The erythropoietin receptor transmembrane domain mediates complex formation with viral anemic and polycythemic gp55 proteins

Discovering immunoreceptor coupling and organization motifs

The recently determined cryo-EM structures of the T cell antigen receptor (TCR) and B cell antigen receptor (BCR) show in molecular details the interactions of the ligand-binding part with the signaling subunits but they do not reveal the signaling mechanism of these antigen receptors. Without knowing the molecular basis of antigen sensing by these receptors, a rational design of optimal vaccines is not possible. The existence of conserved amino acids (AAs) that are not involved in the subunit interaction suggests that antigen receptors form higher complexes and/or have lateral interactors that control their activity. Here, I describe evolutionary conserved leucine zipper (LZ) motifs within the transmembrane domains (TMD) of antigen and coreceptor components that are likely to be involved in the oligomerization and lateral interaction of antigen receptor complexes on T and B cells. These immunoreceptor coupling and organization motifs (ICOMs) are also found within the TMDs of other important receptor types and viral envelope proteins. This discovery suggests that antigen receptors do not function as isolated entities but rather as part of an ICOM-based interactome that controls their nanoscale organization on resting cells and their dynamic remodeling on activated lymphocytes.

Topological control of cytokine receptor signaling induces differential effects in hematopoiesis

Although tunable signaling by G protein-coupled receptors can be exploited through medicinal chemistry, a comparable pharmacological approach has been lacking for the modulation of signaling through dimeric receptors, such as those for cytokines. We present a strategy to modulate cytokine receptor signaling output by use of a series of designed C2-symmetric cytokine mimetics, based on the designed ankyrin repeat protein (DARPin) scaffold, that can systematically control erythropoietin receptor (EpoR) dimerization orientation and distance between monomers. We sampled a range of EpoR geometries by varying intermonomer angle and distance, corroborated by several ligand-EpoR complex crystal structures. Across the range, we observed full, partial, and biased agonism as well as stage-selective effects on hematopoiesis. This surrogate ligand strategy opens access to pharmacological modulation of therapeutically important cytokine and growth factor receptor systems.

Characterization of erythropoietin and hepcidin in the regulation of persistent injury-associated anemia

BACKGROUND

The cause of persistent injury-associated anemia is multifactorial and includes acute blood loss, an altered erythropoietin (EPO) response, dysregulation of iron homeostasis, and impaired erythropoiesis in the setting of chronic inflammation/stress. Hepcidin plays a key role in iron homeostasis and is regulated by anemia and inflammation. Erythropoietin is a main regulator of erythropoiesis induced by hypoxia. A unique rodent model of combined lung injury (LC)/hemorrhagic shock (HS) (LCHS)/chronic restraint stress (CS) was used to produce persistent injury-associated anemia to further investigate the roles of EPO, hepcidin, iron, ferritin, and the expression of EPO receptors (EPOr).

METHODS

Male Sprague-Dawley rats were randomly assigned into one of the four groups of rodent models: naive, CS alone, combined LCHS, or LCHS/CS. Plasma was used to evaluate levels of EPO, hepcidin, iron, and ferritin. RNA was isolated from bone marrow and lung tissue to evaluate expression of EPOr. Comparisons between models were performed by t tests followed by one-way analysis of variance.

RESULTS

After 7 days, only LCHS/CS was associated with persistent anemia despite significant elevation of plasma EPO. Combined LCHS and LCHS/CS led to a persistent decrease in EPOr expression in bone marrow on Day 7. The LCHS/CS significantly decreased plasma hepcidin levels by 75% on Day 1 and 84% on Day 7 compared to LCHS alone. Hepcidin plasma levels are inversely proportional to EPO plasma levels (Pearson R = -0.362, p < 0.05).

CONCLUSION

Tissue injury, hemorrhagic shock, and stress stimulate and maintain high levels of plasma EPO while hepcidin levels are decreased. In addition, bone marrow EPOr and plasma iron availability are significantly reduced following LCHS/CS. The combined deficit of reduced iron availability and reduced bone marrow EPOr expression may play a key role in the ineffective EPO response associated with persistent injury-associated anemia.

Screening for transmembrane association in divisome proteins using TOXGREEN, a high-throughput variant of the TOXCAT assay

TOXCAT is a widely used genetic assay to study interactions of transmembrane helices within the inner membrane of the bacterium Escherichia coli. TOXCAT is based on a fusion construct that links a transmembrane domain of interest with a cytoplasmic DNA-binding domain from the Vibrio cholerae ToxR protein. Interaction driven by the transmembrane domain results in dimerization of the ToxR domain, which, in turn, activates the expression of the reporter gene chloramphenicol acetyl transferase (CAT). Quantification of CAT is used as a measure of the ability of the transmembrane domain to self-associate. Because the quantification of CAT is relatively laborious, we developed a high-throughput variant of the assay, TOXGREEN, based on the expression of super-folded GFP and detection of fluorescence directly in unprocessed cell cultures. Careful side-by-side comparison of TOXCAT and TOXGREEN demonstrates that the methods have comparable response, dynamic range, sensitivity and intrinsic variability both in LB and minimal media. The greatly enhanced workflow makes TOXGREEN much more scalable and ideal for screening, since hundreds of constructs can be rapidly assessed in 96 well plates. Even for small scale investigations, TOXGREEN significantly reduces time, labor and cost associated with the procedure. We demonstrate applicability with a large screening for self-association among the transmembrane domains of bitopic proteins of the divisome (FtsL, FtsB, FtsQ, FtsI, FtsN, ZipA and EzrA) belonging to 11 bacterial species. The analysis confirms a previously reported tendency for FtsB to self-associate, and suggests that the transmembrane domains of ZipA, EzrA and FtsN may also possibly oligomerize.

Synthesis, purification, and characterization of single helix membrane peptides and proteins for NMR spectroscopy

Membrane proteins function as receptors, channels, transporters, and enzymes. These proteins are generally difficult to express and purify in a functional form due to the hydrophobic nature of their membrane spanning sequences. Studies on membrane proteins with a single membrane spanning helix have been particularly challenging. Single-pass membrane proteins will often form dimers or higher order oligomers in cell membranes as a result of sequence motifs that mediate specific transmembrane helix interactions. Understanding the structural basis for helix association provides insights into how these proteins function. Nevertheless, nonspecific association or aggregation of hydrophobic membrane spanning sequences can occur when isolated transmembrane domains are reconstituted into membrane bilayers or solubilized into detergent micelles for structural studies by solid-state or solution NMR spectroscopy. Here, we outline the methods used to synthesize, purify, and characterize single transmembrane segments for structural studies. Two synthetic strategies are discussed. The first strategy is to express hydrophobic peptides as protein chimera attached to the maltose binding protein. The second strategy is by direct chemical synthesis. Purification is carried out by several complementary chromatography methods. The peptides are solubilized in detergent for solution NMR studies or reconstituted into model membranes for solid-state NMR studies. We describe the methods used to characterize the reconstitution of these systems prior to NMR structural studies to establish if there is nonspecific aggregation.

Molecular determinants and thermodynamics of the amyloid precursor protein transmembrane domain implicated in Alzheimer's disease

The deposition of toxic amyloid-β (Aβ) peptide aggregates in the brain is a hallmark of Alzheimer's disease. The intramembrane proteolysis by γ-secretase of the amyloid precursor protein β-carboxy-terminal fragment (APP-βCTF) constitutes the final step in the production of Aβ peptides. Mounting evidence suggests that APP-βCTF is a transmembrane domain (TMD) dimer, and that dimerization might modulate the production of Aβ species that are prone to aggregation and are therefore most toxic. We combined experimental and computational approaches to study the molecular determinants and thermodynamics of APP-βCTF dimerization, and we produced a unifying structural model that reconciles much of the published data. Using a cell assay that exploits a dimerization-dependent activator of transcription, we identified specific dimerization-affecting mutations located mostly at the N-terminus of the TMD of APP-βCTF. The ability of selected mutants to affect the dimerization of full-length APP-βCTF was confirmed by fluorescence resonance energy transfer experiments. Free-energy estimates of the wild type and mutants of the TMD of APP-βCTF derived from enhanced molecular dynamics simulations showed that the dimeric state is composed of different arrangements, in which either (709)GXXXA(713) or (700)GXXXG(704)GXXXG(708) interaction motifs can engage in symmetric or asymmetric associations. Mutations along the TMD of APP-βCTF were found to modulate the relative free energy of the dimeric configurations and to differently affect the distribution of interfaces within the dimeric state. This observation might have important biological implications, since dimers with a different arrangement of the transmembrane helices are likely to be recognized differently by γ-secretase and to lead to a variation in Aβ levels.

Friend Spleen Focus-Forming Virus Activates the Tyrosine Kinase sf-Stk and the Transcription Factor PU.1 to Cause a Multi-Stage Erythroleukemia in Mice

Hematological malignancies in humans typically involve two types of genetic changes: those that promote hematopoietic cell proliferation and survival (often the result of activation of tyrosine kinases) and those that impair hematopoietic cell differentiation (often the result of changes in transcription factors). The multi-stage erythroleukemia induced in mice by Friend spleen focus-forming virus (SFFV) is an excellent animal model for studying the molecular basis for both of these changes. Significant progress has been made in understanding the molecular basis for the multi-stage erythroleukemia induced by Friend SFFV. In the first stage of leukemia, the envelope protein encoded by SFFV interacts with and activates the erythropoietin (Epo) receptor and the receptor tyrosine kinase sf-Stk in erythroid cells, causing their Epo-independent proliferation, differentiation and survival. In the second stage, SFFV integration into the Sfpi1 locus activates the myeloid transcription factor PU.1, blocking erythroid cell differentiation, and in conjunction with the loss of p53 tumor suppressor activity, results in the outgrowth of malignant cells. In this review, we discuss the current level of understanding of how SFFV alters the growth and differentiation of erythroid cells and results in the development of erythroleukemia. Our knowledge of how SFFV causes erythroleukemia in mice may give us clues as to how the highly related human retrovirus XMRV causes malignancies in humans.

The role of individual amino acids in the dimerization of CR4 and ACR4 transmembrane domains

CRINKLY4 is a growth factor-like plant receptor kinase designated as CR4 in Zea mays and ACR4 in Arabidopsis. Using the TOXCAT system, a genetic assay that measures helix interactions in a natural membrane environment, we have previously demonstrated that the dimerization potential of the ACR4 transmembrane (TM) domain is significantly weaker than that of CR4 TM domain, even though 13 of the 24 residues are identical. Neither of the TM domains contain the GxxxG motif that has been shown to be important for the dimerization of the TM segments of several receptors. To further investigate the relationship between protein sequence and dimerization potential, we (a) mutated each of the 11 differing residues in the CR4 TM domain to the corresponding residue of ACR4 (b) made reciprocal mutations in ACR4 and (c) made hybrids consisting of half CR4 and half ACR4 TM domains. Our results suggest that most mutations in ACR4 or CR4 TM domains have low to moderate effects on the dimerization potential and that residues in the N-terminal half of the CR4 TM domain are important for dimerization.

Activation of the N-terminally truncated form of the Stk receptor tyrosine kinase Sf-Stk by Friend virus-encoded gp55 is mediated by cysteine residues in the ecotropic domain of gp55 and the extracellular domain of Sf-Stk

Friend virus induces an erythroleukemia in susceptible mice that is initiated by the interaction of the Friend virus-encoded glycoprotein gp55 with the erythropoietin (Epo) receptor and the product of the host Fv2 gene, a naturally occurring truncated form of the Stk receptor tyrosine kinase (Sf-Stk). We have previously demonstrated that the activation of Sf-Stk, recruitment of a Grb2/Gab2/Stat3 signaling complex, and induction of Pu.1 expression by Stat3 are required for the development of the early stage of Friend disease both in vitro and in vivo. Here we demonstrate that the interaction of gp55 with Sf-Stk is dependent on cysteine residues in the ecotropic domain of gp55 and the extracellular domain of Sf-Stk. Point mutation of these cysteine residues or deletion of these domains inhibits the ability of gp55 to interact with Sf-Stk, resulting in the inability of these proteins to promote the Epo-independent growth of erythroid progenitor cells. We also demonstrate that the interaction of gp55 with Sf-Stk does not promote dimerization of Sf-Stk but results in enhanced phosphorylation of Sf-Stk and the relocalization of Sf-Stk from the cytosol to the plasma membrane. Finally, we demonstrate that a constitutively active form of Sf-Stk (Sf-StkM330T), as well as its human counterpart, Sf-Ron, promotes Epo-independent colony formation in the absence of gp55 and that this response is also dependent on the cysteines in the extracellular domains of Sf-StkM330T and Sf-Ron. These data suggest that the cysteines in the extracellular domains of Sf-Stk and Sf-Ron may also mediate the interaction of these truncated receptors with other cellular factors that regulate their ability to promote cytokine-independent growth.

Stat3 promotes the development of erythroleukemia by inducing Pu.1 expression and inhibiting erythroid differentiation

Leukemogenesis requires two classes of mutations, one that promotes proliferation and one that blocks differentiation. The erythroleukemia induced by Friend virus is a multistage disease characterized by an early proliferative stage driven by the interaction of the viral glycoprotein, gp55, with Sf-Stk and the EpoR, and a late block to differentiation resulting from retroviral insertion in the Pu.1 locus. We demonstrate here that activation of Stat3 by Sf-Stk in the early stage of disease is essential for the progression of erythroleukemia in the presence of differentiation signals induced by the EpoR, but is dispensable in the late stages of the disease. Furthermore, we identify Pu.1 as a Stat3 target gene in the early stages of erythroleukemia development. Our results support a model whereby the activation of Stat3 in the early stage of disease plays a pivotal role in regulating differentiation through the upregulation of Pu.1, thus inhibiting differentiation and favoring the expansion of infected erythroblasts and enhancing the pool of progenitors available for the acquisition of additional mutations, including insertional activation of Pu.1, resulting in full leukemic transformation.

Sequences of polycythemia-type Friend spleen focus-forming virus in clone-745-derived mouse erythroleukemia cells

Friend leukemia virus complex consists of a replication-competent virus plus one of two replication-incompetent viruses, spleen focus-forming virus anemia virus or spleen focus-forming virus polycythemia virus. The replication-incompetent viruses induce rapid malignant transformation of erythroid precursor cells. Transformed cell lines from mice infected with the complex can be induced to undergo erythrodifferentiation in vitro. However, lines containing the anemia-type virus require erythropoietin and another agent such as dimethyl sulfoxide for optimal erythrodifferentiation, whereas those containing the polycythemia-type virus do not require or respond to erythropoietin. Mice infected with the original Friend virus isolates were anemic, so sub-lines derived from these mice should be erythropoietin-dependent for induction of erythrodifferentiation. However, many of the widely studied sub-lines are erythropoietin-independent. In order to clarify this apparent anomaly, the genomes of viruses present in two commonly used erythropoietin-independent sub-lines were sequenced. Sequence analysis demonstrates that they contain the polycythemia-type virus and not the anemia-type virus.

The bovine papillomavirus E5 protein and the PDGF beta receptor: it takes two to tango

The extremely hydrophobic, 44-amino acid bovine papillomavirus (BPV) E5 protein is the smallest known oncoprotein, which orchestrates cell transformation by causing ligand-independent activation of a cellular receptor tyrosine kinase, the platelet-derived growth factor beta receptor (PDGFbetaR). The E5 protein forms a dimer in transformed cells and is essentially an isolated membrane-spanning segment that binds directly to the transmembrane domain of the PDGFbetaR, inducing receptor dimerization, autophosphorylation, and sustained mitogenic signaling. There are few sequence constraints for activity as long as the overall hydrophobicity of the E5 protein and its ability to dimerize are preserved. Nevertheless, the E5 protein is highly specific for the PDGFbetaR and does not activate other cellular proteins. Genetic screens of thousands of small, artificial hydrophobic proteins with randomized transmembrane domains inserted into an E5 scaffold identified proteins with diverse transmembrane sequences that activate the PDGFbetaR, including some activators as small as 32-amino acids. Analysis of these novel proteins has provided new insight into the requirements for PDGFbetaR activation and specific transmembrane recognition in general. These results suggest that small, transmembrane proteins can be constructed and selected that specifically bind to other cellular or viral transmembrane target proteins. By using this approach, we have isolated a 44-amino acid artificial transmembrane protein that appears to activate the human erythropoietin receptor. Studies of the tiny, hydrophobic BPV E5 protein have not only revealed a novel mechanism of viral oncogenesis, but have also suggested that it may be possible to develop artificial small proteins that specifically modulate much larger target proteins by acting within cellular or viral membranes.

Multi-stage Friend murine erythroleukemia: molecular insights into oncogenic cooperation

The Friend virus SFFV (Spleen Focus Forming Virus) provokes an acute erythroblastosis in susceptible strains of mice that progresses to overt erythroleukemia by a multi-step process. For virologists, the Friend virus-induced disease has provided deep insights into the host mechanisms influencing susceptibility to retroviral infection and viremia. These insights have contributed to the understanding of HIV and other human retroviral infections. For cell biologists and oncologists, this leukemia has been a powerful experimental model to identify critical oncogenes involved in a multi-stage process, to understand the contribution of host genes to cancer development, and to investigate the mechanisms leading to cell growth autonomy. This model also provided an example of oncogenic reversion since Friend tumor cells can reinitiate their erythroid differentiation program when exposed in vitro to some chemical inducers. This review highlights recent findings demonstrating that the leukemic progression depends on the cooperation of at least two oncogenic events, one interfering with differentiation and one conferring a proliferative advantage. The Friend model of leukemia progression recapitulates the two phases of human acute myeloid leukemia (AML). Coupling of insights from studies on the Friend erythroleukemia with knowledge on AML might allow a better understanding of the molecular mechanisms involved in the evolution of leukemia in mice and men.

Ligand-independent homomeric and heteromeric complexes between interleukin-2 or -9 receptor subunits and the gamma chain

Signaling via interleukin-2 (IL-2) and interleukin-9 receptors (IL-2R and IL-9R) involves heteromeric interactions between specific interleukin receptor subunits, which bind Janus kinase 1 (JAK1) and the JAK3 binding common gamma chain (gamma c). The potential existence and roles of homomeric and heteromeric complexes before ligand binding and their modulation by ligand and JAK3 are unclear. Using computerized antibody-mediated immunofluorescence co-patching of epitope-tagged receptors at the surface of live cells, we demonstrate that IL-2Rbeta, IL-9Ralpha, and gamma c each display a significant fraction of ligand-independent homomeric complexes (24-28% co-patching), whereas control co-patching levels with unrelated receptors are very low (7%). Heteromeric complex formation of IL2-Rbeta or IL-9Ralpha with gamma c is also observed in the absence of ligand (15-30%). Ligand binding increases this hetero-oligomerization 2-fold but does not affect homo-oligomerization. Co-expression of IL-2Ralpha does not affect the hetero-oligomerization of IL-2Rbeta and gamma c. Recruitment of gamma c into heterocomplexes is partly at the expense of its homo-oligomerization, suggesting that a functional role of the latter may be to keep the receptors inactive in the absence of ligand. At the same time, the preformed complexes between gamma c and IL-2Rbeta or IL-9Ralpha promote signaling by the JAK3 A572V mutant without ligand, supporting a pathophysiological role for the constitutive oligomerization in triggering ligand-independent activation of JAK3 (and perhaps other JAK mutants) mutants identified in several human cancers.

Aromatic and Cation−π Interactions Enhance Helix−Helix Association in a Membrane Environment

The cation-pi interaction is an electrostatic attraction between a positive charge and the conjugated pi electrons of an aromatic ring. These interactions are well documented in soluble proteins and can be both structurally and functionally important. Catalyzed by observations in our laboratory that an Ala- and Ile-rich two-helix transmembrane segment tended to form SDS-resistant dimers upon the incorporation of suitably located Trp residues, here we have constructed a library of related constructs to study systematically the impact of aromatic-aromatic and cation-pi interactions on tertiary structure formation within an Escherichia coli membrane. Using the TOXCAT oligomerization assay with the hydrophobic segment AIAIAIIAZAXAIIAIAIAI, where Z = A, W, Y, or F and X = A, H, R, or K in all possible combinations of cation and/or aromatic pairings, to assess the TM-TM dependent expression of the chloramphenicol acetyltransferase reporter gene, we find that cation-pi interactions, particularly between Lys and Trp, Tyr, or Phe, as well as weakly polar interactions between pairs of aromatic residues, significantly enhance the strength of oligomerization of these hydrophobic helices, in some instances forming oligomers four times stronger than the high-affinity glycophorin A dimer. The contribution of these forces to the tertiary structure formation in designed transmembrane segments suggests that similar forces may also be a significant factor in the folding and stability of native membrane proteins.

The position of the Gly-xxx-Gly motif in transmembrane segments modulates dimer affinityThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

Although the intrinsic low solubility of membrane proteins presents challenges to their high-resolution structure determination, insight into the amino acid sequence features and forces that stabilize their folds has been provided through study of sequence-dependent helix–helix interactions between single transmembrane (TM) helices. While the stability of helix–helix partnerships mediated by the Gly-xxx-Gly (GG4) motif is known to be generally modulated by distal interfacial residues, it has not been established whether the position of this motif, with respect to the ends of a given TM segment, affects dimer affinity. Here we examine the relationship between motif position and affinity in the homodimers of 2 single-spanning membrane protein TM sequences: glycophorin A (GpA) and bacteriophage M13 coat protein (MCP). Using the TOXCAT assay for dimer affinity on a series of GpA and MCP TM segments that have been modified with either 4 Leu residues at each end or with 8 Leu residues at the N-terminal end, we show that in each protein, centrally located GG4 motifs are capable of stronger helix–helix interactions than those proximal to TM helix ends, even when surrounding interfacial residues are maintained. The relative importance of GG4 motifs in stabilizing helix–helix interactions therefore must be considered not only in its specific residue context but also in terms of the location of the interactive surface relative to the N and C termini of α-helical TM segments.

Lipid Solvation Effects Contribute to the Affinity of Gly-xxx-Gly Motif-Mediated Helix−Helix Interactions†

Interactions between transmembrane helices are mediated by the concave Gly-xxx-Gly motif surface. Whether Gly residues per se are sufficient for selection of this motif has not been established. Here, we used the in vivo TOXCAT assay to measure the relative affinities of all 18 combinations of Gly, Ala, and Ser "small-xxx-small" mutations in glycophorin A (GpA) and bacteriophage M13 major coat protein (MCP) homodimers. Affinity values were compared with the accessibility to a methylene-sized probe of the total surface area of each helix monomer as a measure of solvation by membrane components. A strong inverse correlation was found between nonpolar-group lipid accessibility and dimer affinity (R = 0.75 for GpA, p = 0.013, and R = 0.81 for MCP, p = 0.004), suggesting that lipid as a poor membrane protein solvent, conceptually analogous to water in soluble protein folding, can contribute to dimer stability and help to define helix-helix interfaces.

Modulation of cell function by small transmembrane proteins modeled on the bovine papillomavirus E5 protein

Viruses have been subjected to intense study because of their medical importance and because they can provide fundamental insights into normal and pathological cellular processes. Indeed, much of our knowledge about basic cellular biology and biochemistry was acquired through the study of viruses, and some of medicine's greatest triumphs and challenges involve viruses. Since viruses have evolved to exploit important cell processes, they can provide tools and approaches to manipulate cell function. The small transmembrane E5 protein of bovine papillomavirus type 1 transforms cells by a unique mechanism involving ligand-independent activation of the platelet-derived growth factor beta receptor. Experiments summarized in this review suggest that it may be possible to use the E5 protein as a model to design an entirely new class of small, modular transmembrane proteins with novel biological activities.

Friend spleen focus-forming virus transforms rodent fibroblasts in cooperation with a short form of the receptor tyrosine kinase Stk

Friend spleen focus-forming virus (SFFV) causes rapid erythroleukemia in mice due to expression of its unique envelope glycoprotein, gp55. Erythroid cells expressing SFFV gp55 proliferate in the absence of their normal regulator erythropoietin (Epo) because of constitutive activation of Epo signal transduction pathways. Although SFFV infects many cell types, deregulation of cell growth occurs only when SFFV infects erythroid cells, suggesting that these cells express unique proteins that the virus requires to mediate its biological effects. Not only do erythroid cells express the Epo receptor (EpoR), but those from mice susceptible to SFFV-induced erythroleukemia also express a short form of the receptor tyrosine kinase Stk (sf-Stk). In erythroid cells, SFFV gp55 interacts with the EpoR complex and sf-Stk, leading to activation of the kinase and constitutive activation of signal transducing molecules. In this study, we demonstrate that SFFV gp55 can also deregulate the growth of nonerythroid cells when it is coexpressed with sf-Stk. Expression of SFFV gp55 in rodent fibroblasts engineered to express sf-Stk induced their transformation, as demonstrated by focus formation and anchorage-independent growth in vitro. This transformation by SFFV gp55 requires the kinase activity of sf-Stk and the presence of its extracellular domain but not expression of the EpoR or the tyrosine kinase Jak2, which is required for activation of signal transduction pathways through the EpoR. Thus, expression of SFFV gp55 in nonerythroid cells coexpressing sf-Stk results in their uncontrolled growth, demonstrating a previously unrecognized mechanism for retrovirus transformation of rodent fibroblasts and providing insight into SFFV-induced disease.

Helix packing and orientation in the transmembrane dimer of gp55-P of the spleen focus forming virus

gp55-P is a dimeric membrane protein with a single transmembrane helix that is coded by the env gene of the polycythemic strain of the spleen focus forming virus. gp55-P activates the erythropoietin (Epo) receptor through specific transmembrane helix interactions, leading to Epo-independent growth of erythroid progenitors and eventually promoting erythroleukemia. We describe the use of magic angle spinning deuterium NMR to establish the structure of the transmembrane dimer of gp55-P in model membranes. Comparison of the deuterium lineshapes of leucines in the center (Leu(396-399)) and at the ends (Leu(385), Leu(407)) of the transmembrane sequence shows that gp55-P has a right-handed crossing angle with Leu(399) packed in the dimer interface. We discuss the implications of the structure of the gp55-P transmembrane dimer for activation of the Epo receptor.