fusions to VirB10 truncation derivatives. Together, our findings establish a proof-of-concept for the use of cell-location-specific proteins for studies of interactions among cytosolic and membrane proteins in diverse bacterial species.

Characterization of a membrane-linked Ser/Thr protein kinase in Bacillus subtilis, implicated in developmental processes

PrkC was shown to be a eukaryotic-like (Hanks-type) protein kinase from Bacillus subtilis with a structural organization similar to that of the eukaryotic sensor Ser/Thr or Tyr kinases (e.g. the TGF beta or PDGF receptors). The molecule consists of a catalytic domain located in the cytoplasm, joined by a single transmembrane-spanning region (TMD) to a large extracellular domain. Using a genetic reporter system, involving the cI repressor of lambda, evidence was obtained indicating that PrkC forms a dimer, involving both the TMD and the external domain in dimerization. The purified catalytic domain of PrkC was shown to autophosphorylate and to phosphorylate an external target, MBP, in both cases on threonine. These two functions require the completely conserved K40 residue in subdomain II, which is essential for enzymatic activity. Importantly, both the mutant deleted for prkC and a K40R mutant exhibit decreased efficiency of sporulation and a significant reduction in biofilm formation, demonstrating that the catalytic activity of PrkC is necessary for these two developmental processes. In addition, we showed that the product of prpC, a PPM phosphatase encoded by the adjacent gene, co-transcribed with prkC, is also required for normal biofilm and spore formation.

Genetic selection for and molecular dynamic modeling of a protein transmembrane domain multimerization motif from a random Escherichia coli genomic library

In order to identify new transmembrane helix packing motifs in naturally occurring proteins, we have selected transmembrane domains from a library of random Escherichia coli genomic DNA fragments and screened them for homomultimerization via their abilities to dimerize the bacteriophage lambda cI repressor DNA-binding domain. Sequences were isolated using a modified lambda cI headpiece dimerization assay system, which was shown previously to measure transmembrane helix-helix association in the E. coli inner membrane. Screening resulted in the identification of several novel sequences that appear to mediate helix-helix interactions. One sequence, representing the predicted sixth transmembrane domain (TM6) of the E. coli protein YjiO, was chosen for further analysis. Using site-directed mutagenesis and molecular dynamics, a small set of models for YjiO TM6 multimerization interface interactions were generated. This work demonstrates the utility of combining in vivo genetic tools with computational systems for understanding membrane protein structure and assembly.

Thermodynamic stability of the bacteriorhodopsin lattice as measured by lipid dilution

To determine the strength of noncovalent interactions that stabilize a membrane protein complex, we have developed an in vitro method for quantifying the dissociation of the bacteriorhodopsin (BR) lattice, a naturally occurring two-dimensional crystal. A lattice suspension was titrated with a short- and long-chain phosphatidylcholine mixture to dilute BR within the lipid bilayer. The fraction of BR in the lattice form as a function of added lipid was determined by visible circular dichroism spectroscopy and fit with a cooperative self-assembly model to obtain a critical concentration for lattice assembly. Critical concentration values of wild-type and mutant proteins were used to calculate the change in lattice stability upon mutation (DeltaDeltaG). By using this method, a series of mutant proteins was examined in which residues at the BR-BR interface were replaced with smaller amino acids, either Ala or Gly. Most of the mutant lattices were destabilized, with DeltaDeltaG values of 0.2-1.1 kcal/mol at 30 degrees C, consistent with favorable packing of apolar residues in the membrane. One mutant, I45A, was stabilized by approximately 1.0 kcal/mol, possibly due to increased lipid entropy. The DeltaDeltaG values agreed well with previous in vivo measurements, except in the case of I45A. The ability to measure the change in stability of mutant protein complexes in a lipid bilayer may provide a means of determining the contributions of specific protein-protein and protein-lipid interactions to membrane protein structure.

Genetic systems for analyzing protein-protein interactions in bacteria

Analysis of protein-protein interactions has been revolutionized by the yeast two-hybrid system introduced by Fields and coworkers. In recent years, similar genetic assays have been developed in bacteria. We describe here several of these systems and highlight some potential applications of these technologies.

Influence of the C-terminus of the glycophorin A transmembrane fragment on the dimerization process

The monomer-dimer equilibrium of the glycophorin A (GpA) transmembrane (TM) fragment has been used as a model system to investigate the amino acid sequence requirements that permit an appropriate helix-helix packing in a membrane-mimetic environment. In particular, we have focused on a region of the helix where no crucial residues for packing have been yet reported. Various deletion and replacement mutants in the C-terminal region of the TM fragment showed that the distance between the dimerization motif and the flanking charged residues from the cytoplasmic side of the protein is important for helix packing. Furthermore, selected GpA mutants have been used to illustrate the rearrangement of TM fragments that takes place when leucine repeats are introduced in such protein segments. We also show that secondary structure of GpA derivatives was independent from dimerization, in agreement with the two-stage model for membrane protein folding and oligomerization.

Escherichia coli one- and two-hybrid systems for the analysis and identification of protein-protein interactions

Genetic methods based on fusion proteins allow the power of a genetic approach to be applied to the self-assembly of proteins or protein fragments, regardless of whether or not the normal function of the fused assembly domains is either known or amenable to selection or screening. The widespread adoption of variations of the yeast two-hybrid system originally described by S. Fields and O. Song (1989, Nature 340, 245-246) demonstrates the usefulness of these kinds of assays. This review describes some of the many systems used to select or screen for protein-protein interactions based on the regulation of reporter constructs by hybrid proteins expressed in bacteria, including recent implementations of generalizable two-hybrid systems for Escherichia coli.

Detergents modulate dimerization, but not helicity, of the glycophorin A transmembrane domain

Understanding how the lipid environment influences transmembrane helix association requires thermodynamic measurements that can be interpreted in terms of specific chemical interactions. We have used Förster resonance energy transfer to measure dimerization of the glycophorin A transmembrane helix in detergent micelles. The observed Kd is at least two orders of magnitude weaker in sodium dodecyl sulfate than it is in zwitterionic detergents. In contrast, neither dimerization nor the detergent affects the secondary structure of the glycophorin A helix as measured by far-UV circular dichroism. These measurements support a long standing assumption about the glycophorin A transmembrane domain, that detergents uncouple helix formation from helix dimerization. The approach is applicable to a variety of systems in diverse environments, extending our ability to measure how interactions with complex solvents affect the thermodynamics of oligomerization.

A method for determining transmembrane helix association and orientation in detergent micelles using small angle x-ray scattering

Solution small angle x-ray scattering can be used to study the association of transmembrane proteins solubilized in detergent micelles. We have used the alpha-helical transmembrane domain of the human erythrocyte glycophorin A (GpA) fused to the carboxyl terminus of monomeric staphylococcal nuclease (SN/GpA) as a model system for study. By matching the average electron density of the detergent micelles to that of the buffer solution, the micelle contribution to the small angle scattering vanishes, and the molecular weight and the radius of gyration of the proteins can be determined. SN/GpA has been found to dimerize in a zwitterionic detergent micelle, N-dodecyl-N,N-(dimethylammonio)butyrate (DDMAB), whose average electron density naturally matches the electron density of an aqueous buffer. The dimerization occurs through the transmembrane domains of GpA. With the aid of the nuclease domain scattering, the orientation of the helices within a dimer can be determined to be parallel by radius of gyration analysis. The association constant of a mutant (G83I) that weakens the GpA dimerization has been determined to be 24 microM in the DDMAB environment. The experimental methods established here could be used to apply solution small angle x-ray scattering to studying the association and interactions of other membrane proteins.

Role of helix-helix interactions in assembly of the bacteriorhodopsin lattice

The purple membrane of Halobacterium salinarium is a two-dimensional lattice of lipids and the integral membrane protein bacteriorhodopsin (BR). To determine whether helix-helix interactions within the membrane core stabilize this complex, we substituted amino acid residues at the helix-helix interface between BR monomers and examined the assembly of the protein into the lattice. Lattice assembly was demonstrated to fit a cooperative self-assembly model that exhibits a critical concentration in vivo. Using this model as the basis for a quantitative assay of lattice stability, bulky substitutions at the helix-helix interface between BR monomers within the membrane core were shown to be destabilizing, probably due to steric clash. Ala substitutions of two residues at the helix-helix interface also reduced stability, suggesting that the side chains of these residues participate in favorable van der Waals packing interactions. However, the stabilizing interactions were restricted to a small region of the interface, and most of the substitutions had little effect. Thus, the contribution of helix-helix interactions to lattice stability appears limited, and favorable interactions between other regions of neighboring BR monomers or between BR and lipid molecules must also contribute.

TOXCAT: a measure of transmembrane helix association in a biological membrane

The noncovalent association of transmembrane alpha-helices is a fundamental event in the folding of helical membrane proteins. In this work, a system (TOXCAT) is developed for the study of transmembrane helix-helix oligomerization in a natural membrane environment. This assay uses a chimeric construct composed of the N-terminal DNA binding domain of ToxR (a dimerization-dependent transcriptional activator) fused to a transmembrane domain (tm) of interest and a monomeric periplasmic anchor (the maltose binding protein). Association of the tms results in the ToxR-mediated activation of a reporter gene encoding chloramphenicol acetyltransferase (CAT). The level of CAT expression indicates the strength of tm association. The assay distinguishes between a known dimerizing tm and a mutant in which dimerization is disrupted. In addition, modulation of the chimera concentration shows that the dimerization exhibits concentration dependence in membranes. TOXCAT also is used to select oligomeric tms from a library of randomized sequences, demonstrating the potential of this system to reveal novel oligomerization motifs. The TOXCAT system has been used to investigate glycophorin A tm-mediated dimerization. Although the overall sensitivity of glycophorin A tm dimerization to mutagenesis is found to be similar in membranes and in detergent micelles, several significant differences exist. Mutations to polar residues, which are generally disruptive in SDS, exhibit sequence specificity in membranes, demonstrating both the limitations of detergent micelles and the wider range of application of the TOXCAT system.