env genes of avian retroviruses: nucleotide sequence and molecular recombinants define host range determinants

Genetic mapping of the cloned subgroup A avian sarcoma and leukosis virus receptor gene to the TVA locus

A chicken gene conferring susceptibility to subgroup A avian sarcoma and leukosis viruses (ASLV-A) was recently identified by a gene transfer strategy. Classical genetic approaches had previously identified a locus, TVA, that controls susceptibility to ASLV-A. Using restriction fragment length polymorphism (RFLP) mapping in inbred susceptible (TVA*S) and resistant (TVA*R) chicken lines, we demonstrate that in 93 F2 progeny an RFLP for the cloned receptor gene segregates with TVA. From these analyses we calculate that the cloned receptor gene lies within 5 centimorgans of TVA, making it highly probable that the cloned gene is the previously identified locus TVA. The polymorphism that distinguishes the two alleles of TVA in these inbred lines affects the encoded amino acid sequence of the region of Tva that encompasses the viral binding domain. However, analysis of the genomic sequence encoding this region of Tva in randomly bred chickens suggests that the altered virus binding domain is not the basis for genetic resistance in the chicken lines analyzed.

Use of reverse transcriptase polymerase chain reaction for detection of vaccine contamination by avian leukosis virus

A reverse transcriptase polymerase chain reaction (RT-PCR) for avian leukosis virus (ALV) was developed for the detection of contamination of vaccines produced in embryonated eggs and cell cultures derived from chicken. ALV is highly pathogenic and induces a wide spectrum of disease in infected animals. ALV can be divided into five subgroups (A-E). The envelope glycoprotein (env gp85) is the main antigen determinant and responsible for subgroup classification. Viral RNA of all subgroups (A-E) was isolated and amplified using three sets of primers. Subsequently, restriction endonuclease analysis confirmed the product identity and discriminated between subgroups. In specific pathogen free (SPF) eggs experimentally inoculated with ALV, viral RNA was found in allantoic fluids, as well as in vaccines spiked with different subgroups of ALV. No adventitious virus was detected in commercially available preparations. This system provides a rapid and specific in vitro method for the detection of ALV RNA as an extraneous agent and may be applied for quality control of avian vaccines.

Role of basic residues in the subgroup-determining region of the subgroup A avian sarcoma and leukosis virus envelope in receptor binding and infection

Receptor specificity in avian sarcoma and leukosis viruses (ASLV) maps to the central region of the envelope surface protein, SU. Two hypervariable regions, hr1 and hr2, within this region of SU are the principal determinants of receptor specificity. The cellular receptor for subgroup A ASLV, Tva, utilizes a 40-residue, acidic, cysteine-rich sequence for viral binding and entry. This domain in Tva is closely related to the ligand-binding domain of the low-density lipoprotein receptor (LDLR). Ligands bind to LDLR via the interaction of clustered basic residues in the ligand with the acidic cysteine-rich domains of the receptor. Analysis of the ASLV envelope sequences revealed a cluster of basic residues within hr2 that is unique to the subgroup A viruses, suggesting a possible role for these residues in receptor recognition. Therefore, the effects of altering these basic residues on subgroup A envelope expression, receptor binding, and infectivity were examined. Most of the mutant proteins were transported to the cell surface and processed normally. Receptor binding was diminished approximately 50% by alanine substitution at amino acid R213 or K227, whereas substitution by alanine at R210, R223, or R224 had no effect. However, when coupled with mutations at R213 or K227, changes at R223,R224 reduced envelope binding by 90%. Mutation of all five basic residues abrogated receptor binding. The effect of the hr2 mutations on ASLV envelope-mediated infection did not parallel the effect on receptor binding. Residues 210, 213, 223, and 224 were important for efficient infection, while mutations at residue 227 had little effect on infectivity. These results demonstrate that the basic residues in the ASLV envelope have roles in both receptor recognition and post-receptor binding events during viral entry.

The noncoding and surface envelope coding sequences of myeloblastosis-associated virus are respectively responsible for nephroblastoma development and renal hyperplasia

The use of chimeric viruses allowed us to establish that myeloblastosis-associated virus long terminal repeat sequences are necessary and sufficient for induction of nephroblastoma in chickens and that the blastemal hyperplasia induced by env SU is not a prerequisite for tumor development but rather constitutes a predisposing stage.

Presence of a hypervariable region within the hr2 domain of the host range determining sequences of the envelope protein gp85 (SU) of subgroup-A avian sarcoma-leukosis viruses

The nucleotide sequence of the gene for the envelope protein gp85 (SU) of the Schmidt-Ruppin subgroup A (NY) strain of Rous sarcoma virus [SRA(NY)] was determined, and the deduced amino acid sequence was compared with those of other avian sarcoma-leukosis viruses. Among the five host-range determinant sequences (vr1, vr2, hr1, hr2, and vr3), the host-range determinant sequence hr2 of SRA(NY) showed a significant deviation from the hr2 sequences of other subgroup A viruses namely, RAV-1 and SRA(SF). A phylogenetic analysis of the amino acid sequence of this region indicated that this intra-subgroup diversity was as great as or even greater than the inter-subgroup diversity found among other subgroups of ASLV. Within the hr2 region, we found a short hypervariable segment that differs in length and sequence from hr2 of other subgroup-A viruses. The difference in the hr2 amino acid sequence between SRA(NY) and SRA(SF) is reflected in the predicted protein secondary structure of this region.

HPRS-103 (exogenous avian leukosis virus, subgroup J) has an env gene related to those of endogenous elements EAV-0 and E51 and an E element found previously only in sarcoma viruses

The avian leukosis and sarcoma virus (ALSV) group comprises eight subgroups based on envelope properties. HPRS-103, an exogenous retrovirus recently isolated from meat-type chicken lines, is similar to the viruses of these subgroups in group antigen but differs from them in envelope properties and has been assigned to a new subgroup, J. HPRS-103 has a wide host range in birds, and unlike other nontransforming ALSVs which cause late-onset B-cell lymphomas, HPRS-103 causes late-onset myelocytomas. Analysis of the sequence of an infectious clone of the complete proviral genome indicates that HPRS-103 is a multiple recombinant of at least five ALSV sequences and one EAV (endogenous avian retroviral) sequence. The HPRS-103 env is most closely related to the env gene of the defective EAV-E51 but divergent from those of other ALSV subgroups. Probing of restriction digests of line 0 chicken genomic DNA has identified a novel group of endogenous sequences (EAV-HP) homologous to that of the HPRS-103 env gene but different from sequences homologous to EAV and E51. Unlike other replication-competent nontransforming ALSVs, HPRS-103 has an E element in its 3' noncoding region, as found in many transforming ALSVs. A deletion found in the HPRS-103 U3 EFII enhancer factor-binding site is also found in all replication-defective transforming ALSVs (including MC29, which causes rapid-onset myelocytomas).

The receptor for the subgroup A avian leukosis-sarcoma viruses binds to subgroup A but not to subgroup C envelope glycoprotein

The putative subgroup A avian leukosis-sarcoma virus (ALSV) receptor (Tva) was recently cloned by gene transfer (P. Bates, J. A. Young, and H. E. Varmus, Cell 74:1043-1051, 1993; J. A. T. Young, P. Bates, and H. E. Varmus, J. Virol. 67:1811-1816, 1993). Susceptibility to infection by subgroup A ALSV is conferred on cells upon transfection with cDNAs encoding tva. The hypothesis that tva encodes a specific receptor for subgroup A ALSV predicts that the Tva protein should bind to subgroup A, but not to subgroup C, envelope glycoprotein. In this study, we examined this prediction by using several biochemical assays. We established stable NIH 3T3 cell lines expressing either Tva, the subgroup A envelope glycoprotein (Env-A), or the subgroup C envelop glycoprotein (Env-C) and used them in conjunction with soluble forms of these molecules to demonstrate specific binding. When cell lysates containing Tva were mixed with lysates of either Env-A or Env-C, an immunoprecipitable complex formed between Tva and Env-A but not between Tva and Env-C. A soluble, oligomeric form, of Env-A, not Env-C, binds to cells expressing Tva. Reciprocally, a secreted form of Tva can bind to cells expressing Env-A but not to cells expressing Env-C. A specific and stable complex formed between soluble Env-A and secreted Tva as demonstrated by sucrose density gradient centrifugation. Thus, by three kinds of assays, Tva appears to bind specifically to Env-A, which is consistent with genetic evidence that it serves as the cell surface receptor of subgroup A ALSV and the main determinant of subgroup specificity.

Modifications in the binding domain of avian retrovirus envelope protein to redirect the host range of retroviral vectors

On the basis of theoretical structural and comparative studies of various avian leukosis virus SU (surface) envelope proteins, we have identified four small regions (I, II, III, and IV) in their receptor-binding domains that could potentially be involved in binding to receptors. From the envelope gene of an avian leukosis virus of subgroup A, we have constructed a set of SU mutants in which these regions were replaced by the coding sequence of FLA16, a 16-amino-acid RGD-containing peptide known to be the target for several cellular integrin receptors. Helper-free retroviral particles carrying a neo-lacZ retroviral vector were produced with the mutant envelopes. SU mutants in which regions III and IV were substituted yielded normal levels of envelope precursors but were not detectably processed or incorporated in viral particles. In contrast, substitutions in regions I and II did not affect the processing and the viral incorporation of SU mutants. When FLA16 was inserted in region II, it could be detected with antibodies against FLA16 synthetic peptide, but only when viral particles were deglycosylated. Viral particles with envelopes mutated in region I or II were able to infect avian cells through the subgroup A receptor at levels similar to those of the wild type. When viruses with envelopes containing FLA16 peptide in region II were applied to plastic dishes, they were found to promote binding of mammalian cells resistant to infection by subgroup A avian leukosis viruses but expressing the integrins recognized by FLA16. Deglycosylated helper-free viruses obtained by mild treatment with N-glycosidase F have been used to infect these mammalian cells, and infections have been monitored by neomycin selection. No neomycin-resistant clones could be obtained after infection by viruses with wild-type envelopes. Conversely, colonies were obtained after infection by viruses with envelopes bearing FLA16 in region II, and the genome of the retroviral vector was found correctly integrated in cell DNA of these colonies. By using a blocking peptide containing the minimal adhesive RGD sequence contained in FLA16, we have shown that preincubation of target cells could specifically inhibit infection by viruses with FLA16.

A receptor for subgroup A Rous sarcoma virus is related to the low density lipoprotein receptor

Cellular receptors are required for efficient entry of retroviruses into cells. We previously cloned a chicken gene responsible for susceptibility to the retrovirus subgroup A Rous sarcoma virus (RSV(A)). Here we have isolated the quail homolog and generated two alternatively spliced processed genes encoding cellular receptors for RSV(A). Predicted products of the processed genes appear to be small membrane-associated proteins with identical 83 amino acid extracellular domains but different membrane anchors. Within the extracellular domain is a region closely related to the ligand-binding repeat of the low density lipoprotein receptor (LDLR). Expression of either processed gene renders mammalian cells specifically susceptible to RSV(A). Antibodies directed against the receptor block subgroup A infection of avian cells via endogenous receptors and have no effect on entry of other RSV subgroups. Thus, small LDLR-related proteins are cellular receptors for RSV(A).

Inversion of a chicken ets-1 proto-oncogene segment in avian leukemia virus E26

The segment of the avian leukemia virus E26 genome near the termination of the p135gag-myb-ets open reading frame contains an inversion of the chicken ets-1 sequence. The inversion contains at least 41 bp and may be as large as 46 bp. This results in the replacement of 13 amino acids of chicken ets-1, with 16 amino acids derived from reverse complement of the normal ets-1 coding strand or read-through into E26 env sequences. At least 13 of these codons are specified by the inverted ets sequences. This represents the first reported occurrence of inverted oncogene sequences in a natural retrovirus. The inverted ets sequences are immediately followed by sequences homologous to the Rous sarcoma virus Prague B env gene. Since the E26 env sequence is more closely related to subgroup B avian retroviruses than to avian retroviruses from subgroups A, C, D, or E, the progenitor of E26 was a virus belonging to avian retrovirus subgroup B.

Basis for receptor specificity of nonecotropic murine leukemia virus surface glycoprotein gp70SU

Murine leukemia viruses (MuLVs) initiate infection of NIH 3T3 cells by binding of the viral envelope (Env) protein to a cell surface receptor. Interference assays have shown that MuLVs can be divided into four groups, each using a distinct receptor: ecotropic, polytropic, amphotropic, and 10A1. In this study, we have attempted to map the determinants within viral Env proteins by constructing chimeric env genes. Chimeras were made in all six pairwise combinations between Moloney MCF (a polytropic MuLV), amphotropic MuLV, and 10A1, using a conserved EcoRI site in the middle of the Env coding region. The receptor specificity of each chimera was determined by using an interference assay. We found that amphotropic receptor specificity of each chimera was determined by using an interference assay. We found that amphotropic receptor specificity seems to map to the N-terminal portion of surface glycoprotein gp70SU. The difference between amphotropic and 10A1 receptor specificity can be attributed to one or more of only six amino acid differences in this region. Nearly all other cases showed evidence of interaction between Env domains in the generation of receptor specificity. Thus, a chimera composed exclusively of MCF and amphotropic sequences was found to exhibit 10A1 receptor specificity. None of the chimeras were able to infect cells by using the MCF receptor; however, two chimeras containing the C-terminal portion of MCF gp70SU could bind to this receptor, while they were able to infect cells via the amphotropic receptor. This result raises the possibility that receptor binding maps to the C-terminal portion of MCF gp70SU but requires MCF N-terminal sequences for a functional interaction with the MCF receptor.

Identification and structure analysis of endogenous proviral sequences in a Brown Leghorn chicken strain

In a Brown Leghorn chicken strain, four endogenous proviral loci have been identified. The DNA mapping data show strong homology between their structures and that of the Rous-associated virus O (RAV-O) genome. Two of them seem similar to ev3 and ev6 loci previously described in White Leghorn chickens; the two others are unknown in White Leghorns. Using DNA amplification methods, envelope genes of these endogenous viral structures have been partially sequenced. The results demonstrate that subgroup-specific sequences of the endogenous loci were largely homologous with those of RAV-O.

Genetic analysis of the Rous sarcoma virus subgroup D env gene: mammal tropism correlates with temperature sensitivity of gp85

Subgroup D avian sarcoma and leukosis viruses can penetrate a variety of mammalian cells in addition to cells from their natural host, chickens. Sequences derived from the gp85-coding domain within the env gene of a mammal-tropic subgroup D virus (Schmidt-Ruppin D strain of Rous sarcoma virus [SR-D RSV]) and a non-mammal-tropic subgroup B virus (Rous-associated virus type 2) were recombined to map genetic determinants that allow penetration of mammalian cells. The following conclusions were based on host range analysis of the recombinant viruses. (i) The determinants of gp85 that result in the mammal tropism phenotype of SR-D RSV are encoded within the 160 codons that lie 3' of codon 121 from the corresponding amino terminus of the gp85 protein. (ii) Small linear domains of the SR-D RSV gp85-coding domain placed in the subgroup B background did not yield viruses with titers equal to that of the subgroup D virus in a human cell line. (iii) Recombinant viruses that contained subgroup D sequences within the hr1 variable domain of gp85 showed modest-to-significant increases in infectivity on human cells relative to chicken cells. A recombinant virus that contained three fortuitous amino acid substitutions in the gp85-coding domain was found to penetrate the human cell line and give a titer similar to that of the subgroup D virus. In addition, we found that the subgroup D virus, the mutant virus, and recombinant viruses with an increased mammal tropism phenotype were unstable at 42 degrees C. These results suggest that the mammal tropism of the SR-D strain is not related to altered receptor specificity but rather to an unstable and fusogenic viral glycoprotein. A temperature sensitivity phenotype for infectivity of mammalian cells was also observed for another mammal-tropic avian retrovirus, the Bratislava 77 strain of RSV, a subgroup C virus, but was not seen for any other avian retrovirus tested, strengthening the correlation between mammal tropism and temperature sensitivity.

Rearrangements in unintegrated retroviral DNA are complex and are the result of multiple genetic determinants

We used a replication-competent retrovirus shuttle vector based on a DNA clone of the Schmidt-Ruppin A strain of Rous sarcoma virus to characterize rearrangements in circular viral DNA. In this system, circular molecules of viral DNA present after acute infection of cultured cells were cloned as plasmids directly into bacteria. The use of a replication-competent shuttle vector permitted convenient isolation of a large number of viral DNA clones; in this study, over 1,000 clones were analyzed. The circular DNA molecules could be placed into a limited number of categories. Approximately one-third of the rescued molecules had deletions in which one boundary was very near the edge of a long terminal repeat (LTR) unit. Subtle differences in the patterns of deletions in circular DNAs with one versus two copies of the LTR sequence were observed, and differences between deletions emanating from the right and left boundaries of the LTR were seen. A virus with a missense mutation in the region of the pol gene responsible for integration and exhibiting a temperature sensitivity phenotype for replication had a marked decrease in the number of rescued molecules with LTR-associated deletions when infection was performed at the nonpermissive temperature. This result suggests that determinants in the pol gene, possibly in the integration protein, play a role in the generation of LTR-associated deletions. Sequences in a second region of the genome, probably within the viral gag gene, were also found to affect the types of circular viral DNA molecules present after infection. Sequences in this region from different strains of avian sarcoma-leukosis viruses influenced the fraction of circular molecules with LTR-associated deletions, as well as the relative proportion of circular molecules with either one or two copies of the LTR. Thus, the profile of rearrangements in unintegrated viral DNA is complex and dependent upon the nature of sequences in the gag and pol regions.

Unique sequences in the env gene of avian hemangioma retrovirus are responsible for cytotoxicity and endothelial cell perturbation

An avian retrovirus isolated from spontaneous cavernous hemangiomas of layer hens codes for an env protein that induces a cytopathic effect on a wide variety of cultured avian and mammalian cells and also causes thrombogenicity of endothelial cells. Sequence analysis of the avian hemangioma inducing virus revealed unique elements in both its env gene and its LTR. We propose that these elements are responsible for the biological and pathogenic characteristics of the virus.

Sequence analysis of amphotropic and 10A1 murine leukemia viruses: close relationship to mink cell focus-inducing viruses

Viral interference studies have demonstrated the existence of four distinct murine leukemia virus (MuLV) receptors on NIH 3T3 mouse cells. The four viral interference groups are ecotropic MuLV; mink cell focus inducing virus (MCF); amphotropic MuLV; and 10A1, a recombinant derivative of amphotropic MuLV that uses a unique receptor but also retains affinity for the amphotropic MuLV receptor. We report here that 10A1 infects rat and hamster cells, unlike its amphotropic parent. We isolated an infectious molecular clone of 10A1 and present here the sequences of the env genes and enhancer regions of amphotropic MuLV and 10A1. The deduced amino acid sequences of amphotropic MuLV and 10A1 gp70su are remarkably similar to those of MCF and xenotropic MuLV (for which mouse cells lack receptors), with 64% amino acids identical in the four groups. We generated a consensus from these comparisons. Further, the differences are largely localized to a few discrete regions: (i) amphotropic MuLV has two short insertions relative to MCF, at residues 87 to 92 and 163 to 169, and (ii) amphotropic MuLV and MCF are totally different in a hypervariable region, which is greater than 30% proline, at residues approximately 253 to 304. 10A1 closely resembles amphotropic MuLV in its N terminus but contains an MCF-type hypervariable region. These results suggest the possibility that receptor specificity is localized in these short variable regions and further that the unique receptor specificity of 10A1 is due to the novel combination of amphotropic MuLV and MCF sequences rather than to the presence of any novel sequences. The Env proteins of ecotropic MuLV are far more distantly related to those of the other four groups than the latter are to each other. We also found that the enhancer regions of amphotropic MuLV and 10A1 are nearly identical, although 10A1 is far more leukemogenic than amphotropic MuLV.

v-maf, a viral oncogene that encodes a "leucine zipper" motif

We have molecularly cloned the provirus of the avian musculoaponeurotic fibrosarcoma virus AS42. Nucleotide sequence analysis of a biologically active clone of AS42 showed that this virus encodes a viral oncogene, maf. The deduced amino acid sequence of the v-maf gene product contains a "leucine zipper" motif similar to that found in a number of DNA binding proteins, including the gene products of the fos, jun, and myc oncogenes. However, unlike these oncogenes, the cellular maf gene was not transcriptionally activated by growth stimulation of cultured cells.

Long terminal repeat (LTR) sequences, env, and a region near the 5' LTR influence the pathogenic potential of recombinants between Rous-associated virus types 0 and 1

A series of recombinants between Rous-associated virus type 0 (RAV-0), RAV-1, and a replication-competent avian leukosis virus vector (RCAN) have been tested for disease potential in day-old inoculated K28 chicks. RAV-0 is a benign virus, whereas RAV-1 and RCAN induce lymphoma and a low incidence of a variety of other neoplasms. The results of the oncogenicity tests indicate that (i) the long terminal repeat regions of RAV-1 and RCAN play a major role in disease potential, (ii) subgroup A envelope glycoproteins are associated with a two- to fourfold higher incidence of lymphoma than subgroup E glycoproteins, and (iii) certain combinations of 5' viral and env sequences cause osteopetrosis in a highly context-dependent manner. Long terminal repeat and env sequences appeared to influence lymphomogenic potential by determining the extent of bursal infection within the first 2 to 3 weeks of life. This would suggest that bursal but not postbursal stem cells are targets for avian leukosis virus-induced lymphomogenesis. The induction of neutralizing antibody had no obvious influence on the incidence of lymphoma.

The avian retrovirus env gene family: molecular analysis of host range and antigenic variants

The nucleotide sequence of the env gp85-coding domain from two avian sarcoma and leukosis retrovirus isolates was determined to identify host range and antigenic determinants. The predicted amino acid sequence of gp85 from a subgroup D virus isolate of the Schmidt-Ruppin strain of Rous sarcoma virus was compared with the previously reported sequences of subgroup A, B, C, and E avian sarcoma and leukosis retroviruses. Subgroup D viruses are closely related to the subgroup B viruses but have an extended host range that includes the ability to penetrate certain mammalian cells. There are 27 amino acid differences shared between the subgroup D sequence and three subgroup B sequences. At 16 of these sites, the subgroup D sequence is identical to the sequence of one or more of the other subgroup viruses (A, C, and E). The remaining 11 sites are specific to subgroup D and show some clustering in the two large variable regions that are thought to be major determinants of host range. Biological analysis of recombinant viruses containing a dominant selectable marker confirmed the role of the gp85-coding domain in determining the host range of the subgroup D virus in the infection of mammalian cells. We also compared the sequence of the gp85-coding domain from two subgroup A viruses, Rous-associated virus type 1 and a subgroup A virus of the Schmidt-Ruppin strain of Rous sarcoma virus. The comparison revealed 24 nonconservative amino acid changes, of which 6 result in changes in potential glycosylation sites. The positions of 10 amino acid differences are coincident with the positions of 10 differences found between two subgroup B virus env gene sequences. These 10 sites identify seven domains in the sequence which may constitute determinants of type-specific antigenicity. Using a molecular recombinant, we demonstrated that type-specific neutralization of two subgroup A viruses was associated with the gp85-coding domain of the virus.

Role of RAV-0 genes in the permissive replication of subgroup E avian leukosis viruses on line 15Bev1 CEF

Rous associated virus type-0 (RAV-0) is a replication-competent endogenous virus of chickens which grows more efficiently on chick embryo fibroblasts (CEFs) from line 15B chickens than on CEFs from line K28. Differences in viral growth on these two sources of cells have been attributed to an early event in the retrovirus life-cycle, at or before viral DNA synthesis. Five in vitro constructed avian leukosis viruses (ALVs) as well as RAV-0 (a subgroup E ALV), RAV-1 (subgroup A), and RAV-2 (subgroup B) have been assessed for their relative growth on 15Bev1 and K28 CEFs. More efficient replication on 15Bev1 CEFs than on K28 CEFs was determined by subgroup E-encoding sequences in env. Subgroup A and B envelope sequences as well as viral LTR, gag, and pol sequences did not obviously bias relative rates of viral replication on the two cell types. We suggest that the unusually permissive replication of subgroup E viruses on 15B CEFs is a receptor-mediated phenomenon and that the line 15B receptor for subgroup E ALVs is more efficient than that of line K28.

Adaptor plasmids simplify the insertion of foreign DNA into helper-independent retroviral vectors

We have previously described several helper independent vector constructions (S. Hughes and E. Kosik, Virology 136:89-99, 1984; J. Sorge and S. H. Hughes, J. Mol. Appl. Genet. 1:547-599, 1982; J. Sorge, B. Ricci, and S. Hughes, J. Virol. 48:667-675, 1983), all of which derive from Rous sarcoma virus. In this report we describe three improvements in the earlier constructions. First, the vectors have been restructured as proviruses, which considerably improves the efficiency of virus production following acute transfection. Second, a series of miniplasmids have been developed, which we call adaptors, and these miniplasmids can be used to convert virtually any DNA segment into a ClaI fragment suitable for insertion into the retroviral (or other) vectors. Adaptors have been developed that supply regions of functional significance, including a splice acceptor and an initiator ATG. Finally, the region of env defining subgroup specificity, A in the original vectors, has been substituted by the corresponding regions of subgroup B and D viruses, giving vectors with additional subgroup specificities and increased host ranges.

Domains of virus glycoproteins

This chapter reviews current information about the structure and function of virus glycoproteins. There are few virus glycoproteins that provide prototypes for illustrating important relationships between the functions and glycoprotein structure. The discussion presented in the chapter concentrates on those viral glycoproteins that (1) span the lipid bilayer once, (2) are oriented such that the carboxy terminus comprises the cytoplasmic domain, and (3) contain asparagine-linked oligosaccharides. There are also viral glycoproteins with extensive O-linked glycosylation, some of which are also presented in the discussion. The chapter also focuses on the studies involving directed mutagenesis and construction of chimeric proteins. The effects of altering specific amino acid sequences, of swapping domains, and of adding a new domain to a protein serve to define the functions of a domain and to show that a domain can be independently associated with a specific function. The experiments described have been carried out by inserting the genes of particular viral glycoproteins—such as cDNAs—into expression vectors and transcribing the cDNAs from the promoter provided by the expression vector. This approach established that localization and functions such as the fusogenic activity are properties of the viral glycoprotein per se and do not require other viral-coded components.