Structures of activin ligand traps using natural sets of type I and type II TGFβ receptors

The 30+ unique ligands of the TGFβ family signal by forming complexes using different combinations of type I and type II receptors. Therapeutically, the extracellular domain of a single receptor fused to an Fc molecule can effectively neutralize subsets of ligands. Increased ligand specificity can be accomplished by using the extracellular domains of both the type I and type II receptor to mimic the naturally occurring signaling complex. Here, we report the structure of one "type II-type I-Fc" fusion, ActRIIB-Alk4-Fc, in complex with two TGFβ family ligands, ActA, and GDF11, providing a snapshot of this therapeutic platform. The study reveals that extensive contacts are formed by both receptors, replicating the ternary signaling complex, despite the inherent low affinity of Alk4. Our study shows that low-affinity type I interactions support altered ligand specificity and can be visualized at the molecular level using this platform.

ActRIIB:ALK4-Fc alleviates muscle dysfunction and comorbidities in murine models of neuromuscular disorders

Patients with neuromuscular disorders suffer from a lack of treatment options for skeletal muscle weakness and disease comorbidities. Here, we introduce as a potential therapeutic agent a heterodimeric ligand-trapping fusion protein, ActRIIB:ALK4-Fc, which comprises extracellular domains of activin-like kinase 4 (ALK4) and activin receptor type IIB (ActRIIB), a naturally occurring pair of type I and II receptors belonging to the TGF-β superfamily. By surface plasmon resonance (SPR), ActRIIB:ALK4-Fc exhibited a ligand binding profile distinctly different from that of its homodimeric variant ActRIIB-Fc, sequestering ActRIIB ligands known to inhibit muscle growth but not trapping the vascular regulatory ligand bone morphogenetic protein 9 (BMP9). ActRIIB:ALK4-Fc and ActRIIB-Fc administered to mice exerted differential effects - concordant with SPR results - on vessel outgrowth in a retinal explant assay. ActRIIB:ALK4-Fc induced a systemic increase in muscle mass and function in wild-type mice and in murine models of Duchenne muscular dystrophy (DMD), amyotrophic lateral sclerosis (ALS), and disuse atrophy. Importantly, ActRIIB:ALK4-Fc improved neuromuscular junction abnormalities in murine models of DMD and presymptomatic ALS and alleviated acute muscle fibrosis in a DMD model. Furthermore, in combination therapy ActRIIB:ALK4-Fc increased the efficacy of antisense oligonucleotide M12-PMO on dystrophin expression and skeletal muscle endurance in an aged DMD model. ActRIIB:ALK4-Fc shows promise as a therapeutic agent, alone or in combination with dystrophin rescue therapy, to alleviate muscle weakness and comorbidities of neuromuscular disorders.

Follistatin-based ligand trap ACE-083 induces localized hypertrophy of skeletal muscle with functional improvement in models of neuromuscular disease

Skeletal muscle is under inhibitory homeostatic regulation by multiple ligands of the transforming growth factor-β (TGFβ) superfamily. Follistatin is a secreted protein that promotes muscle growth and function by sequestering these ligands extracellularly. In the present study, we evaluated the potential of ACE-083 – a locally acting, follistatin-based fusion protein – as a novel therapeutic agent for focal or asymmetric myopathies. Characterization of ACE-083 in vitro revealed its high affinity for heparin and extracellular matrix while surface plasmon resonance and cell-based assays confirmed that ACE-083 binds and potently neutralizes myostatin, activin A, activin B and growth differentiation factor 11 (GDF11). Intramuscular administration of ACE-083 caused localized, dose-dependent hypertrophy of the injected muscle in wild-type mice and mouse models of Charcot-Marie-Tooth disease (CMT) and Duchenne muscular dystrophy, with no evidence of systemic muscle effects or endocrine perturbation. Importantly, ACE-083 also increased the force of isometric contraction in situ by the injected tibialis anterior muscle in wild-type mice and disease models and increased ankle dorsiflexion torque in CMT mice. Our results demonstrate the potential of ACE-083 as a therapeutic agent for patients with CMT, muscular dystrophy and other disorders with focal or asymmetric muscle atrophy or weakness.

Transforming growth factor-β superfamily ligand trap ACE-536 corrects anemia by promoting late-stage erythropoiesis

Erythropoietin (EPO) stimulates proliferation of early-stage erythrocyte precursors and is widely used for the treatment of chronic anemia. However, several types of EPO-resistant anemia are characterized by defects in late-stage erythropoiesis, which is EPO independent. Here we investigated regulation of erythropoiesis using a ligand-trapping fusion protein (ACE-536) containing the extracellular domain of human activin receptor type IIB (ActRIIB) modified to reduce activin binding. ACE-536, or its mouse version RAP-536, produced rapid and robust increases in erythrocyte numbers in multiple species under basal conditions and reduced or prevented anemia in murine models. Unlike EPO, RAP-536 promoted maturation of late-stage erythroid precursors in vivo. Cotreatment with ACE-536 and EPO produced a synergistic erythropoietic response. ACE-536 bound growth differentiation factor-11 (GDF11) and potently inhibited GDF11-mediated Smad2/3 signaling. GDF11 inhibited erythroid maturation in mice in vivo and ex vivo. Expression of GDF11 and ActRIIB in erythroid precursors decreased progressively with maturation, suggesting an inhibitory role for GDF11 in late-stage erythroid differentiation. RAP-536 treatment also reduced Smad2/3 activation, anemia, erythroid hyperplasia and ineffective erythropoiesis in a mouse model of myelodysplastic syndromes (MDS). These findings implicate transforming growth factor-β (TGF-β) superfamily signaling in erythroid maturation and identify ACE-536 as a new potential treatment for anemia, including that caused by ineffective erythropoiesis.

A novel therapeutic approach to treating obesity through modulation of TGFβ signaling

Obesity results from disproportionately high energy intake relative to energy expenditure. Many therapeutic strategies have focused on the intake side of the equation, including pharmaceutical targeting of appetite and digestion. An alternative approach is to increase energy expenditure through physical activity or adaptive thermogenesis. A pharmacological way to increase muscle mass and hence exercise capacity is through inhibition of the activin receptor type IIB (ActRIIB). Muscle mass and strength is regulated, at least in part, by growth factors that signal via ActRIIB. Administration of a soluble ActRIIB protein comprised of a form of the extracellular domain of ActRIIB fused to a human Fc (ActRIIB-Fc) results in a substantial muscle mass increase in normal mice. However, ActRIIB is also present on and mediates the action of growth factors in adipose tissue, although the function of this system is poorly understood. In the current study, we report the effect of ActRIIB-Fc to suppress diet-induced obesity and linked metabolic dysfunctions in mice fed a high-fat diet. ActRIIB-Fc induced a brown fat-like thermogenic gene program in epididymal white fat, as shown by robustly increased expression of the thermogenic genes uncoupling protein 1 and peroxisomal proliferator-activated receptor-γ coactivator 1α. Finally, we identified multiple ligands capable of reducing thermogenesis that represent likely target ligands for the ActRIIB-Fc effects on the white fat depots. These data demonstrate that novel therapeutic ActRIIB-Fc improves obesity and obesity-linked metabolic disease by both increasing skeletal muscle mass and by inducing a gene program of thermogenesis in the white adipose tissues.

Specificity and structure of a high affinity activin receptor-like kinase 1 (ALK1) signaling complex

Activin receptor-like kinase 1 (ALK1), an endothelial cell-specific type I receptor of the TGF-β superfamily, is an important regulator of normal blood vessel development as well as pathological tumor angiogenesis. As such, ALK1 is an important therapeutic target. Thus, several ALK1-directed agents are currently in clinical trials as anti-angiogenic cancer therapeutics. Given the biological and clinical importance of the ALK1 signaling pathway, we sought to elucidate the biophysical and structural basis underlying ALK1 signaling. The TGF-β family ligands BMP9 and BMP10 as well as the three type II TGF-β family receptors ActRIIA, ActRIIB, and BMPRII have been implicated in ALK1 signaling. Here, we provide a kinetic and thermodynamic analysis of BMP9 and BMP10 interactions with ALK1 and type II receptors. Our data show that BMP9 displays a significant discrimination in type II receptor binding, whereas BMP10 does not. We also report the crystal structure of a fully assembled ternary complex of BMP9 with the extracellular domains of ALK1 and ActRIIB. The structure reveals that the high specificity of ALK1 for BMP9/10 is determined by a novel orientation of ALK1 with respect to BMP9, which leads to a unique set of receptor-ligand interactions. In addition, the structure explains how BMP9 discriminates between low and high affinity type II receptors. Taken together, our findings provide structural and mechanistic insights into ALK1 signaling that could serve as a basis for novel anti-angiogenic therapies.

Soluble endoglin specifically binds bone morphogenetic proteins 9 and 10 via its orphan domain, inhibits blood vessel formation, and suppresses tumor growth

Endoglin (CD105), a transmembrane protein of the transforming growth factor β superfamily, plays a crucial role in angiogenesis. Mutations in endoglin result in the vascular defect known as hereditary hemorrhagic telangiectasia (HHT1). The soluble form of endoglin was suggested to contribute to the pathogenesis of preeclampsia. To obtain further insight into its function, we cloned, expressed, purified, and characterized the extracellular domain (ECD) of mouse and human endoglin fused to an immunoglobulin Fc domain. We found that mouse and human endoglin ECD-Fc bound directly, specifically, and with high affinity to bone morphogenetic proteins 9 and 10 (BMP9 and BMP10) in surface plasmon resonance (Biacore) and cell-based assays. We performed a function mapping analysis of the different domains of endoglin by examining their contributions to the selectivity and biological activity of the protein. The BMP9/BMP10 binding site was localized to the orphan domain of human endoglin composed of the amino acid sequence 26-359. We established that endoglin and type II receptors bind to overlapping sites on BMP9. In the in vivo chick chorioallantoic membrane assay, the mouse and the truncated human endoglin ECD-Fc both significantly reduced VEGF-induced vessel formation. Finally, murine endoglin ECD-Fc acted as an anti-angiogenic factor that decreased blood vessel sprouting in VEGF/FGF-induced angiogenesis in in vivo angioreactors and reduced the tumor burden in the colon-26 mouse tumor model. Together our findings indicate an important role of soluble endoglin ECD in the regulation of angiogenesis and highlight efficacy of endoglin-Fc as a potential anti-angiogenesis therapeutic agent.

A soluble activin receptor type IIb prevents the effects of androgen deprivation on body composition and bone health

Androgen deprivation, a consequence of hypogonadism, certain cancer treatments, or normal aging in men, leads to loss of muscle mass, increased adiposity, and osteoporosis. In the present study, using a soluble chimeric form of activin receptor type IIB (ActRIIB) we sought to offset the adverse effects of androgen deprivation on muscle, adipose tissue, and bone. Castrated (ORX) or sham-operated (SHAM) mice received either TBS [vehicle-treated (VEH)] or systemic administration of ActRIIB-mFc, a soluble fusion protein comprised of a form of the extracellular domain of ActRIIB fused to a murine IgG2aFc subunit. In vivo body composition imaging demonstrated that ActRIIB-mFc treatment results in increased lean tissue mass of 23% in SHAM mice [19.02 +/- 0.42 g (VEH) versus 23.43 +/- 0.35 g (ActRIIB-mFc), P < 0.00001] and 26% in ORX mice [15.59 +/- 0.26 g (VEH) versus 19.78 +/- 0.26 g (ActRIIB-mFc), P < 0.00001]. Treatment also caused a decrease in adiposity of 30% in SHAM mice [5.03 +/- 0.48 g (VEH) versus 3.53 +/- 0.19 g (ActRIIB-mFc), NS] and 36% in ORX mice [7.12 +/- 0.53 g (VEH) versus 4.57 +/- 0.28 g (ActRIIB-mFc), P < 0.001]. These changes were also accompanied by altered serum levels of leptin, adiponectin, and insulin, as well as by prevention of steatosis (fatty liver) in ActRIIB-mFc-treated ORX mice. Finally, ActRIIB-mFc prevented loss of bone mass in ORX mice as assessed by whole body dual x-ray absorptiometry and micro-computed tomography of proximal tibias. The data demonstrate that treatment with ActRIIB-mFc restored muscle mass, adiposity, and bone quality to normal levels in a mouse model of androgen deprivation, thereby alleviating multiple adverse consequences of such therapy.

Characterization of the ligand binding functionality of the extracellular domain of activin receptor type IIb

The single transmembrane domain serine/threonine kinase activin receptor type IIB (ActRIIB) has been proposed to bind key regulators of skeletal muscle mass development, including the ligands GDF-8 (myostatin) and GDF-11 (BMP-11). Here we provide a detailed kinetic characterization of ActRIIB binding to several low and high affinity ligands using a soluble activin receptor type IIB-Fc chimera (ActRIIB.Fc). We show that both GDF-8 and GDF-11 bind the extracellular domain of ActRIIB with affinities comparable with those of activin A, a known high affinity ActRIIB ligand, whereas BMP-2 and BMP-7 affinities for ActRIIB are at least 100-fold lower. Using site-directed mutagenesis, we demonstrate that ActRIIB binds GDF-11 and activin A in different ways such as, for example, substitutions in ActRIIB Leu(79) effectively abolish ActRIIB binding to activin A yet not to GDF-11. Native ActRIIB has four isoforms that differ in the length of the C-terminal portion of their extracellular domains. We demonstrate that the C terminus of the ActRIIB extracellular domain is crucial for maintaining biological activity of the ActRIIB.Fc receptor chimera. In addition, we show that glycosylation of ActRIIB is not required for binding to activin A or GDF-11. Together, our findings reveal binding specificity and activity determinants of the ActRIIB receptor that combine to effect specificity in the activation of distinct signaling pathways.

ALK1-Fc inhibits multiple mediators of angiogenesis and suppresses tumor growth

Activin receptor-like kinase-1 (ALK1) is a type I, endothelial cell-specific member of the transforming growth factor-beta superfamily of receptors known to play an essential role in modulating angiogenesis and vessel maintenance. In the present study, we sought to examine the angiogenic and tumorigenic effects mediated upon the inhibition of ALK1 signaling using a soluble chimeric protein (ALK1-Fc). Of 29 transforming growth factor-beta-related ligands screened by surface plasmon resonance, only bone morphogenetic protein (BMP9) and BMP10 displayed high-affinity binding to ALK1-Fc. In cell-based assays, ALK1-Fc inhibited BMP9-mediated Id-1 expression in human umbilical vein endothelial cells and inhibited cord formation by these cells on a Matrigel substrate. In a chick chorioallantoic membrane assay, ALK1-Fc reduced vascular endothelial growth factor-, fibroblast growth factor-, and BMP10-mediated vessel formation. The growth of B16 melanoma explants was also inhibited significantly by ALK1-Fc in this assay. Finally, ALK1-Fc treatment reduced tumor burden in mice receiving orthotopic grafts of MCF7 mammary adenocarcinoma cells. These data show the efficacy of chimeric ALK1-Fc proteins in mitigating vessel formation and support the view that ALK1-Fc is a powerful antiangiogenic agent capable of blocking vascularization.

Abstract B5: ACE-041, a soluble activin receptor-like kinase-1 fusion protein, is a novel antiangiogenic agent

The therapeutic targeting of angiogenesis has been associated with clinical benefit for a number of tumors including colorectal carcinoma, non-small cell lung cancer, breast cancer, renal cell carcinoma and glioblastoma. Activin receptor-like kinase-1, an endothelium specific member of the TGFβ-type I receptor family, is believed to play a role in modulating angiogenesis. Homozygous inactivation of Alk1 in mice leads to embryonic lethality resulting from severe vascular abnormalities, while heterozygous mice exhibit chronic hemorrhaging and arteriovenous malformation similar to hereditary hemorrhagic telangiectasia-2 (HHT-2) patients harboring mutations in ALK1. To examine the potential of ALK1 inhibition as a novel anti-angiogenic therapy for cancer, we developed a soluble form of the ALK1 receptor (ACE-041) by fusing the extracellular region of human ALK1 to the Fc of human IgG1. ACE-041 was shown to bind only BMP-9 and BMP-10 out of 29 ligands tested in TGFβ family. ALK1 inhibition blocks SMAD 1/5/8 signaling and blocks in vitro cord formation of endothelial cells. ACE-041 inhibited in vivo blood vessel formation using Matrigel plug assay and chick chorioallantoic assay (CAM). Moreover, ACE-041 decreased tumor volume in a modified tumor CAM assay and in an orthotopic breast cancer model induced by MDA-MD-231 and MCF-7 mammary adenocarcinoma cells. These data suggest that ALK1 inhibition may be a promising therapeutic strategy for patients with breast and other solid tumors and support advancement of ACE-041 into clinical trials.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B5.

Visualization of protein interactions in living cells using bimolecular fluorescence complementation (BiFC) analysis

Protein interactions integrate stimuli from different signaling pathways and developmental programs. Bimolecular fluorescence complementation (BiFC) analysis has been developed for visualization of protein interactions in living cells. This approach is based on complementation between two fragments of a fluorescent protein when they are brought together by an interaction between proteins fused to the fragments, and it enables visualization of the subcellular locations of protein interactions in the normal cellular environment. It can be used for the analysis of many protein interactions and does not require information about the structures of the interaction partners. A multicolor BiFC approach has been developed for simultaneous visualization of interactions with multiple alternative partners in the same cell, based on complementation between fragments of engineered fluorescent proteins that produce bimolecular fluorescent complexes with distinct spectral characteristics. This enables comparison of subcellular distributions of different protein complexes in the same cell and allows analysis of competition between mutually exclusive interaction partners.

Visualization of protein interactions in living cells using bimolecular fluorescence complementation (BiFC) analysis

Protein interactions integrate stimuli from different signaling pathways and developmental programs. Bimolecular fluorescence complementation (BiFC) analysis has been developed for visualization of protein interactions in living cells. This approach is based on complementation between two fragments of a fluorescent protein when they are brought together by an interaction between proteins fused to the fragments, and it enables visualization of the subcellular locations of protein interactions in the normal cellular environment. It can be used for the analysis of many protein interactions and does not require information about the structures of the interaction partners. A multicolor BiFC approach has been developed for simultaneous visualization of interactions with multiple alternative partners in the same cell, based on complementation between fragments of engineered fluorescent proteins that produce bimolecular fluorescent complexes with distinct spectral characteristics. This enables comparison of subcellular distributions of different protein complexes in the same cell and allows analysis of competition between mutually exclusive interaction partners.

Visualization of Myc/Max/Mad family dimers and the competition for dimerization in living cells

Myc and Mad family proteins play opposing roles in the control of cell growth and proliferation. We have visualized the subcellular locations of complexes formed by Myc/Max/Mad family proteins using bimolecular fluorescence complementation (BiFC) analysis. Max was recruited to different subnuclear locations by interactions with Myc versus Mad family members. Complexes formed by Max with Mxi1, Mad3, or Mad4 were enriched in nuclear foci, whereas complexes formed with Myc were more uniformly distributed in the nucleoplasm. Mad4 was localized to the cytoplasm when it was expressed separately, and Mad4 was recruited to the nucleus through dimerization with Max. The cytoplasmic localization of Mad4 was determined by a CRM1-dependent nuclear export signal located near the amino terminus. We compared the relative efficiencies of complex formation among Myc, Max, and Mad family proteins in living cells using multicolor BiFC analysis. Max formed heterodimers with the basic helix-loop-helix leucine zipper (bHLHZIP) domain of Myc (bMyc) more efficiently than it formed homodimers. Replacement of two amino acid residues in the leucine zipper of Max reversed the relative efficiencies of homo- and heterodimerization in cells. Surprisingly, Mad3 formed complexes with Max less efficiently than bMyc, whereas Mad4 formed complexes with Max more efficiently than bMyc. The distinct subcellular locations and the differences between the efficiencies of dimerization with Max indicate that Mad3 and Mad4 are likely to modulate transcription activation by Myc at least in part through distinct mechanisms.

Both Max and TFE3 cooperate with Smad proteins to bind the plasminogen activator inhibitor-1 promoter, but they have opposite effects on transcriptional activity

Transforming growth factor (TGF)-beta regulates gene expression in large part through combinatorial interactions between members of the Smad family and other transcription factors. The basic helix-loop-helix leucine zipper (bHLHZIP) protein TFE3 and Smad3 synergistically activate transcription of the plasminogen activator inhibitor-1 (PAI-1) as well as other genes. We investigated interactions among different bHLHZIP and Smad family proteins. TFE3, TFEB, and Max associated with Smad3 and Smad4 in the absence of DNA and at the PE2.1 element of the PAI-1 promoter. These interactions were mediated by the leucine zipper and MH1 regions of the respective proteins. No interactions were observed with the E47 bHLH family protein. Chimeric proteins, in which leucine zippers from bHLHZIP or bZIP proteins were fused to heterologous bHLH domains, associated with Smad proteins both in the absence of DNA and at the PE2.1 element. The kinetics of bHLHZIP and Smad protein binding at the PE2.1 element were examined using surface plasmon resonance analysis. TFE3 exhibited cooperative DNA binding with Smad proteins, whereas no cooperativity was observed between E47 and Smads. Max inhibited transcription activation by Smad3 and TGF-beta at the PAI-1 promoter, whereas TFE3 and TFEB stimulated transcription activation. These results suggest that Smad family proteins can interact with several bHLHZIP proteins, resulting in different transcriptional outcomes.

Contribution of a salt bridge to the thermostability of adrenodoxin determined by site-directed mutagenesis

We identified a unique conserved salt bridge Arg89-Glu74 inside the protein core of adrenodoxin, which ensures proper orientation between the [2Fe-2S] cluster-containing domain and the recognition helix. Incorporation and geometry of the redox center were essentially preserved in the mutants E74D, R89A, and R89K as judged by EPR spectroscopy. However, absorption and CD spectra pointed out essential conformational changes in the protein vicinity of the [2Fe-2S] cluster. Judged by essentially increased K(m) and K(d) values and changed redox properties, mutations resulted in displacement of the recognition helix and hindered proper docking of the protein with both adrenodoxin reductase and CYP11A1. Substitutions of Arg89 and Glu74 induce thermodynamic destabilization attested by dramatically decreased unfolding temperature (T(d)) and enthalpy (Delta(d)H(T(d))). The heat capacity change of denaturation (Delta(d)C(p)) was significantly decreased for the mutants, suggesting that parts of the polypeptide chain normally hidden inside the protein core are exposed to the solvent in these variants.

The thermal unfolding and domain structure of Na+/K+-exchanging ATPase. A scanning calorimetry study

The thermal unfolding and domain structure of Na+/K+-ATPase from pig kidney were studied by high-sensitivity differential scanning calorimetry (HS-DSC). The excess heat capacity function of Na+/K+-ATPase displays the unfolding of three cooperative domains with midpoint transition temperatures (Td) of 320.6, 327.5, 331.5 K, respectively. The domain with Td = 327.5 K was identified as corresponding to the beta subunit, while two other domains belong to the alpha subunit. The thermal unfolding of the low-temperature domain leads to large changes in the amplitude of the short-circuit current, but has no effect on the ATP hydrolysing activity. Furthermore, dithiothreitol or 2-mercaptoethanol treatment causes destruction of this domain, accompanied by significant disruption of the ion transporting function and a 25% loss of ATPase activity. The observed total unfolding enthalpy of the protein is rather low (approximately 12 J.g-1), suggesting that thermal denaturation of Na+/K+-ATPase does not lead to complete unfolding of the entire molecule. Presumably, transmembrane segments retain most of their secondary structure upon thermal denaturation. The binding of physiological ligands results in a pronounced increase in the conformational stability of both enzyme subunits.

DNA sequence-dependent folding determines the divergence in binding specificities between Maf and other bZIP proteins

Maf family transcription factors are atypical basic region-leucine zipper (bZIP) proteins that contain a variant basic region and an ancillary DNA-binding region. These proteins recognize extended DNA sequence elements flanking the core recognition element bound by canonical bZIP proteins. We have investigated the causes for the differences in DNA recognition between Maf and other bZIP family proteins through studies of Maf secondary structure, trypsin sensitivity, binding affinity, dissociation rate and DNA contacts. Our results show that specific DNA binding by Maf is coupled to a conformational change involving both the basic and ancillary DNA-binding regions that depends on the extended DNA sequence elements. Two basic region amino acid residues that differ between Maf and canonical bZIP proteins facilitate the conformational change required for Maf recognition of the extended elements. Nucleotide base contacts made by Maf differ from those made by canonical bZIP proteins. Taken together, our results suggest that the unusual DNA binding specificity of Maf family proteins is mediated by concerted folding of structurally unrelated DNA recognition motifs.

Adrenodoxin: structure, stability, and electron transfer properties

Adrenodoxin is an iron-sulfur protein that belongs to the broad family of the [2Fe-2S]-type ferredoxins found in plants, animals and bacteria. Its primary function as a soluble electron carrier between the NADPH-dependent adrenodoxin reductase and several cytochromes P450 makes it an irreplaceable component of the steroid hormones biosynthesis in the adrenal mitochondria of vertebrates. This review intends to summarize current knowledge about structure, function, and biochemical behavior of this electron transferring protein. We discuss the recently solved first crystal structure of the vertebrate-type ferredoxin, the truncated adrenodoxin Adx(4-108), that offers the unique opportunity for better understanding of the structure-function relationships and stabilization of this protein, as well as of the molecular architecture of [2Fe-2S] ferredoxins in general. The aim of this review is also to discuss molecular requirements for the formation of the electron transfer complex. Essential comparison between bacterial putidaredoxin and mammalian adrenodoxin will be provided. These proteins have similar tertiary structure, but show remarkable specificity for interactions only with their own cognate cytochrome P450. The discussion will be largely centered on the protein-protein recognition and kinetics of adrenodoxin dependent reactions.

Effect of replacing a conserved proline residue on the function and stability of bovine adrenodoxin

A proline residue in the C-terminal part of the polypeptide chain is highly conserved among many [2Fe-2S] ferredoxins. To investigate the requirement for proline at this position, we constructed steric (4-108W), charged (4-108K), polar (4-108S) and non-polar (4-108A) truncated mutants of adrenodoxin and studied them for biological function and stability. Although the variants were expressed in Escherichia coli with a significantly lower yield compared with wild-type adrenodoxin, successful incorporation of the iron-sulfur cluster suggested their proper folding. Similar absorption, CD and EPR spectra indicated that the cluster environment was not affected by the mutations. No evidence for an essential role of Pro108 in determining the redox potential of adrenodoxin or its interactions with the redox partners was found. However, replacement of this residue results in a dramatic decrease in the overall protein stability. The differences in the Gibbs energy of unfolding at 37 degrees C, delta[delta(d)G(37 degrees C)], are -5.0, -7.8, -10.1 and -10.7 kJ/mol for 4-108A, 4-108S, 4-108W and 4-108K mutants, respectively, compared with 4-108P as a control. We conclude that the principle function of Pro108 is to stabilize adrenodoxin threefold: (i) through limitation of the conformation of the polypeptide chain in this region, (ii) through a hydrogen bond to Arg14 and (iii) favorable hydrophobic contacts.