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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Mol Biol. Author manuscript; available in PMC 2010 September 11.
Published in final edited form as:
PMCID: PMC2739586
NIHMSID: NIHMS130463

Crystal Structure of Miner1: The Redox Active 2Fe-2S Protein Causative in Wolfram Syndrome 2

Abstract

The endoplasmic reticulum protein Miner1 is essential for health and longevity. Mis-splicing of CISD2, which codes for Miner1, is causative in Wolfram Syndrome 2 (WFS2) resulting in early onset optic atrophy, diabetes mellitus, deafness and decreased lifespan. In knock-out studies, disruption of CISD2 leads to accelerated aging, blindness and muscle atrophy. In this work, we characterized the soluble region of human Miner1 and solved its crystal structure to a resolution of 2.1 Å (R-factor=17%). Although originally annotated as a zinc finger, we show that Miner1 is a homodimer harboring two redox active 2Fe-2S clusters indicating for the first time an association of a redox active FeS protein with WFS2. Each 2Fe-2S cluster is bound by a rare 3Cys-1His motif within a 17 amino acid segment. Miner1 is the first functionally different protein that shares the unique NEET fold similar of its recently identified paralog mitoNEET, an outer mitochondrial membrane protein. We also report the first measurement of the redox potentials (Em) of Miner1 and mitoNEET showing that they are proton-coupled with Em ~ 0 mV at pH 7.5. Changes in the pH sensitivity of their cluster stabilities are attributed to significant differences in the electrostatic distribution and surfaces between the two proteins. The structural and biophysical results are discussed in relation to possible roles of Miner1 in cellular Fe-S management and redox reactions.

Keywords: diabetes, membrane bound, oxidative stress, CDGSH, ER stress

Introduction

Miner1 , originally annotated as a zinc finger protein of unknown function, is actually a unique redox active Fe-S protein. The gene encoding Miner1, CISD2 (CDGSH iron sulfur domain 2),|| found at chromosomal location 4q24, was recently identified as the carrier of a mutation that causes Wolfram Syndrome 2 (WFS2) 1. The pre-mRNA of CISD2 has two splice sites, creating 3 exons and 2 introns (Fig. 1). Correct splicing is essential for the proper translation and function of Miner1. It localizes to the endoplasmic reticulum (ER) when properly expressed and folded. The mutation found in WFS2 patients is a G to C base pair transversion in pre-mRNA CISD2 that causes a splicing error resulting in removal of the second exon and introduction of a premature stop codon, that eliminate 75% of the protein transcript (Fig. 1) 1. Indeed, the region binding the 2Fe-2S cluster is completely eliminated. Although initially healthy, patients with WFS2 experience a decreased life expectancy, early onset diabetes mellitus, optical atrophy, sensorineural deafness, and a significant bleeding tendency 1. Thus, studies of structure and function of Miner1 are expected to provide critical information on the fundamental cause of the disease and reasons for the decrease in life expectancy.

Figure 1
Elimination of functional Miner1 protein leads to multiple symptoms in both human and mice models

CISD2 lies on chromosome 4, a genetic locus associated with longevity. Investigations focused on the longevity locus where researchers disrupted CISD2 in mice found that the elimination of Miner1 resulted in decreased life expectancy and reduced general health 2. These knockout mice show accelerated aging, blindness, an abnormal skeleton, and muscle atrophy 2, effects that are very similar to those described in the WFS2 patients. The mouse ortholog is 96% identical to the human Miner1 and is highly conserved (94–100%) with other mammals (supplemental Fig. S1). Obtaining the Miner1 protein structure provides a basis for understanding functional consequences of mutations in human and animal studies.

Miner1 belongs to a newly discovered family, which includes the 2Fe-2S containing outer mitochondrial membrane (OMM) protein mitoNEET 3; 4. In contrast, Miner1 localizes to the ER 1. It contains an atypical ER localization sequence, an N-terminal transmembrane domain, and a CDGSH 2Fe-2S domain. To facilitate yields and structural studies, we focused on the soluble domain of the protein and made a point mutation at the only non-conserved free cysteine (supplemental Fig. S1). In this work, we show that the Cys mutation does not affect the properties and report the crystal structure of the soluble region of Miner1 (C92S) solved at 2.1 Å resolution using Fe-MAD phasing 5. The structure is homodimeric with a scaffold similar to the OMM protein mitoNEET, but has a distinct surface topology and differences in the charge distribution. We also report and compare redox and stability measurements as functions of pH for both Miner1 and its paralog mitoNEET 6.

Results

Isolation and UV-Vis Spectroscopy of the ER protein Miner1

In an effort to understand the properties of this protein we produced a soluble form of recombinant human Miner1 that corresponds to amino acids 57-135 (lacking the amino-terminal targeting and transmembrane sequences). The protein was fused to a cleavable His-tag to facilitate purification. As a result, the construct includes an additional four amino acids (GSHM) at the N-terminus. Early attempts at purification were hindered by protein instability and aggregation. Although use of reducing agent decreased aggregation, yields remained insufficient for structural studies. Therefore, we replaced the one free nonconserved cysteine (Supplemental Fig. S1) with the isosteric serine (C92S). The resultant mutant protein, Miner1 (C92S), has the same optical signature of the native Miner1 (Fig. 2). Furthermore, the construct had the desirable effects of improved yields of purified protein and a decreased tendency to aggregate. Since the point mutation does not change the spectra of Miner1, we will refer to Miner1(C92S) as Miner1 hereon.

Figure 2
Miner1 (C92S) has essentially the same UV-Vis spectrum as WT Miner1

The Overall Fold of Miner1

The isolated purified recombinant protein showed CD spectra (Fig. S2) indicative of a well folded protein with a chiral center. The protein was amenable to crystallization. X-ray diffraction intensities were collected at Stanford Synchrotron Radiation Laboratory (SSRL) BL7-1 and BL9-2 to 2.1 Å (Table 1). Miner1 crystallized in the orthorhombic space group P212121 with unit-cell parameters a =49.90 Å, b=48.58 Å, c=74.10 Å, α=β=γ=90° (Table 1). The Matthews coefficient (Vm) of the crystal was 2.0 Å3/Dalton with an estimated solvent content of 37%.

Table 1
Summary of crystal parameters, data collection, and refinement statistics for Miner1

Experimental phases for model building were determined by using Fe-MAD 5. The model was refined using 2.1 Å data to an R-factor of 17.0% (Rfree=21.6%). The high quality of the electron density is shown in Fig. 3A. The refined model reveals a parallel homodimeric structure that includes the cytoplasmic fragment of each protomer from Asp68 to Glu134 on Protomer A and to Val135 on Protomer B (Fig. 3A). Fifteen amino acids at the N-termini are disordered and not resolved in the electron density. The homodimer is tightly packed with 1800 Å2 of buried surface area at the interface as calculated using PISA 7. Model validation using the MOLPROBITY 8 structure validation tool indicates that 97% of the amino acid residues are in the favored region of Φ/Ψ space.

Figure 3
Structural organization and domain topology of dimeric Miner1

Miner1 is folded into two spatially distinct sub-regions: a beta rich or “Beta-Cap” domain and a helical 2Fe-2S binding or “Cluster-Binding” domain (Fig. 3B). The beta rich domain contains a strand swap from opposite ends of the primary sequence to form the Beta-Cap structure. This domain contains twenty-eight residues within beta-strands with two antiparallel strands coming from one protomer and the third parallel coming from the other (Fig. 3B). These two strand-swapped sheets pack together to form the Beta-Cap sandwich domain and form the narrowest part (~ 15 Å across) of the structure (Fig. 3B). The structure confirms the presence of two 2Fe-2S clusters expected from the optical spectrum (Fig. 2). These clusters are separated by approximately 16 Å (center-to-center) within the larger helical Cluster-Binding domain (~ 30 Å across) (Fig. 3B). A structural similarity search using the DALI server 9 revealed that this fold shows significant similarity only to the recently determined new structural “NEET” fold of mitoNEET 6; 10; 11. This is the first example of another protein that shares the “NEET” fold.

Structure of the 2Fe-2S Clusters

The Cluster binding domain consists of the sequences Asp68-Val83 and Cys99-Asn124 on each protomer (Fig. 3B). Each 2Fe-2S cluster is cradled by a relatively short polypeptide chain from Cys99-Gly112. This segment contains the three coordinating Cys ligands (Cys99, Cys101, Cys110) and the single coordinating His ligand (His114). His114 is near the N-terminus of the α-helix within the Cluster-Binding domain (Ala113-Thr121) (Fig. 3).

Cys110 and His114, which coordinate the outermost Fe, are solvent accessible as illustrated in Figure 4 by their semitransparent surfaces. In contrast, Cys99 and Cys101, which coordinate the innermost Fe, are buried within the structure (Fig. 4). Located sequentially between the innermost Cys ligands is Arg100. It forms interprotomer interactions near the 2Fe-2S clusters. Its guanidium group forms a direct interprotomer hydrogen bond with the amide side chain of Asn84 of the other protomer. In addition, Arg100 interacts with the backbone oxygens of Cys99 and Pro108 of the other protomer via an internal water molecule (Fig. 4) locking Pro108 into a Cis conformation.

Figure 4
The 2Fe-2S cluster binding cradle

The Proton-Coupled Redox Potential of the 2Fe-2S clusters of Miner1 and its Paralog mitoNEET

A property of Miner1 that relates to possible redox activity is its redox potential Em. The Em values of both Miner1 and mitoNEET, i.e. [2Fe-2S]2+/[2Fe-2S]1+, were measured spectroscopically by the change in visible absorbances that occur upon reduction (Fig. S3). The fraction oxidized was plotted as a function of the measured ambient redox potential (Fig. S3) and fitted by the Nernst equation as described 12 yielding an Em of ~ 0 mV (± 10 mV) at pH 7.5 for both Miner1 and mitoNEET.

In order to determine the influence of charges and titratable groups on Em, the pH dependence was measured. The Em values of both Miner1 and mitoNEET are pH dependent from pH 7.5 to 10.0 decreasing 50 mV per pH unit (Fig. 5A). The observed dependence shows the reduction is proton-coupled, which is often of functional relevance 13.

Figure 5
Redox potential and Stability of the 2Fe-2S clusters of Miner1 and mitoNEET

Drastic difference in the Half-lifes of the 2Fe-2S Clusters of Miner1 and mitoNEET

Investigations of the release of the 2Fe-2S clusters can provide information on key interactions between the clusters and the protein. The half-life of the 2Fe-2S clusters was determined from monitoring the cluster absorbance from 320nm to 600 nm as a function of time at 35°C (Fig. S4). A half-life of 700 minutes (pH 7.1) was determined from the observed signal change as a function of time. Surprisingly, given the similar fold, Miner1 was 10-fold less stable than mitoNEET 14 (Fig. 5B). In contrast, Miner1 was more stable at pH 5.3 (Fig. S4). In addition, the decay is more non-exponential suggesting a more complex unfolding energy landscape 15. To further explore differences, we performed a more detailed investigation of the stability as a function of pH.

Investigation of the stability as a function of pH provides additional information on interactions between titrating (charged) residues and the cluster. In contrast to mitoNEET, the lifetime for Miner1 varies by only ~5-fold from pH 5.3 to 7.1 compared to ~300-fold for mitoNEET (Fig. 5B). Thus, the release of the 2Fe-2S clusters of Miner1 is orders of magnitude less sensitive to pH as those of mitoNEET.

Prominent Differences in the Charge Distribution of Miner1 and mitoNEET and in the Packing of the Beta-Cap Domain

The two proteins display prominent differences in the distribution of charges. Overall Miner1 has both fewer charged (Asp and Lys) and His residues (Fig. 6B). The homologous sites of polar Asn87 and apolar Leu93 and Leu120 in Miner1 are acidic (Asp61, Asp67 and Asp93) in mitoNEET. Other differences include (i) polar Thr106 in Miner1 replaced with a basic Lys79 in mitoNEET, (ii) acidic Glu85 by neutral Ala59, and (iii) Lys74 and Asn84 by His48 and His58. These changes result in a more positively charged and soluble domain for Miner1.

Figure 6
Differences in the backbone and charge distributions of Miner1 and related mitoNEET structures

The similarities and differences in the properties of Miner1 and mitoNEET are evident when the two structures are superimposed (Fig. 6). Miner1 is the second known member of the family first established by the determination the mitoNEET structure 6; 10; 11. (Fig. 6). The Cluster-Binding domains of Miner1 and mitoNEET share the same 3Cys-1His 2Fe-2S coordination and are structurally the same within uncertainty (rmsd ~ 0.3, pdb ID 2QH7 6).

However, superposition of the dimeric structures reveals significant differences in the outermost strands. The backbone rmsd of the Beta-Cap domain is ~ 1.1 Å, c.a. 4-fold greater than that of the cluster binding domain (Fig. 6A). These differences result in a more open “V” shape appearance in the Beta-Cap of Miner1. As an additional test of the significance of this apparent difference, we superimposed the protomor structures of Miner1 and mitoNEET (Fig. 7A). Strikingly, the central strand along with the Cluster -Binding domain of the protomers are superimposable. However, the outer strands (circled) are shifted with respect to one another. Interestingly, Miner1 has an amino acid insertion at the top of the Beta-Cap domain (Thr 94 Fig 7 highlighted) that appears to act as a spacer extending the distance between the central and the outer strand. In addition, several key variations in aromatic residues (Phe60/Ile86 and Phe82/Ala109) not only create differences in side chain orientation, but also appear to affect the position of Tyr 71/Tyr98, with a relative displacement of 2.2Å.

Figure 7
Protomer superposition of Miner1 and mitoNEET highlighting distinct backbone and side chain conformations

Discussion

We report here the crystal structure determined to 2.1 Å resolution of the ER protein Miner1. We show that contrary to its original annotation, it contains redox active 2Fe-2S clusters with an Em ~ 0mV at pH 7.5. Miner1 is a member of the novel 2Fe-2S CDGSH protein family and the first FeS protein localized to the ER. Mis-splicing of the mRNA that codifies this protein is causative in the genetic disease Wolfram Syndrome 2 (WFS2). FeS proteins participate in many types of biochemical reactions, the most predominantly characterized are oxidation-reduction reactions, but they also participate in Fe-S cluster assembly and gene expression regulation 16; 17; 18; 19. Although not yet determined in vivo, Miner1 has biophysical properties necessary to participate in the functions that play a vital role in homeostasis required for proper health and longevity. The importance of Miner1 in cellular health is evidenced in the deleterious effects of its absence in WFS2 and knockout mice 1; 2. The potential role this protein plays in human health and defense against environmental stresses is suggested by the fact that patients with WFS2 can live to 15 years before becoming symptomatic.

Miner1 is the first 2Fe-2S protein found in the ER

Miner1 was identified as one of a small family of proteins that share the CDGSH sequence domain 4. Based on strong perinuclear staining and its localization to a lacy network in V5 epitope studies performed in transiently expressed COS-7 cells, Wiley et al. suggested that Miner1 localizes to the ER 4. Amr et al. subsequently confirmed this suggestion in mouse P19 cells in which N-FLAG tagged Miner1 colocalized with calnexin, a known ER marker 1. Taken together with our structural studies (Fig. 3), Miner1 is to our knowledge the first 2Fe-2S containing protein localized to the ER membrane.

Proton-Coupled Redox Activity of Miner1

One of the best characterized functions of FeS proteins is their role in redox reactions. A primary property that relates to this function is the redox potential Em. The Em of Miner1 and mitoNEET were measured via equilibrium titration to be essentially the same at ~ 0 mV at pH 7.5. The similarity of their Em values is expected as both Miner1 and mitoNEET have nearly superimposable structures of the Cluster binding domains which includes the rare 3 Cys-1 His coordination. The Em values varies by −50mV/pH unit indicating that the reductions are proton-coupled (Fig. 5A). Since both Miner1 and mitoNEET display the same behavior, the site of protonation is also likely the same. Given the magnitude of the change in redox potential (> 150 meV), the protonation site must be located near the 2Fe-2S cluster. In addition to the inorganic sulfide of the cluster, the 2Fe-2S clusters share ligation by Cys110 and His114 and interact with the nearby Asp111 and Ser104. Further localization of the proton requires additional investigations.

The most common redox-active motifs for 2Fe-2S proteins are ferredoxin-like whose clusters are coordinated by 4 Cys and Rieske-like whose clusters are coordinated by 2 Cys and 2 His. The Fd-like proteins tend to have negative Em (~ − 300 mV) and the Rieske-like positive Em (~ + 300 mV) although some can be in the same range as Miner1 17. Miner1 and mitoNEET have similar redox potentials as with known electron transfer species such as cytochrome b and quinones 20; 21. Thus, their potentials are within the physiological range consistent with Miner1 and mitoNEET participating in electron transfer or oxidation/reduction reactions in the cell.

Different pH Sensitivities of the 2Fe-2S Clusters of Miner1 and mitoNEET

Another known functional role for FeS proteins is cluster assembly and transfer 19. The Miner1 family shares the unusual 3Cys-1His coordination with only one other known protein structure – that of an IscU D38A mutant from a hyperthermophilic bacteria 22, a protein that participates in cluster assembly. In order for this function to be plausible for Miner1, the Fe-S cluster must be easily transferable or labile. The measured lifetime of the 2Fe-2S clusters of Miner1 was ~ 10-fold shorter at pH 7.1 than that of mitoNEET 4. This lability is accelerated by orders of magnitude in cellular extracts (data not shown), suggesting a possible role of Miner1 in FeS cluster assembly and/or transfer.

Surprisingly, given the high structural similarity of Miner1 and mitoNEET, the lability of the two proteins displayed drastically different pH dependences. Whereas the lability of Miner1 varied by only ~5-fold from pH 5.3 to 7.1, mitoNEET varied ~ 300-fold over the same range (Fig. 5). This difference in the lability is likely due to differences in the charged or titratable groups of the proteins. A site specific comparison revealed seven differences between the proteins (Figure 6) – Lys74, Asn84, Glu85, Asn87, Leu93, Thr106 and Leu120. One or more of these changes are likely responsible for the altered pH sensitivity.

Miner1, ER Function and Health and Disease

Miner1 is targeted to the ER membrane 1; 4, the organelle in which lipid synthesis, protein folding and protein maturation take place. A number of post-translational modifications are performed here including lipidation, hydroxylation, glycosylation, methionine and cysteine oxidation 23; 24. The ER also participates in quality control by facilitating the recognition and targeting of aberrant proteins for degradation 25. When its capacity is exceeded, a signaling network called the unfolded protein response is activated 24. The redox ability of Miner1 could allow it to participate in protein modification and possible ER stress responses.

Recent studies on the tethering of the ER and mitochondria via the protein mitofusin indicate that a direct interaction between the organelles is essential for Ca2+ transfer and cellular function 26; 27. Increasing evidence also links altered communication between the ER and mitochondria to apoptosis and disease 28; 29; 30. Mitochondrial-ER crosstalk is implicated in type 2 diabetes 23; 31; 32; 33; 34. Interestingly, Miner1 is situated on the surface of the ER and mitoNEET is on the surface of the OMM. The similarities in their Em indicate that they would respond to similar redox stimuli and could possibly communicate through electron transfer thus providing a means of ER-mitochondrial crosstalk.

Proper biological function of Miner1 is necessary for cellular health, as the absence of Miner1 has deleterious effects such as WFS2 1 or decreased longevity 2 (Fig. 1). The similarities of Miner1 with its paralog mitoNEET also strengthen the link between proper function of this novel family of 2Fe-2S proteins and general health. Thus, further investigation of the interplay between the functional and biophysical properties of this family are anticipated to advance our understanding of these complex mechanisms involved in health and metabolic disease.

Materials and Methods

Expression and Purification

The soluble domain of miner1 (residues 57-135) (cDNA purchased from Open Biosystems) was amplified by PCR and subcloned into bacterial expression vector pET28a(+) (Novagen) containing an N-terminal, thrombin cleavable his-tag. The C92S mutation was made by site-directed mutagenesis using PCR. This plasmid was transformed into BL21-RIL(DE3) (Stratagene) and grown as described 6. Harvested cells were resuspended in binding buffer (20mM Tris-HCl pH7.9, 5mM Imidazole, 500mM NaCl). After centrifugation, the lysate was added to Ni-NTA resin. After 10x volume wash in wash buffer (20mM Tris-HCl pH 7.9, 30mM Imidazole, 500mM NaCl), equilibrated in thrombin cleavage buffer (25mM Tris-HCl pH 8.0, 100mM NaCl, 2.5mM CaCl2), and cleaved upon addition of thrombin at room temperature 16–24 hours. Miner1 was eluted with wash buffer and subsequently further purified by size exclusion chromatography (S-200 GE Healthcare) in 25mM Tris-HCl pH 8.0, 100mM NaCl. For crystallographic experiments, the protein was further purified by cation-exchange chromatography (HiTrap, GE Healthcare) as described 6.

Optical Spectroscopy and Stability Measurements

All UV-Visible absorption spectra were measured from the near UV to the near IR (250–700 nm) on a Cary50 spectrometer (Varian Inc, Palo Alto CA) equipped with at temperature controlled cell (T = 35°C) (protein concentration 10–20 μM in 25 mM Tris-HCl, pH 8.0 and 100 mM NaCl).

The stabilities of the 2Fe-2S clusters were determined from monitoring their characteristic absorbance at 460nm as a function of time following dilution of the protein into the assay buffer (~20 μM protein in 100mM buffer at 35°C) (Fig. S3). The following buffers were used at the pH indicated: Tris-HCl at pH 7.1 and 6.8 and bis-Tris-HCl at pH 6.3, 5.8 and 5.3. Both the buffer and the protein were filtered prior to the start of the measurement and a layer of 100% paraffin oil was placed over the solution to minimize evaporation. The half life, corresponding to decay by 50% was determined at each pH.

Redox Measurements

The reduction state was determined by monitoring the absorbance near 460nm with 100 μM protein in 25 mM Tris-HCl pH 7.5 and 100 mM NaCl at 25°C using a quartz cuvette containing a side arm under positive Argon pressure (Airgas-West, San Diego CA) (Fig. S4). A Ag/AgCl dual reference and working electrode (Microelectrodes Inc, Bedford NH) was used to measure the ambient redox potential. Its calibration was checked using quinhydrone at pH 4.0 and pH 7.0 as recommended. 50 mM pH buffered dithionite (Fisher Scientific, Pittsburgh PA) was added via a Hamilton syringe to adjust the redox potential. 50 μM ubiquinone (Sigma-Aldrich), 50 μM duroquinone (Sigma-Aldrich), 100 μM menadione (Sigma-Aldrich), and 100 μM naphthaquinone (Sigma-Aldrich) were added to ensure efficient electron transfer between the solution components and the electrode. The midpoint/redox potential (Em) was determined from a fit of the fraction oxidized to the Nernst equation (A= Aox/{1 + 10 ^ [(Em − E) (n/59.1 mV)]} + Ared) using the graphing program Origin 6.1 (OriginLab Corporation) where Aox is the absorbance of the fully oxidized sample, Em is the midpoint potential in mV, E is the measured cell potential in mV, n is the number of electrons being transferred in the redox reaction, and Ared is the lowest absorbance level corresponding to the fully reduced 2Fe-2S center in either native or mutant mitoNEET. Potentials were adjusted to SHE for presentation. Samples were tested for their ability to reoxidize upon exposure to ambient oxygen.

Crystallization

Crystallization was achieved as described previously for mitoNEET 6. Final conditions were 100 mM Tris-HCl pH 8.0, 100 mM NaCl, and 15–20% PEG 3000 in the protein well, equilibrating against 100 mM Tris-HCl pH 8.0 and 30–33% PEG 3000 in the reservoir. Crystals were frozen (77 K) after a 1-min soak in 100 mM Tris-HCl pH 8.0, 40% PEG3000 and sent frozen (77 K) to SSRL in an SSRL-supplied cassette system for x-ray data collection and analysis.

X-Ray Data Collection and Structural Determination

Frozen crystals were screened using the Stanford Automated Mounter 35 operated by Blu-Ice 36. The data were recorded using a Rayonix MX-325 CCD detector at BL9-2 and an ADSC Q315R CCD detector at BL7-1. Data were processed with XDS 37. The structure of Miner1 was determined by MAD phasing 5; 38. Data reduction and primary phasing at a resolution of 2.1 Å were accomplished as previously described 6.

Data Deposition

The atomic coordinates have been deposited in the Protein Data Bank, www.pdb.org (PDB ID code 3FNV; RCSB ID code rcsb050821).

Supplementary Material

01

Acknowledgments

Supported by grants from the NIH [grants GM 41637 (to M. Okamura and M.L. Paddock), and GM54038 and DK54441 (to P.A.J.)]. Students were supported by the HEME and CMG training grants. R. N. thanks the Zevi Hermann Shapira Foundation for supporting the collaborative USA-Israeli efforts. We thank Christopher L. Rife at the Joint Center for Structural Genomics for providing an automated programming script for coordinate validation, Mitch Miller at SSRL for helpful discussions on the data collection and processing and Mel Okamura for helpful discussions and support. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and the National Institute of General Medical Sciences.

Abbreviations

ER
endoplasmic reticulum
MAD
multiwavelength anomalous dispersion
SSRL
Stanford Synchrotron Radiation Laboratory
OMM
outer mitochondrial membrane

Footnotes

Also known as ERIS, CDGSH2

||Previous symbol ZCD2

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