The EGF receptor transmembrane domain: peptide-peptide interactions in fluid bilayer membranes

Chronicles of EGFR Tyrosine Kinase Inhibitors: Targeting EGFR C797S Containing Triple Mutations

Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase widely expressed in many cancers such as non-small cell lung cancer (NSCLC), pancreatic cancer, breast cancer, and head and neck cancer. Mutations such as L858R in exon 21, exon 19 truncation (Del19), exon 20 insertions, and others are responsible for aberrant activation of EGFR in NSCLC. First-generation EGFR tyrosine kinase inhibitors (TKIs) such as gefitinib and erlotinib have clinical benefits for EGFR-sensitive (L858R and Del19) NSCLC patients. However, after 10-12 months of treatment with these inhibitors, a secondary T790M mutation at the gatekeeper position in the kinase domain of EGFR was identified, which limited the clinical benefits. Second-generation EGFR irreversible inhibitors (afatinib and dacomitinib) were developed to overcome this T790M mutation. However, their lack of selectivity toward wild-type EGFR compromised their clinical benefits due to serious adverse events. Recently developed third-generation irreversible EGFR TKIs (osimertinib and lazertinib) are selective toward driving mutations and the T790M mutation, while sparing wildtype EGFR activity. The latest studies have concluded that their efficacy was also compromised by additional acquired mutations, including C797S, the key residue cysteine that forms covalent bonds with irreversible inhibitors. Because second- and thirdgeneration EGFR TKIs are irreversible inhibitors, they are not effective against C797S containing EGFR triple mutations (Del19/T790M/C797S and L858R/T790M/C797S). Therefore, there is an urgent unmet medical need to develop next-generation EGFR TKIs that selectively inhibit EGFR triple mutations via a non-irreversible mechanism.

Active S2168 and inactive S21IRS pinholin interact differently with the lipid bilayer: A 31P and 2H solid state NMR study

Pinholins are a family of lytic membrane proteins responsible for the lysis of the cytosolic membrane in host cells of double stranded DNA bacteriophages. Protein-lipid interactions have been shown to influence membrane protein topology as well as its function. This work investigated the interactions of pinholin with the phospholipid bilayer while in active and inactive confirmations to elucidate the different interactions the two forms have with the bilayer. Pinholin incorporated into deuterated DMPC-d₅₄ lipid bilayers, along with ³¹P and ²H solid state NMR (SS-NMR) spectroscopy were used to probe the protein-lipid interactions with the phosphorus head group at the surface of the bilayer while interactions with the ²H nuclei were used to study the hydrophobic core. A comparison of the ³¹P chemical shift anisotropy (CSA) values of the active S²¹68 pinholin and inactive S²¹IRS pinholin indicated stronger head group interactions for the pinholin in its active form when compared to that of the inactive form supporting the model of a partially externalized peripheral transmembrane domain (TMD) of the active S²¹68 instead of complete externalized TMD1 as suggested by Ahammad et al. JPC B 2019. The ²H quadrupolar splitting analysis showed a decrease in spectral width for both forms of the pinholin when compared to the empty bilayers at all temperatures. In this case the decrease in the spectral width of the inactive S²¹IRS form of the pinholin showed stronger interactions with the acyl chains of the bilayer. The presence of the inactive form's additional TMD within the membrane was supported by the loss of peak resolution observed in the ²H NMR spectra.

Membrane solid-state NMR in Canada: A historical perspective

This manuscript presents an overview of more than 40years of membrane solid-state nuclear magnetic resonance (NMR) research in Canada. This technique is a method of choice for the study of the structure and dynamics of lipid bilayers; bilayer interactions with a variety of molecules such as membrane peptides, membrane proteins and drugs; and to investigate membrane peptide and protein structure, dynamics, and topology. Canada has a long tradition in this field of research, starting with pioneering work on natural and model membranes in the 1970s in a context of emergence of biophysics in the country. The 1980s and 1990s saw an emphasis on studying lipid structures and dynamics, and peptide-lipid and protein-lipid interactions. The study of bicelles began in the 1990s, and in the 2000s there was a rise in the study of membrane protein structures. Novel perspectives include using dynamic nuclear polarization (DNP) for membrane studies and using NMR in live cells. This article is part of a Special Issue entitled: Biophysics in Canada, edited by Lewis Kay, John Baenziger, Albert Berghuis and Peter Tieleman.

Epidermal Growth Factor Receptor Cell Proliferation Signaling Pathways

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that is commonly upregulated in cancers such as in non-small-cell lung cancer, metastatic colorectal cancer, glioblastoma, head and neck cancer, pancreatic cancer, and breast cancer. Various mechanisms mediate the upregulation of EGFR activity, including common mutations and truncations to its extracellular domain, such as in the EGFRvIII truncations, as well as to its kinase domain, such as the L858R and T790M mutations, or the exon 19 truncation. These EGFR aberrations over-activate downstream pro-oncogenic signaling pathways, including the RAS-RAF-MEK-ERK MAPK and AKT-PI3K-mTOR pathways. These pathways then activate many biological outputs that are beneficial to cancer cell proliferation, including their chronic initiation and progression through the cell cycle. Here, we review the molecular mechanisms that regulate EGFR signal transduction, including the EGFR structure and its mutations, ligand binding and EGFR dimerization, as well as the signaling pathways that lead to G1 cell cycle progression. We focus on the induction of CYCLIN D expression, CDK4/6 activation, and the repression of cyclin-dependent kinase inhibitor proteins (CDKi) by EGFR signaling pathways. We also discuss the successes and challenges of EGFR-targeted therapies, and the potential for their use in combination with CDK4/6 inhibitors.

Epidermal Growth Factor Receptor Cell Proliferation Signaling Pathways

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that is commonly upregulated in cancers such as in non-small-cell lung cancer, metastatic colorectal cancer, glioblastoma, head and neck cancer, pancreatic cancer, and breast cancer. Various mechanisms mediate the upregulation of EGFR activity, including common mutations and truncations to its extracellular domain, such as in the EGFRvIII truncations, as well as to its kinase domain, such as the L858R and T790M mutations, or the exon 19 truncation. These EGFR aberrations over-activate downstream pro-oncogenic signaling pathways, including the RAS-RAF-MEK-ERK MAPK and AKT-PI3K-mTOR pathways. These pathways then activate many biological outputs that are beneficial to cancer cell proliferation, including their chronic initiation and progression through the cell cycle. Here, we review the molecular mechanisms that regulate EGFR signal transduction, including the EGFR structure and its mutations, ligand binding and EGFR dimerization, as well as the signaling pathways that lead to G1 cell cycle progression. We focus on the induction of CYCLIN D expression, CDK4/6 activation, and the repression of cyclin-dependent kinase inhibitor proteins (CDKi) by EGFR signaling pathways. We also discuss the successes and challenges of EGFR-targeted therapies, and the potential for their use in combination with CDK4/6 inhibitors.

Orientation and dynamics of synthetic transbilayer polypeptides containing GpATM dimerization motifs

Deuterium NMR spectroscopy was used to study how the positioning of a dimerization motif within a transbilayer polypeptide influences its orientation and dynamics in bilayers. Three polypeptide variants comprising glycophorin A transmembrane (GpATM) dimerization motifs incorporated into lysine-terminated poly-leucine-alanine helices were mixed into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine multilamellar vesicles. The variants differed in orientation of the motif segment around the helix axis with respect to the peptide ends. Polypeptides were labeled with methyl-deuterated alanines at positions that were identically situated relative to the peptide ends (Ala-20 and Ala-22) and at two positions within the motif. An analysis of quadrupole splittings revealed similar tilts and orientations of the peptide ends for all three variants, suggesting that average orientations were dominated by interactions at the bilayer surface. For one variant, however, fast orientational fluctuations about the helix axis were significantly smaller. This may indicate some perturbation of peptide dynamics and conformation by interactions that are sensitive to the motif orientation relative to the peptide ends. For the variant that displayed distinct dynamics, one orientation consistent with observed splittings corresponded to the motif being situated such that its two glycines were particularly accessible to adjacent peptides.

Solid state NMR analysis of peptides in membranes: Influence of dynamics and labeling scheme

The functional state of a membrane-active peptide is often defined by its conformation, molecular orientation, and its oligomeric state in the lipid bilayer. These "static" structural properties can be routinely studied by solid state NMR using isotope-labeled peptides. In the highly dynamic environment of a liquid crystalline biomembrane, however, the whole-body fluctuations of a peptide are also of paramount importance, although difficult to address and most often ignored. Yet it turns out that disregarding such motional averaging in calculating the molecular alignment from orientational NMR-constraints may give a misleading, if not false picture of the system. Here, we demonstrate that the reliability of a simplified static or an advanced dynamic data analysis depends critically on the choice of isotope labeling scheme used. Two distinctly different scenarios have to be considered. When the labels are placed on the side chains of a helical peptide (such as a CD(3)- or CF(3)-group attached to the C(alpha)C(beta) bond), their nuclear spin interaction tensors are very sensitive to motional averaging. If this effect is not properly accounted for, the helix tilt angle tends to be severely underestimated. At the same time, the analysis of labels in the side chains allows to extract valuable dynamical information about whole-body fluctuations of the peptide helix in the membrane. On the other hand, the alternative labeling scheme where (15)N-labels are accommodated within the peptide backbone, will yield nearly correct helix tilt angles, irrespective as to whether dynamics are taken into account or not.

Phospholamban and its phosphorylated form interact differently with lipid bilayers: a 31P, 2H, and 13C solid-state NMR spectroscopic study

Phospholamban (PLB) is a 52-amino acid integral membrane protein that helps to regulate the flow of Ca(2+) ions in cardiac muscle cells. Recent structural studies on the PLB pentamer and the functionally active monomer (AFA-PLB) debate whether its cytoplasmic domain, in either the phosphorylated or dephosphorylated states, is alpha-helical in structure as well as whether it associates with the lipid head groups (Oxenoid, K. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 10870-10875; Karim, C. B. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 14437-14442; Andronesi, C.A. (2005) J. Am. Chem. Soc. 127, 12965-12974; Li, J. (2003) Biochemistry 42, 10674-10682; Metcalfe, E. E. (2005) Biochemistry 44, 4386-4396: Clayton, J. C. (2005) Biochemistry 44, 17016-17026). Comparing the secondary structure of the PLB pentamer and its phosphorylated form (P-PLB) as well as their interaction with the lipid bilayer is crucial in order to understand its regulatory function. Therefore, in this study, the full-length wild-type (WT) PLB and P-PLB were incorporated into 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) phospholipid bilayers and studied utilizing solid-state NMR spectroscopy. The analysis of the (2)H and (31)P solid-state NMR data of PLB and P-PLB in POPC multilamellar vesicles (MLVs) indicates that a direct interaction takes place between both proteins and the phospholipid head groups. However, the interaction of P-PLB with POPC bilayers was less significant compared that with PLB. Moreover, the secondary structure using (13)C=O site-specific isotopically labeled Ala15-PLB and Ala15-P-PLB in POPC bilayers suggests that this residue, located in the cytoplasmic domain, is a part of an alpha-helical structure for both PLB and P-PLB.

Dynamic and 2D NMR Studies on Hydrogen-Bonding Aggregates of Cholesterol in Low-Polarity Organic Solvents

Self-diffusion coefficients (D) are measured for normal (nondeuterated) and deuterated cholesterol-d(6) (C26 and C27 methyl groups deuterated) in 1-octanol, chloroform, and cyclohexane at concentrations of 1-700 mM by varying the impurity water concentration (>2 mM) and temperature (30-50 degrees C). The pulsed field gradient spin-echo (PGSE) (1)H and (2)H NMR were used, respectively, at 600 and 92 MHz. At 30 degrees C, the hydrodynamic radius (R) obtained at 20 mM from the D value and solvent viscosity is 5.09, 7.07, and 6.17 A, respectively, in 1-octanol, chloroform, and cyclohexane when the impurity water is negligible. The R value in 1-octanol is the smallest and comparable with the average length of the molecular axes for the cholesterol molecule. In 1-octanol, R is invariant against the concentration variation, whereas in chloroform, R is larger and increases almost linearly with cholesterol concentration. At the highest concentration, 700 mM, the R in chloroform is 13.5 and 16.7 A, respectively, when the impurity water is at negligible and saturated concentrations. The R value larger than that in hydrogen-bonding 1-octanol indicates that cholesterol forms an aggregate through hydrogen bonding. The aggregate structure is confirmed by comparing NOESY spectra in chloroform and 1-octanol. The NOESY analysis reveals the presence of one extra cross peak (C4-C19) in chloroform compared to 1-octanol. Because the carbon atoms related to the cross peak are close to the hydroxyl group (C3-OH), cholesterol molecules are considered to be not piled but are found to be OH-centered in the aggregate. This is supported also by larger rotational hydrodynamic radii measured on cholesterol deuterated at positions C2, C3, C4, and C6. This shows that the aggregate formation is driven by the hydrogen-bonding between cholesterol molecules.

Solid-state NMR studies of a diverged microsomal amino-proximate delta12 desaturase peptide reveal causes of stability in bilayer: tyrosine anchoring and arginine snorkeling

This study reports the solid-state NMR spectroscopic characterization of the amino-proximate transmembrane domain (TM-A) of a diverged microsomal delta12-desaturase (CREP-1) in a phospholipid bilayer. A series of TM-A peptides were synthesized with 2H-labeled side chains (Ala-53, -56, and -63, Leu-62, Val-50), and their dynamic properties were studied in 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) bilayers at various temperatures. At 6 mol % peptide to lipid, 31P NMR spectra indicated that the peptides did not significantly disrupt the phospholipid bilayer in the L(alpha) phase. The 2H NMR spectra from Ala-53 and Ala-56 samples revealed broad Pake patterns with quadrupolar splittings of 16.9 kHz and 13.3 kHz, respectively, indicating restricted motion confined within the hydrocarbon core of the phospholipid bilayer. Conversely, the deuterated Ala-63 sample revealed a peak centered at 0 kHz with a linewidth of 1.9 kHz, indicating increased side-chain motion and solvent exposure relative to the spectra of the other Ala residues. Val-50 and Leu-62 showed Pake patterns, with quadrupolar splittings of 3.5 kHz and 3.7 kHz, respectively, intermediate to Ala-53/Ala-56 and Ala-63. This indicates partial motional averaging and supports a model with the Val and Leu residues embedded inside the lipid bilayer. Solid-state NMR spectroscopy performed on the 2H-labeled Ala-56 TM-A peptide incorporated into magnetically aligned phospholipid bilayers indicated that the peptide is tilted 8 degrees with respect to the membrane normal of the lipid bilayer. Snorkeling and anchoring interactions of Arg-44 and Tyr-60, respectively, with the polar region or polar hydrophobic interface of the lipid bilayer are suggested as control elements for insertional depth and orientation of the helix in the lipid matrix. Thus, this study defines the location of key residues in TM-A with respect to the lipid bilayer, describes the conformation of TM-A in a biomembrane mimic, presents a peptide-bilayer model useful in the consideration of local protein folding in the microsomal desaturases, and presents a model of arginine and tyrosine control of transmembrane protein stability and insertion.

The transmembrane domains of ErbB receptors do not dimerize strongly in micelles

The epidermal growth factor receptors (erbB) constitute an important class of single pass transmembrane receptors involved in the transduction of signals important for cell proliferation and differentiation. Receptor association is a key event in the signal transduction process, but the molecular basis of this interaction is not fully understood. Previous biochemical and genetic studies have suggested that the single transmembrane helices of these receptor proteins might play a role in stabilizing the receptor complexes. To determine if the erbB transmembrane domains could provide a driving force to stabilize the receptor dimers, we carried out a thermodynamic study of these domains expressed as C-terminal fusion proteins with staphylococcal nuclease. Similar fusion constructs have been used successfully to investigate the oligomerization and association thermodynamics of a number of transmembrane sequences, including that of glycophorin A. Using SDS-PAGE analysis and sedimentation equilibrium analytical ultracentrifugation, we do not find strong, specific homo or hetero-interactions between the transmembrane domains of the erbB receptors in micellar solutions. Our results indicate that any preferential interactions between these domains in micellar solutions are extremely modest, of the order of 1 kcal mol(-1) or less. We applied a thermodynamic formalism to assess the effect of weakly interacting TM segments on the behavior of a covalently attached soluble domain. In the case of the ligand-bound EGFR ectodomain, we find that restriction of the ectodomain to the micellar phase by a hydrophobic TM, even in the absence of strong specific interactions, is largely sufficient to account for the previously reported increase in dimerization affinity.

Tilt angles of transmembrane model peptides in oriented and non-oriented lipid bilayers as determined by 2H solid-state NMR

Solid-state NMR methods employing (2)H NMR and geometric analysis of labeled alanines (GALA) were used to study the structure and orientation of the transmembrane alpha-helical peptide acetyl-GWW(LA)(8)LWWA-amide (WALP23) in phosphatidylcholine (PC) bilayers of varying thickness. In all lipids the peptide was found to adopt a transmembrane alpha-helical conformation. A small tilt angle of 4.5 degrees was observed in di-18:1-PC, which has a hydrophobic bilayer thickness that approximately matches the hydrophobic length of the peptide. This tilt angle increased slightly but systematically with increasing positive mismatch to 8.2 degrees in di-C12:0-PC, the shortest lipid used. This small increase in tilt angle is insufficient to significantly change the effective hydrophobic length of the peptide and thereby to compensate for the increasing hydrophobic mismatch, suggesting that tilt of these peptides in a lipid bilayer is energetically unfavorable. The tilt and also the orientation around the peptide axis were found to be very similar to the values previously reported for a shorter WALP19 peptide (GWW(LA)(6)LWWA). As also observed in this previous study, the peptide rotates rapidly around the bilayer normal, but not around its helix axis. Here we show that these properties allow application of the GALA method not only to macroscopically aligned samples but also to randomly oriented samples, which has important practical advantages. A minimum of four labeled alanine residues in the hydrophobic transmembrane sequence was found to be required to obtain accurate tilt values using the GALA method.

Molecular dynamics simulation of transmembrane polypeptide orientational fluctuations

The orientation and motion of a model lysine-terminated transmembrane polypeptide were investigated by molecular dynamics simulation. Recent 2H NMR studies of synthetic polypeptides with deuterated alanine side chains suggest that such transmembrane polypeptides undergo fast, axially symmetric reorientation about the bilayer normal but have a preferred average azimuthal orientation about the helix axis. In this work, interactions that might contribute to this behavior were investigated in a simulated system consisting of 64 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and one alpha-helical polypeptide with the sequence acetyl-KK-(LA)11-KK-amide. In one simulation, initiated with the peptide oriented along the bilayer normal, the system was allowed to evolve for 8.5 ns at 1 atm of pressure and a temperature of 55 degrees C. A second simulation was initiated with the peptide orientation chosen to match a set of experimentally observed alanine methyl deuteron quadrupole splittings and allowed to proceed for 10 ns. Simulated alanine methyl group orientations were found to be inequivalent, a result that is consistent with 2H NMR observations of specifically labeled polypeptides in POPC bilayers. Helix tilt varied substantially over the durations of both simulations. In the first simulation, the peptide tended toward an orientation about the helix axis similar to that suggested by experiment. In the second simulation, orientation about the helix axis tended to return to this value after an excursion. These results provide some insight into how interactions at the bilayer surface can constrain reorientation about the helix axis while accommodating large changes in helix tilt.

Investigating structural changes in the lipid bilayer upon insertion of the transmembrane domain of the membrane-bound protein phospholamban utilizing 31P and 2H solid-state NMR spectroscopy

Phospholamban (PLB) is a 52-amino acid integral membrane protein that regulates the flow of Ca(2+) ions in cardiac muscle cells. In the present study, the transmembrane domain of PLB (24-52) was incorporated into phospholipid bilayers prepared from 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC). Solid-state (31)P and (2)H NMR experiments were carried out to study the behavior of POPC bilayers in the presence of the hydrophobic peptide PLB at temperatures ranging from 30 degrees C to 60 degrees C. The PLB peptide concentration varied from 0 mol % to 6 mol % with respect to POPC. Solid-state (31)P NMR spectroscopy is a valuable technique to study the different phases formed by phospholipid membranes. (31)P NMR results suggest that the transmembrane protein phospholamban is incorporated successfully into the bilayer and the effects are observed in the lipid lamellar phase. Simulations of the (31)P NMR spectra were carried out to reveal the formation of different vesicle sizes upon PLB insertion. The bilayer vesicles fragmented into smaller sizes by increasing the concentration of PLB with respect to POPC. Finally, molecular order parameters (S(CD)) were calculated by performing (2)H solid-state NMR studies on deuterated POPC (sn-1 chain) phospholipid bilayers when the PLB peptide was inserted into the membrane.

Seeing the light: preassembly and ligand-induced changes of the interferon gamma receptor complex in cells

Our experiments were designed to test the hypothesis that the cell surface interferon gamma receptor chains are preassembled rather than associated by ligand and to assess the molecular changes on ligand binding. To accomplish this, we used fluorescence resonance energy transfer, a powerful spectroscopic technique that has been used to determine molecular interactions and distances between the donor and acceptor. However, current commercial instruments do not provide sufficient sensitivity or the full spectra to provide decisive results of interactions between proteins labeled with blue and green fluorescent proteins in living cells. In our experiments, we used the blue fluorescent protein and green fluorescent protein pair, attached a monochrometer and charge-coupled device camera to a modified confocal microscope, reduced background fluorescence with the use of two-photon excitation, and focused on regions of single cells to provide clear spectra of fluorescence resonance energy transfer. In contrast to the prevailing view, the results demonstrate that the receptor chains are preassociated and that the intracellular domains move apart on binding the ligand interferon gamma. Application of this technology should lead to new rapid methods for high throughput screening and delineation of the interactome of cells.

Organization of model helical peptides in lipid bilayers: insight into the behavior of single-span protein transmembrane domains

Selectively deuterated transmembrane peptides comprising alternating leucine-alanine subunits were examined in fluid bilayer membranes by solid-state nuclear magnetic resonance (NMR) spectroscopy in an effort to gain insight into the behavior of membrane proteins. Two groups of peptides were studied: 21-mers having a 17-amino-acid hydrophobic domain calculated to be close in length to the hydrophobic thickness of 1-palmitoyl-2-oleoyl phosphatidylcholine and 26-mers having a 22-amino-acid hydrophobic domain calculated to exceed the membrane hydrophobic thickness. (2)H NMR spectral features similar to ones observed for transmembrane peptides from single-span receptors of higher animal cells were identified which apparently correspond to effectively monomeric peptide. Spectral observations suggested significant distortion of the transmembrane alpha-helix, and/or potential for restriction of rotation about the tilted helix long axis for even simple peptides. Quadrupole splittings arising from the 26-mer were consistent with greater peptide "tilt" than were those of the analogous 21-mer. Quadrupole splittings associated with monomeric peptide were relatively insensitive to concentration and temperature over the range studied, indicating stable average conformations, and a well-ordered rotation axis. At high peptide concentration (6 mol% relative to phospholipid) it appeared that the peptide predicted to be longer than the membrane thickness had a particular tendency toward reversible peptide-peptide interactions occurring on a timescale comparable with or faster than approximately 10(-5) s. This interaction may be direct or lipid-mediated and was manifest as line broadening. Peptide rotational diffusion rates within the membrane, calculated from quadrupolar relaxation times, T(2e), were consistent with such interactions. In the case of the peptide predicted to be equal to the membrane thickness, at low peptide concentration spectral lineshape indicated the additional presence of a population of peptide having rotational motion that was restricted on a timescale of 10(-5) s.

Evidence of a tendency to self-association of the transmembrane domain of ErbB-2 in fluid phospholipid bilayers

The transmembrane domains of receptor tyrosine kinases are single-span helical structures suggested to participate directly in the formation of side-to-side receptor homodimers/homooligomers that modulate signal transduction. Transmembrane peptides from the class I receptor tyrosine kinase, ErbB-2, were examined directly by 2H NMR spectroscopy as a means of following such phenomena under the dynamic conditions that characterize fluid, fully hydrated bilayers of natural phospholipids. Appropriate peptides were expressed as 50-mers, containing the transmembrane domain of ErbB-2 plus contiguous stretches of amino acids from the cytoplasmic and extracellular domains. Deuterium probes were incorporated in place of 1H at a site within the helical intramembranous portion (the -CH3 side chain of Ala657), and the peptides were assembled into bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) for study. An analogous peptide corresponding to the oncogenic variant characterized by a Val659-->Glu point mutation was also examined. At high peptide concentration, prominent spectral features could be assigned to rapidly rotating transmembrane monomers and to large oligomers rotating very slowly relative to a time scale of 10(-5) s. As peptide concentration was lowered, the latter feature was greatly reduced, and an additional population of mobile species became identifiable, consistent with the presence of homodimers and/or small oligomers. The defined nature of these latter spectral features suggests that preferred interaction sites exist on the peptides. The appearance of similar phenomena in the case of transmembrane peptides from both wild-type ErbB-2 and the transforming mutant argues for the involvement of additional factors in signal modulation, such as limitations normally imposed by the missing extramembranous portions.

The single transmembrane domains of ErbB receptors self-associate in cell membranes

Members of the epidermal growth factor receptor, or ErbB, family of receptor tyrosine kinases have a single transmembrane (TM) alpha-helix that is usually assumed to play a passive role in ligand-induced dimerization and activation of the receptor. However, recent studies with the epidermal growth factor receptor (ErbB1) and the erythropoietin receptor have indicated that interactions between TM alpha-helices do contribute to stabilization of ligand-independent and/or ligand-induced receptor dimers. In addition, not all of the expected ErbB receptor ligand-induced dimerization events can be recapitulated using isolated extracellular domains, suggesting that other regions of the receptor, such as the TM domain, may contribute to dimerization in vivo. Using an approach for analyzing TM domain interactions in Escherichia coli cell membranes, named TOXCAT, we find that the TM domains of ErbB receptors self-associate strongly in the absence of their extracellular domains, with the rank order ErbB4-TM > ErbB1-TM equivalent to ErbB2-TM > ErbB3-TM. A limited mutational analysis suggests that dimerization of these TM domains involves one or more GXXXG motifs, which occur frequently in the TM domains of receptor tyrosine kinases and are critical for stabilizing the glycophorin A TM domain dimer. We also analyzed the effect of the valine to glutamic acid mutation in ErbB2 that constitutively activates this receptor. Contrary to our expectations, this mutation reduced rather than increased ErbB2-TM dimerization. Our findings suggest a role for TM domain interactions in ErbB receptor function, possibly in stabilizing inactive ligand-independent receptor dimers that have been observed by several groups.