Oligomerization of the EGF receptor transmembrane domain: a 2H NMR study in lipid bilayers

Membrane domain formation-a key factor for targeted intracellular drug delivery

Protein molecules, toxins and viruses internalize into the cell via receptor-mediated endocytosis (RME) using specific proteins and lipids in the plasma membrane. The plasma membrane is a barrier for many pharmaceutical agents to enter into the cytoplasm of target cells. In the case of cancer cells, tissue-specific biomarkers in the plasma membrane, like cancer-specific growth factor receptors, could be excellent candidates for RME-dependent drug delivery. Recent data suggest that agent binding to these receptors at the cell surface, resulting in membrane domain formation by receptor clustering, can be used for the initiation of RME. As a result, these pharmaceutical agents are internalized into the cells and follow different routes until they reach their final intracellular targets like lysosomes or Golgi. We propose that clustering induced formation of plasma membrane microdomains enriched in receptors, sphingolipids, and inositol lipids, leads to membrane bending which functions as the onset of RME. In this review we will focus on the role of domain formation in RME and discuss potential applications for targeted intracellular drug delivery.

Understanding cytokine and growth factor receptor activation mechanisms

Our understanding of the detailed mechanism of action of cytokine and growth factor receptors - and particularly our quantitative understanding of the link between structure, mechanism and function - lags significantly behind our knowledge of comparable functional protein classes such as enzymes, G protein-coupled receptors, and ion channels. In particular, it remains controversial whether such receptors are activated by a mechanism of ligand-induced oligomerization, versus a mechanism in which the ligand binds to a pre-associated receptor dimer or oligomer that becomes activated through subsequent conformational rearrangement. A major limitation to progress has been the relative paucity of methods for performing quantitative mechanistic experiments on unmodified receptors expressed at endogenous levels on live cells. In this article, we review the current state of knowledge on the activation mechanisms of cytokine and growth factor receptors, critically evaluate the evidence for and against the different proposed mechanisms, and highlight other key questions that remain unanswered. New approaches and techniques have led to rapid recent progress in this area, and the field is poised for major advances in the coming years which promise to revolutionize our understanding of this large and biologically and medically important class of receptors.

Dynamics and Cleavability at the alpha-cleavage site of APP(684-726) in different lipid environments

The occurrence of late-onset Alzheimer's disease has been related to the lipid homeostasis. We tested whether the membrane lipid environment affects the dynamics and cleavability of a model peptide corresponding to the amino acid sequence 684-726 of the amyloid precursor protein APP reconstituted in liposomes. Solid-state NMR with (2)H-Ala(713), which is located within the putative transmembrane domain, suggested that the peptide observes less rotational motion in egg phosphatidylcholine (PhC) membranes than in dimyristoyl-phosphatidylcholine (DMPC) bilayers above the main phase transition temperature T(c). The residue (15)N-Ala(692), which is in the vicinity of the alpha-cleavage site, i.e., Lys(687), showed less motion after reconstitution in distearoyl-phosphatidylcholine liposomes <T(c) than in PhC, DMPC, or sphingomyelin vesicles. In all tested liposomal systems the alpha-cleavage site was accessible for hydrolysis by trypsin. However, the catalytic rate constant was higher in the PhC and DMPC than in the sphingomyelin and distearoyl-phosphatidylcholine systems. In conclusion, the dynamics of APP(684-726) on the transmembrane level as well as the motion of the alpha-cleavage site and its hydrolysis by a model enzyme are dependent on the bilayer characteristics. This could be relevant for the processing of APP in vivo.

Side chain and backbone dynamics of phospholamban in phospholipid bilayers utilizing 2H and 15N solid-state NMR spectroscopy

2H and 15N solid-state NMR spectroscopic techniques were used to investigate both the side chain and backbone dynamics of wild-type phospholamban (WT-PLB) and its phosphorylated form (P-PLB) incorporated into 1-palmitoyl-2-oleoyl-sn-glycerophosphocholine (POPC) phospholipid bilayers. 2H NMR spectra of site-specific CD3-labeled WT-PLB (at Leu51, Ala24, and Ala15) in POPC bilayers were similar under frozen conditions (-25 degrees C). However, significant differences in the line shapes of the 2H NMR spectra were observed in the liquid crystalline phase at and above 0 degrees C. The 2H NMR spectra indicate that Leu51, located toward the lower end of the transmembrane (TM) helix, shows restricted side chain motion, implying that it is embedded inside the POPC lipid bilayer. Additionally, the line shape of the 2H NMR spectrum of CD3-Ala24 reveals more side chain dynamics, indicating that this residue (located in the upper end of the TM helix) has additional backbone and internal side chain motions. 2H NMR spectra of both WT-PLB and P-PLB with CD3-Ala15 exhibit strong isotropic spectral line shapes. The dynamic isotropic nature of the 2H peak can be attributed to side chain and backbone motions to residues located in an aqueous environment outside the membrane. Also, the spectra of 15N-labeled amide WT-PLB at Leu51 and Leu42 residues showed only a single powder pattern component indicating that these two 15N-labeled residues located in the TM helix are motionally restricted at 25 degrees C. Conversely, 15N-labeled amide WT-PLB at Ala11 located in the cytoplasmic domain showed both powder and isotropic components at 25 degrees C. Upon phosphorylation, the mobile component contribution increases at Ala11. The 2H and 15N NMR data indicate significant backbone motion for the cytoplasmic domain of WT-PLB when compared to the transmembrane section.

Dimerization of Neu/Erb2 transmembrane domain is controlled by membrane curvature

Secondary structures of the proto-oncogenic Neu/ErbB2 transmembrane segment and its mutant analogue have been determined in phospholipids. It is found that the mutated peptide possesses less helical character possibly due to the valine/glutamic acid point mutation. Embedding peptides in lipid systems whose topology can change from small (100-200 A) tumbling objects to bilayer discs of 450 A diameter leads to the finding that coiled-coil interactions are only observed in the presence of a bilayer membrane of low curvature, independent of mutation. This strongly suggests that any event that may change membrane topology can therefore perturb the dimerization/ologomerization and subsequent phosphorylation cascade leading to cell growth or cancer processes.

Signaling through ERBB receptors: Multiple layers of diversity and control

The four known ERBB receptors in humans are involved in a broad range of cellular responses, and their deregulation is a significant aspect in a large number of disease states. However, their mechanism of action and modes of control are still poorly understood. This is largely due to the fact that the control of ERBB activity is a multilayered process with significant differences between the various ERBB members. In contrast to other receptor tyrosine kinases, the kinase domain of EGFR (ERBB1) does not require phosphorylation for activation. Consequently, the overall activation state of the receptor is controlled by constant balancing of activity favoring and activity suppressing actions within the receptor molecule. Influences of the membrane microenvironment and context dependent interactions with varying sets of signaling partners are superimposed on this system of intramolecular checks and balances. We will discuss current models of the control of ERBB signaling with an emphasis on the multilayered nature of activation control and aspects that give rise to diversity between ERBB receptors.

Revisited and large-scale synthesis and purification of the mutated and wild type neu/erbB-2 membrane-spanning segment

Solid-phase syntheses of the hydrophobic peptides Neu(TM35) ((1)EQRASPVTFIIATVVGVLLFLILVVVVGILIKRRR(35)) and Neu*(TM35) ((1)EQRASPVTFIIATVEGVLLFLILVVVVGILIKRRR(35)), corresponding to the native and mutated (V15E) transmembrane domain of the neu/erbB-2 tyrosine kinase receptor, respectively, were accomplished using Fmoc chemistry. The use of a new resin and cleavage and purification conditions led to large increases in yields and peptide purity. Two (15)N-labelled versions of both wild type and mutated peptides were also synthesized. Approximately 20-40 mg of peptide was obtained using a small-scale synthesis, whereas ca 100 mg of pure peptide was collected on a medium scale. Peptide purity, as monitored by HPLC and mass spectrometry, ranged from 95 to 98% for the six peptides synthesized. Secondary structure as determined by UV circular dichroism (CD) in trifluoroethanol (TFE) showed ca 74% alpha-helical content for the native peptide and ca 63% for that bearing the mutation. Secondary structure of Neu(TM35) was retained in DMPC (dimyristoylphosphatidylcholine)/DCPC (dicaproylphosphatidylcholine) membrane bicelles, and evidences for dimers/oligomers in the lipid bilayer were found.

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.

Interaction between ErbB-1 and ErbB-2 transmembrane domains in bilayer membranes

The transmembrane domains of ErbB receptor tyrosine kinases are monotopic helical structures proposed to be capable of direct side-to-side contact with related receptors. Formation of the resulting homo- or hetero-oligomeric complexes is considered a key step in ligand-mediated signalling. ErbB-2, which has not been observed to form active homo-dimers in a ligand dependent manner, has been implicated as an important partner for formation of hetero-dimers with other ErbB receptors. Recent work has shown that the ErbB-2 transmembrane domain is capable of forming homo-oligomeric species in lipid bilayers, while a similar domain from ErbB-1 appears to have a lesser tendency to such interactions. Here, 2H nuclear magnetic resonance was used to investigate the role of the ErbB-2 transmembrane domain in hetero-oligomerisation with that of ErbB-1. At low total concentrations of peptide in the membrane, ErbB-2 transmembrane domains were found to decrease the mobility of corresponding ErbB-1 domains. The results are consistent with the existence of direct transmembrane domain involvement in hetero-oligomer formation within the ErbB receptor family.

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.

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

A peptide containing the transmembrane domain of the human EGF receptor was studied in fluid lipid bilayers for insight into receptor tyrosine kinase lateral associations in cell membranes. The peptide comprised the 23-amino acid hydrophobic segment thought to span the membrane (Ile(622) to Met(644) of the EGF receptor), plus the first 10 amino acids of the receptor's cytoplasmic domain (Arg(645) to Thr(654)). Probes for solid-state NMR spectroscopy were incorporated by deuteration of the methyl side chains of alanine at positions 623 and 637. (2)H-NMR spectra were recorded from 25 to 65 degrees C in membranes composed of 1-palmitoyl-2-oleoyl phosphatidylcholine, with and without 33% cholesterol, and relaxation times were measured. Peptide concentration ranged from 0. 5 to 10 mol %. The peptide behaved as predominant monomers undergoing rapid symmetric rotational diffusion; however, there was evidence of reversible side-to-side interaction among the hydrophobic transmembrane domains, particularly at physiological temperatures and in the presence of natural concentrations of cholesterol. The results of these experiments in fluid membranes are consistent with the existence of lipid-protein interactions that would predispose to receptor microdomain formation in membranes of higher animal cells.

A transmembrane peptide from the human EGF receptor: behaviour of the cytoplasmic juxtamembrane domain in lipid bilayers

Solid state (2)H NMR spectroscopy was employed to study peptides related to the transmembrane domain of the human epidermal growth factor receptor, for insight into the interaction of its cytoplasmic juxtamembrane domain with the membrane surface. Since such receptors have clusters of (+)charged amino acids in this region, the effect of (-)charged phosphatidylserine at the concentration found naturally in the cytoplasmic leaflet (15 mol%) was considered. Each peptide contained 34 amino acids, which included the hydrophobic 23 amino acid stretch thought to span the membrane and a ten amino acid segment beyond the 'cytoplasmic' surface. Non-perturbing deuterium probe nuclei were located within alanine side chains in intramembranous and extramembranous portions. (2)H NMR spectra were recorded at 35 degrees C and 65 degrees C in fluid lipid bilayers consisting of (zwitterionic) 1-palmitoyl-2-oleoylphosphatidylcholine, with and without 15 mol% (anionic) phosphatidylserine. The cationic extramembranous portion of the receptor backbone was found to be highly rotationally mobile on a time scale of 10(-4)-10(-5) s in both types of membrane - as was the alpha-helical intramembranous portion. Deuterium nuclei in alanine side chains (-CD(3)) detected modest changes in peptide backbone orientation and/or dynamics related to the presence of 1-stearoyl-2-oleoylphosphatidylserine: in the case of the extramembranous portion of the peptide these seemed related to lipid charge. Temperature effects on the peptide backbone external to the membrane were qualitatively different from effects on the helical transmembrane domain - likely reflecting the different physical constraints on these peptide regions and the greater flexibility of the extramembranous domain. Effects related to lipid charge could be detected in the spectrum of CD(3) groups on the internally mobile side chain of Val(650), six residues beyond the membrane surface.

Raman spectroscopy of model membrane monolayers of dipalmitoylphosphatidic acid at the air-water interface using surface enhancement from buoyant thin silver films

A novel method for the acquisition of surface enhanced Raman (SER) spectra of model membranes of dipalmitoylphosphatidic acid (DPPA) in Langmuir layers at the air-water interface is reported. The approach is based on the electrochemical formation of a buoyant thin layer of coalesced silver colloids in the vicinity of the phosphatidic acid head groups at the interface. This Ag layer is an excellent platform for SER scattering, which shows the spectral features from all parts of the molecule and water between the Ag surface and the DPPA layer. The observation of the spectral response from the phosphatidic acid head groups is of particular significance, allowing insight into their chemical state and orientation at the air-water interface.

EGF receptors of hepatocytes from rats treated with phenobarbital are sensitized to down-regulation by phenobarbital in culture

Hepatocytes from Fisher 344 rats treated with the liver tumor promoter phenobarbital (PhB; 0.1% in the drinking water, 2-3 months) exhibit reduced epidermal growth factor (EGF) binding and EGF-induced mitogenesis in culture. Similar responses are induced by >1 mM PhB added to the culture medium of hepatocytes from untreated rats. In this study, we demonstrated that hepatocyte EGFr protein, as determined by immunoblotting, was unchanged by treatment of rats with PhB. However, hepatocytes from PhB-treated rats are more sensitive to PhB in culture in that decreased EGF binding occurred with 0.05 mM PhB, a concentration also attained in plasma of rats exposed to PhB. Sensitization was reversible, as is tumor promotion, since hepatocytes from rats withdrawn from PhB for 1 month were unresponsive to <3 mM PhB. EGFr down-regulation by a series of barbiturates correlated well with their known activities as tumor promoters and CYP2B1/2 inducers, with pentobarbital and PhB yielding high activities, while barbital was intermediate and barbituric acid, 5-phenylbarbituric acid, and 5-ethylbarbituric acid were ineffective. Differentiated hepatocyte function was required for PhB-induced EGFr down-regulation since HepG2 and rat liver epithelial cells were unresponsive, but involvement of CYP2B1/2 activity was discounted by the failure of metyrapone to inhibit the response in PhB-induced hepatocytes. These studies support a role for impaired EGFr function in PhB liver tumor promotion due to effects on existing EGFr protein and suggest that EGFr down-regulation by PhB in culture is independent of CYP2B1/2 activity but shares mechanistic components involved in its transcriptional activation by PhB.

A detergent-insoluble membrane compartment contains A beta in vivo

Ordered assembly of the amyloid-beta protein (A beta) into amyloid fibrils is a critical step in Alzheimer's disease (AD). To release the amyloidogenic peptide A beta from the Alzheimer amyloid precursor protein (APP), two secretases act sequentially: first, beta-secretase cleaves close to the membrane within the ectodomain and then gamma-secretase cuts within the transmembrane domain. The sites of gamma-secretase cleavage are after residues 40 or 42 of A beta. Except in those rare cases of AD caused by a mutation, levels of secreted A beta are not elevated; thus, the secretory pathway may be unaffected, and factors other than the extracellular concentration of A beta may contribute to the aggregation properties of the peptide. A beta is also present in intracellular compartments. The two gamma-secretase cleavage products, A beta42 and A beta40, were found in different compartments: A beta42 in the endoplasmic reticulum (ER)/intermediate compartment, and A beta40 in the trans-Golgi network (TGN). The cellular compartments that harbor A beta are target sites for therapeutic intervention. Here we report that in the brain, the principal compartment in which A beta resides is a detergent-insoluble glycolipid-enriched membrane domain (DIG). Also present in the DIG fractions are the endoproteolytic fragments of presenilin-1 (PS1) and APP. The presence of these proteins, which all contribute to the generation of A beta, indicates that the DIG fraction is probably where the intramembranous cleavage of APP occurs.

Sequence-related behaviour of transmembrane domains from class I receptor tyrosine kinases

2H NMR spectroscopy and freeze-fracture electron microscopy were used to compare the transmembrane domains of two Class I protein receptor tyrosine kinases (the EGF receptor and Neu/erbB-2) regarding overall behaviour in fluid lipid bilayer membranes. The 34-residue peptide, EGFRtm, was synthesised to contain the 23 amino acid hydrophobic stretch (Ile622 to Met644) thought to span the membrane of the human EGF receptor, plus the first 10 amino acids (Arg645 to Thr654) of the cytoplasmic domain. Deuterium probes replaced selected 1H nuclei at sites corresponding to Ala623, Met644, and Val650. The 38-residue peptide, Neutm, was synthesised having the 21 residue hydrophobic stretch (Ile660 to Ile680) calculated to span the membrane in rat Neu/erbB-2, plus residues Lys681 to Thr691 of the contiguous cytoplasmic domain. Deuterium probes replaced selected 1H nuclei at Ala661, Leu667, and Val676. A third peptide, Neutm*, was also prepared, corresponding to the transmembrane domain of a constitutively-activating Neu/erbB-2 transformant in which Val664 is replaced by Glu: it was deuterated in a manner identical to Neutm. Peptides were studied by 2H NMR spectroscopy at 1 mol% and 6 mol% in unsonicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and in POPC containing 33 mol% cholesterol, over the range 12 degrees to 65 degreesC. Overall motion was found to be different for each of the three peptides under a given set of conditions. EGFRtm spectra were characteristic of axially symmetric motion in membranes of POPC alone, and in POPC/cholesterol at 35 degreesC and above. In contrast, spectra of the transmembrane peptides, Neutm and Neutm*, were characteristic of significantly axially asymmetric motion under all conditions studied (and regardless of sample preparation method). Addition of 33% cholesterol to membranes was accompanied by spectral changes consistent with increased formation of peptide dimers/oligomers in all cases. The transformant peptide, Neutm*, showed greater spectral evidence of immobilisation than did the wild type - probably reflecting a greater tendency to form large oligomers. Sequence-related details within the transmembrane domains of Class I receptor tyrosine kinases appear to exert important control over their associations within membranes. Freeze-fracture electron microscopy of the NMR samples demonstrated their liposomal nature. Peptide-related intramembranous particles (IMPs) were present which likely represent oligomers of the transmembrane peptide. IMP size and distribution were similar under a given set of conditions for all three peptides, suggesting that the differences seen by NMR spectroscopy reflect structures smaller than the 2 nm resolution limit of freeze-fracture EM and peptide relationships within its 20 nm accuracy of identifying lateral position.