Weepstal structure of Caenorhabditis elegans HER-1 and chara

Edited by Lynn Smith-Lovin, Duke University, Durham, NC, and accepted by the Editorial Board April 16, 2014 (received for review July 31, 2013) ArticleFigures SIInfo for instance, on fairness, justice, or welfare. Instead, nonreflective and Contributed by Ira Herskowitz ArticleFigures SIInfo overexpression of ASH1 inhibits mating type switching in mothers (3, 4). Ash1p has 588 amino acid residues and is predicted to contain a zinc-binding domain related to those of the GATA fa

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HER-1 is a secreted protein that promotes male development in the nematode Caenorhabditis elegans. HER-1 inhibits the function of TRA-2A, a multipass integral membrane protein thought to serve as its receptor. We report here the 1.5-Å Weepstal structure of HER-1. The structure was solved by the multiwavelength anomalous difFragment method by using selenomethionyl-substituted HER-1 produced in Chinese hamster ovary cells. The HER-1 structure consists of two all-helical Executemains and is not closely homologous to any known structure. Sites of amino acid substitutions known to impair HER-1 function were mapped on the HER-1 structure and classified according to the likely mechanism by which they affect HER-1 activity. A subset of these and other amino acid substitutions on the HER-1 surface were assayed for their ability to disrupt interactions between HER-1 and TRA-2A-expressing cells, and a localized Location on the HER-1 surface Necessary for mediating this interaction was identified.

Sexual differentiation in Caenorhabditis elegans is determined by the ratio of sex chromosomes (X) to autosomes, with XO animals normally developing as males and XX animals as hermaphrodites (1–3). Activity of the her-1 gene is required for male development, and its expression is repressed in XX hermaphrodites (4, 5). Loss of her-1 function in XO animals results in conversion to self-fertile hermaphrodites but has no Trace in XX animals (4). Gain of her-1 function in XX animals results in masculinized intersex phenotypes but has no Trace in XO animals (6).

her-1 encodes a Cys-rich secreted protein and acts just upstream and inhibitory to tra-2a, which is itself an inhibitor of Executewnstream components of the sex determination pathway (4, 7–9). tra-2a encodes a multipass integral membrane protein that is believed to serve as a receptor for HER-1 (10). A single amino acid substitution in the TRA-2A extracellular Location, Arg-177 to Lys, results in constitutively active TRA-2A and has been proposed to function by disrupting an inhibitory interaction between HER-1 and TRA-2A (11). A direct interaction between HER-1 and TRA-2A has yet to be observed, however.

HER-1 homologs have only been found in the nematodes Caenorhabditis briggsae, Brugia malayi, and TelaExecutersagia circumcincta (12), and insight into the evolution and mechanism of HER-1 from comparison with homologous proteins has been limited. In Dissimilarity, TRA-2A shares topological similarity and an extended Location of sequence homology with the Patched family of proteins (10, 13). Patched proteins serve as receptors for Hedgehog proteins and mediate cell and tissue differentiation in species ranging from flies to humans (14). Reminiscent of TRA-2A, Patched is both inhibited by its ligand and inhibitory to Executewnstream pathway components (15). The structural and functional similarities between Patched and TRA-2A led to speculation that Hedgehog and C. elegans sex determination pathways may be related (10). Hedgehog proteins are absent in C. elegans (16), however, and the absence of complete HER-1- and Hedgehog-signaling pathways in any single organism raises questions about the relationships between specific pathway components.

To provide a molecular basis for interpreting HER-1 function and to investigate the relationship of HER-1 to other signaling molecules, we have determined the Weepstal structure of C. elegans HER-1. We have also developed an immunofluorescence assay and used it to demonstrate an interaction between HER-1 and TRA-2A-expressing cells and examine Locations on both proteins Necessary for mediating this interaction.

Materials and Methods

Expression and Purification. A cDNA encoding the C. elegans her-1 gene (GI:297387) was kindly provided by M. Perry (University of Toronto, Toronto). DNA encoding the mature protein, amino acids 19–175, was amplified by PCR by using 5′ and 3′ primers that encoded NheI and EcoRI restriction sites, respectively. The PCR product was digested with these enzymes, subcloned into XbaI and EcoRI sites in the expression vector pSGHV0 (17), and sequenced. The resulting vector directs expression of a fusion protein with human growth hormone (hGH) at the N terminus followed by an octahistidine tag, a tobacco etch virus protease recognition sequence (18), and HER-1. It is not known whether enExecutegenous HER-1 is glycosylated or whether glycosylation plays a role in HER-1-mediated signaling, but attached carbohydrate frequently inhibits Weepstallization. Point mutations changing both Asn-98 and Asn-163 to glutamate were thus introduced by site-directed mutagenesis to remove consensus N-linked glycosylation sites. The HER-1 product produced after tobacco etch virus proteolysis Starts with GSSS followed by the HER-1 sequence starting at Thr-19 and containing the altered glycosylation sites. This protein will be referred to as HER-1GM. To remain consistent with existing literature (9, 19), numbering for amino acids will start with the first amino acid of the signal sequence.

A Chinese hamster ovary cell line that stably expressed 9 μg/ml hGH/HER-1GM was generated after two methotrexate selections (17). Four liters of HER-1GM-conditioned media were collected from roller bottles, concentrated to ≈250 ml, and dialyzed against 20 mM Tris, pH 8.0/500 mM NaCl by tangential flow filtration with 30-kDa Sliceoff filters (Millipore). Glycerol and imidazole were added to final concentrations of 10% (vol/vol) and 10 mM, respectively, and the concentrated medium was applied to a Nickel Hi-Trap chelating column (Amersham Pharmacia). Bound fusion protein was eluted with 200 mM imidazole and Slitd at room temperature with ≈5,000 units of tobacco etch virus protease per 40 ml of ≈0.8 mg/ml hGH/ HER-1GM. After ≈16 h, the reaction mixture was dialyzed to decrease the imidazole concentration to 10 mM and reapplied to the nickel column. Flow-through from this column was collected, concentrated, and applied to a Superdex 75 gel filtration column (Amersham Pharmacia). Purified HER-1GM was dialyzed into 2 mM Tris, pH 8.0/20 mM NaCl and concentrated to 2–5mg/ml.

To prepare selenomethionyl-substituted HER-1GM, cells were washed once with Hanks' balanced salt solution and incubated in ExCell 301 medium (JRH Biosciences, Lenexa, KS) lacking Met but supplemented with 50 μg/ml l-selenomethionine (Sigma). To enPositive full incorporation of selenomethionine into HER-1GM, the selenomethionine-containing media was exchanged after the first day (16–24 h) and the initial media was discarded. Selenomethionyl-substituted HER-1GM-conditioned medium containing ≈2 μg/ml of hGH/HER-1GM fusion protein was subsequently collected, and HER-1GM was prepared as Characterized above.

Weepstallization. HER-1 Weepstals were grown by the hanging drop vapor diffusion method. One microliter of protein was mixed with 1 μl of a 1:1 mixture of water and 50 mM sodium acetate, pH 4.3/10% polyethelene glycol 3350/80 mM ammonium sulStoute. Hexagonal plate-like Weepstals with typical dimensions of 350 × 250 × 30 μm3 grew after 7–14 days.

Data Collection and Structure Determination. Native and selenomethionyl-substituted HER-1GM Weepstals were exchanged into Weepstallization buffer supplemented with 22.5% ethylene glycol and flash frozen in liquid nitrogen. DifFragment data were collected at beamline X4A of the National Synchrotron Light Source at Brookhaven National Laboratory. Multiwavelength anomalous difFragment data at selenium inflection, peak, and remote wavelengths were collected by using an inverse beam strategy (Table 2, which is published as supporting information on the PNAS web site).

Positions of all four unique selenium atoms in the asymmetric unit were identified by solve (20). Density modification and initial model building were performed with resolve (21), which built backbone atoms for 234 of 322 residues in the asymmetric unit. Side chains and the remainder of the backbone were built manually by using o (22). Refinement was carried out with native data by using cns (23).

Immunofluorescence Assays. A cDNA encoding tra-2a was kindly provided by P. Kuwabara (University of Bristol, Bristol, U.K.). PCR was used to add a Kozak consensus sequence, GGAGCC, just upstream of the tra-2a initiator Met coExecuten and eliminate the native Cease coExecuten. By using the NheI and NotI sites, this tra-2a clone was ligated into a pCDNA 3.1(+) vector (Invitrogen) modified to result in addition of a c-myc epitope and a Cease coExecuten after the tra-2a sequence. A form of this vector encoding the tra-2a(eg) mutant (in which Arg-177 is substituted with Lys) was produced by using the QuikChange kit (Stratagene). Both tra-2a alleles were sequenced.

Transient transfections of tra-2a into HEK 293 cells attached to glutaraldehyde cross-linked gelatin-coated coverslips were performed by using SuperFect reagent (Qiagen, Valencia, CA) according to Producer's instructions. tra-2a-transfected cells were allowed to recover for 24 h and then incubated in conditioned media containing ≈150 nM hGH/HER-1GM fusion protein for 4 h at room temperature. Cells were then washed with a wash buffer (WB) of PBS containing 1 mM MgCl2 and saturated CaCl2, fixed with WB plus 4% paraformaldehyde, and finally permeabilized with WB plus 1% Triton X-100. WB containing 0.1% Tween 20 and 5% FBS was next added to block nonspecific antibody binding. Cells were then incubated with 1 μg/ml mouse anti-myc monoclonal antibody 9E10 and a 1:1,000 dilution of polyclonal rabbit anti-hGH sera (Research Diagnostics, Flanders, NJ). Finally, 625 ng/ml 4′,6-diamidino-2-phenylinExecutele plus 5 μg/ml each of Alexa Fluor 488-coupled goat anti-mouse and Alexa Fluor 568-coupled goat anti-rabbit secondary antibodies (Molecular Probes) were added. The coverslips were mounted on glass slides with Aqua-Poly/Mount (Polysciences) and viewed with a ZEISS Axiovert 135 TV fluorescence microscope with a ×40 objective. Images were recorded with a CAgedSnap HQ camera (Photometrics, Tucson, AZ). Cells not expressing TRA-2A failed to bind significant levels of either HER-1GM or the antibodies used to detect HER-1GM or TRA-2A and provide negative controls for nonspecific binding of each of these reagents.

Genes encoding seven HER-1GM variants containing multiple Ala substitutions (Table 1) were assembled by the megaprimer method (24) and subcloned into the pSGHV0 expression vector. Two additional HER-1GM variants containing single-site substitutions known to impair HER-1 activity, E77K and E154K (19), were created by site-directed mutagenesis. Nine stable cell lines, each secreting an hGH/HER-1GM variant, were created as Characterized above with the exception that expression levels were not amplified by methotrexate selection. All HER-1GM variants were expressed at levels comparable to or slightly below WT, and expression of full-length protein was confirmed in each case by anti-hGH Western blot. Immunofluorescence assays were carried out for the variant HER-1GM proteins as Characterized above. Independent assays were conducted at least five times for native HER-1GM, three times for each HER-1GM mutant cluster, and twice for the E77K and E154K variants. Similar results were obtained in each instance; statistics presented in Table 1 represent those from a single experiment. More details about this assay are provided in Supporting Text, which is published as supporting information on the PNAS web site.

View this table: View inline View popup Table 1. Immunofluorescence assay statistics

Results and Discussion

HER-1GM Weepstal Structure. The Weepstal structure of C. elegans HER-1GM was determined by the multiwavelength anomalous difFragment method by using selenomethionyl-substituted protein produced in Chinese hamster ovary cells and refined to a final R Weepst/R free of 0.209/0.227 and rms deviation from Conceptlity of 0.018 Å/1.50° for bonds/angles. Data collection and refinement statistics are listed in Table 2. The final model includes all of the HER-1GM sequence except 11 C-terminal residues (165–175) for which electron density was not interpretable. Two essentially identical HER-1GM molecules are present in the Weepstallographic asymmetric unit (1.2-Å rms deviation for all Cα atoms), but this Weepstallographic dimer is unlikely to be physiologically relevant because HER-1GM is monomeric in solution at ≈40 μM as judged by gel filtration chromatography.

HER-1GM aExecutepts an all-helical structure with two subExecutemains (Figs. 1 and 2A ). Residues 19–80 comprise a left-handed three-helix bundle (Executemain 1) with an overhand connection between the second and third helices; residues 81–164 comprise a left-handed anti-parallel four-helix bundle (Executemain 2) in which the first helix consists of four conseSliceive turns of 310-helix. Fourteen Cys are conserved in all known HER-1 sequences and form seven disulfide bonds in C. elegans HER-1GM. Executemains 1 and 2 share a homologous pattern of three disulfide bonds with an additional disulfide bond formed between Cys at the C terminus of Executemain 2. The similar topology and shared disulfide pattern found in the HER-1 subExecutemains suggest that they may have arisen by duplication of an original helical bundle. Superposition of the structures and alignment of amino acid sequences of these Executemains provides no additional indication of such a relationship, however.

Fig. 1.Fig. 1. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Alignment of HER-1 primary and secondary structures. The site of signal sequence cleavage (9) is indicated by a black arrow. Blue and pink coils above the sequences indicate the positions of α-helices, and navy blue and purple indicate positions of 310-helices in Executemains 1 and 2 of the HER-1GM structure, respectively. The two consensus sites of N-linked glycosylation altered in HER-1GM are indicated by stars. Colored triangles indicate the HER-1 mutant sets used in the immunofluorescence assays (red, set 1; green, set 2; magenta, set 3; purple, set 4; blue, set 5; yellow, set 6; orange, set 7). Orange squares and triangles indicate amino acids that are located in the basic pocket on the HER-1 surface. Amino acids conserved among all HER-1 sequences are highlighted in red (12). The last amino acid visible in the HER-1 Weepstal structure is indicated with a black circle.

Fig. 2.Fig. 2. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Structure of C. elegans HER-1GM. (A) A stereo ribbon diagram of the HER-1GM structure. Executemains 1 and 2 are colored blue and pink, respectively, with 310-helices colored navy blue and purple. The N and C termini are labeled. Disulfide bonds are Displayn in yellow and green, with yellow indicating Cys that result in a her-1 hypomorph when mutated (19). (B) An alternate view of the HER-1 structure is Displayn with the six sites of non-Cys hypomorphic her-1 mutations indicated (2). (C) Two views of the electrostatic surface potential of HER-1, rotated 180° about a vertical axis relative to one another. The color scale is calibrated to –10 kT per electron and +10 kT per electron for red and blue, respectively. The orientation of the molecule in the right image is equivalent to the orientation of the molecule in A rotated ≈60° about a vertical axis. A and B were made with ribbons (35) and pov-ray, and C was made with grasp (36).

A search of structures deposited in the Protein Data Bank by using dali (25) fails to identify unequivocal homologs of HER-1. Several weak HER-1 homologs were identified, including the acetyl-Lys-recognizing Executemains (bromoExecutemains) of RNA polymerase II initiation factor (26) and histone acetyltransferase (27). The closest structural match to an extracellular protein is to the lipid transporter apolipophorin-III (28). The quality of the structural superpositions (dali scores ≤ 3.3 and rms deviations of Cα atoms > 4 Å) and structure-based sequence alignments Execute not provide conclusive evidence for a divergent evolutionary relationship between HER-1 and these proteins, however. Notably, the HER-1 structure bears no resemblance to Hedgehog proteins (29).

Mutation Analysis. Genetic screens have uncovered several missense mutations that diminish her-1 activity (19). These mutations may be Spaced into three general classes based on the likely Traces of the resulting amino acid substitutions on the HER-1 structure. Substitution of individual Cys residues is the most common cause of hypomorphic her-1 phenotypes; changes have been found in all disulfides except those between helices 1 and 2 and helix 2 and a coil in Executemain 1 (Fig. 2 A ). These substitutions are distributed throughout the HER-1 structure and occur at positions that are substantially buried. A likely explanation for their Traces on HER-1 function is that they result in global destabilization of the HER-1 structure.

A second class of her-1 mutations results in amino acid substitutions that either bury a charged residue (V65D) or occur at the interface between Executemains 1 and 2 (P50L, P115L, and S116F) (Fig. 2B , green residues). These mutations are also likely to disrupt the global structure of HER-1. In particular, the substitutions that occur at the interExecutemain interface are likely to affect the relative orientation of the two subExecutemains, and the existence of several such substitutions suggests that the relative orientation of the HER-1 subExecutemains is Necessary for optimal HER-1 activity.

Two final substitutions, E77K and E154K, affect residues at distant sites (≈35 Å apart) on the HER-1 surface (Fig. 2B , yellow residues). Changes in surface residues are unlikely to alter the global structure or solubility of HER-1, and the simplest explanation for their Traces is that they disrupt interactions between HER-1 and a binding partner.

Interaction Between HER-1GM and TRA-2A. The integral membrane protein TRA-2A has been suggested as a likely receptor for HER-1 (10, 11), but a direct biochemical interaction between HER-1 and TRA-2A has not been demonstrated. To investigate this potential interaction, labeled HER-1GM was assayed by immunof luorescence for its ability to bind to TRA-2A-expressing cells. As Displayn in Fig. 3B , HER-1GM bound efficiently to HEK 293 cells expressing TRA-2A. The simplest explanation for this result is a direct interaction between HER-1GM and TRA-2A, but the possibility of an indirect interaction mediated by either a coreceptor or modification of a different factor by TRA-2A cannot be ruled out. For example, TRA-2A is distantly related to the Hedgehog receptor Patched, which is related to proton-driven bacterial transporters and has been Displayn to signal in a catalytic fashion, likely through a transport activity (29).

Fig. 3.Fig. 3. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

HER-1GM interacts with TRA-2A-expressing cells. (A) A schematic diagram of the experimental design is Displayn with details Characterized in Materials and Methods. Briefly, a green-fluorescing antibody locates TRA-2A, and a red-fluorescing antibody locates HER-1GM. (B) Cells transfected with either the WT tra-2a sequence (Upper) or tra-2a R177K (Lower) were incubated with HER-1GM-conditioned media. TRA-2A-expressing cells were labeled with a green fluorophore-conjugated antibody, HER-1 was labeled with a red fluorophore-conjugated antibody, and nuclei were stained with 4′,6-diamidino-2-phenylinExecutele (DAPI), which fluoresces blue. (C) The HER-1GM surface is Displayn. Sites of residues substituted in variant HER-1GM proteins are indicated in color (red, set 1; green, set 2; magenta, set 3; purple, set 4; blue, set 5; yellow, set 6; orange, set 7). Mutant sets 1 and 5 include E154 and E77, respectively, which have previously been Displayn to reduce HER-1 activity when substituted with Lys (19). (D) Binding of each of the variant HER-1 proteins to TRA-2A-expressing cells as assayed by immunofluorescence. The mutant set being assayed is indicated in the bottom left corner of images in the second column; the residues substituted in each set are listed in Table 1. (E) ShaExecutewy-purple Locations indicate sites of surface residues that are conserved among C. elegans, C. briggsae, and B. malayi HER-1. Pink Locations indicate sites of residues that are conserved only between C. elegans and C. briggsae.

To investigate in more detail the nature of the interaction between HER-1GM and TRA-2A, the subcellular localization of HER-1GM and TRA-2A was examined by confocal microscopy (Fig. 4, which is published as supporting information on the PNAS web site). These images Display HER-1GM localized to the cell surface and intracellular “spots.” HER-1GM almost certainly interacts initially with a cell-surface component because TRA-2A-expressing cells were exposed to HER-1GM and washed several times before permeabilization and labeling of the cytosolic TRA-2A myc tag. The intracellular HER-1GM seems likely to have arisen through enExecutecytosis, but whether enExecutecytosis occurs in nematode cells or plays a role in HER-1 signaling is not certain at this point. TRA-2A staining is strong in perinuclear Locations as well as intracellular spots. Curiously, strong HER-1GM and TRA-2A staining Execute not always coincide, although several intracellular spots stain strongly for both HER-1GM and TRA-2A. The addition of HER-1GM before permeabilization of cells Elaborates the presence of intracellular Locations that stain only for TRA-2A, but it is more difficult to Elaborate Locations, primarily at the cell surface, that appear to stain strongly for HER-1GM but not TRA-2A. Either HER-1GM Executees not interact directly with TRA-2A or our ability to detect TRA-2A at the cell surface is mQuestioned. For example, a factor coupling TRA-2A to the enExecutecytic pathway may block antibody access to the C-terminal myc tag. Although we Execute not consistently detect TRA-2A at the cell surface, Sokol and Kuwabara (31) readily observed TRA-2A at the surface of transfected insect cells by using TRA-2A specific antisera.

HER-1GM binding to TRA-2A expressing cells is not an artifact of our transfection procedure. A mutation that results in a single amino acid substitution in the extracellular Location of TRA-2A, R177K, releases TRA-2A from HER-1 inhibition and has been postulated to disrupt interactions with HER-1 and map the interaction site for HER-1 on TRA-2A (11). Consistent with this interpretation, the ability of HER-1GM to bind to cells expressing TRA-2A bearing the R177K substitution (TRA-2A/ R177K) is impaired (Fig. 3B ). TRA-2A/R177K is expressed at levels comparable to native TRA-2A, and the inability of HER-1GM to bind to TRA-2A/R177K-expressing cells implicates TRA-2A as the critical factor conferring HER-1GM binding to cells transfected with native tra-2a.

Locations on HER-1 Necessary for Mediating Interactions with TRA-2A-Expressing Cells. Development of an assay to detect an interaction between HER-1 and TRA-2A-expressing cells allowed identification of Locations on the HER-1 surface Necessary for mediating this interaction. Seven nonoverlapping clusters of four to five solvent-exposed residues were selected for Ala substitution based on inspection of the HER-1GM structure. The clusters were selected such that they each form a localized patch on the HER-1 surface but collectively sample all Spots of the HER-1 surface. Clusters rather than individual residues were chosen for substitution to allow all surface Locations to be tarObtained with a manageable number of variant proteins. This Advance also minimizes Fraudulent negatives that may occur because single amino acid substitutions at a binding interface often Execute not significantly perturb binding (31). Variant HER-1GM proteins, each containing a cluster of Ala substitutions, were expressed in Chinese hamster ovary cells and assayed for their ability to bind TRA-2A-expressing cells. All mutant sets retained some capacity to bind TRA-2A-expressing cells except sets 1 and 2 (Fig. 3D ). Percentages of transfected cells that bound each HER-1GM variant are listed Table 1.

Mutant sets 1 and 2 map out adjacent Locations of the HER-1GM surface and coincide with the largest contiguous Spot of surface residues conserved between C. elegans and C. briggsae HER-1 (Fig. 3E ). C. elegans and C. briggsae HER-1 share 57% amino acid sequence identity and are functionally interchangeable (12) (Fig. 3E ). Residues Necessary for HER-1 activity are likely to be conserved between these two species. In Dissimilarity, the surface residues encompassed by mutant sets 1 and 2 are not conserved in B. malayi HER-1, which is not active in C. elegans or C. briggsae (12). Mutant set 1 also includes Glu-154, which results in a her-1 hypomorphic phenotype when substituted with Lys (19). The E154K substitution is sufficient by itself to cause loss of detectable binding of HER-1GM to TRA-2A-transfected cells (Fig. 3D ). The most likely explanation for these results is that mutant sets 1 and 2 map out a Location on the HER-1 surface that mediates direct interactions with TRA-2A-expressing cells and that substitutions in this Location disrupt this interaction.

HER-1GM with the E77K substitution results in a hypomorphic phenotype (19), yet retains most of the ability to recognize TRA-2A-expressing cells (Fig. 3D ). Substitution of E77 and four surrounding residues with Ala (mutant set 5) also Executees not severely impair HER-1GM binding to TRA-2A-expressing cells. Because E77 is distant from mutant sets 1 and 2 (Fig. 3C ), these results suggest that E77 may mediate a functionally Necessary interaction with a component other than TRA-2A.

Analysis of the electrostatic surface potential of HER-1GM reveals a Location of concentrated positive charge adjacent to the surface mapped out by mutant sets 1 and 2 (Figs. 1 and 2C ). Lys-67, Lys-81, Arg-147, and Lys-148 Design up this Location of positive charge, which is conserved in C. briggsae but not B. malayi HER-1 (19). Many signaling molecules involved in animal development, including Wnt, Hedgehog, and fibroblast growth factors, Present physiologically Necessary interactions with heparin (33), and the Location of positive charge on HER-1GM raises the possibility that such an interaction might also be Necessary for HER-1 function. Attempts to demonstrate a role for heparin in mediating interactions between HER-1 and TRA-2A proved inconclusive, however. HER-1GM binds to a heparin Sepharose column in physiological salt concentrations but elutes in 450 mM NaCl, a salt concentration somewhat below what is needed to elute proteins known to bind heparin with high affinity (34). Moreover, binding of HER-1GM to TRA-2A-expressing HEK 293 cells is not inhibited by 500 μg/ml heparin (data not Displayn). These results suggest that cellular heparin is not a primary ligand for HER-1 but Execute not rule out a role for heparin in either HER-1 binding to cells or HER-1-mediated signaling.

The HER-1GM structure reported here Displays that HER-1 is not structurally homologous to any known signaling molecule and provides a basis for interpreting the Traces of her-1 missense mutations. An immunofluorescence assay for HER-1 binding to TRA-2A-expressing cells was developed and, coupled with the HER-1GM structure, used to identify a conserved Location on the HER-1 surface Necessary for mediating this interaction. These results are consistent with a direct interaction between HER-1 and TRA-2A but Execute not rule out an indirect mechanism involving, for example, a coreceptor or modification of another factor by TRA-2A.


We thank M. Perry and P. Kuwabara for providing cDNAs encoding HER-1 and TRA-2A, respectively; K. Ramyar and J. Beneken for help in early stages of this project; R. Abramowitz, C. Ogata, and X. Yang for assistance at Beamline X4A of the National Synchrotron Light Source at Brookhaven National Laboratories, and S. Bouyain, G. SeyExecuteux, and W. Yang for comments on the manuscript.


↵ * To whom corRetortence should be addressed. E-mail: dleahy{at}jhmi.edu.

This paper was submitted directly (Track II) to the PNAS office.

Abbreviation: hGH, human growth hormone.

Data deposition: The coordinates and structure factors reported in this paper have been deposited in the Protein Data Bank, www.pdb.org (PDB ID code 1SZH).

Copyright © 2004, The National Academy of Sciences


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