Identification of the messenger RNAs coding for the gag and env gene products of the murine mammary tumor virus

META-controlled env-initiated transcripts encoding superantigens of murine Mtv29 and Mtv7 and their possible role in B cell lymphomagenesis

Spontaneous germinal center (GC)-derived B cell lymphomas of SJL mice (RCS) transcribe a 1.8-kb Mtv-29 mRNA under control of the META-env promoter. The encoded vSAg29 stimulates syngeneic Vbeta16(+) CD4(+) T cells, thereby acquiring T cell help necessary for RCS growth. Other strains of B cell lymphoma-prone mice include Mtv29(+) C57L and MA/MyJ, and the Mtv29(-) Mtv7(+)-recombinant inbred strain, SW x J-1. The lymphomas of these mice produce similar mouse mtv-vSAg-encoding mRNA, as characterized by Northern blotting, PCR, and RNase protection. A 1.8-kb mRNA in C57L/J and MA/MyJ lymphomas hybridized with an Mtv29-specific oligonucleotide, whereas SW x J-1 lymphomas produced 1.8-kb transcripts hybridizing with an Mtv7-specific oligonucleotide. Similar META-env-initiated transcripts were absent from LPS-activated B cells from any strain examined but were detected in Peyer's patch RNA from SJL mice. Like typical SJL-derived RCS, all these lymphomas stimulated syngeneic CD4(+) T cells and Vbeta16(+) T hybridoma cells. Immunohistochemical staining of primary tumors showed the presence of peanut agglutinin binding (PNA(+)) highly mitotic lymphoblasts, suggesting their GC derivation. The findings indicate that this novel mRNA for Mtv29 is present in B cell lymphomas from several Mtv29(+) mouse strains. Additionally, this is the first description of the ability of Mtv7 to produce transcripts that are controlled and spliced identically to those of Mtv29 and that are expressed in SW x J-1, I-A(s+), lymphomas that also stimulate Vbeta16(+) T cells. Our results suggest an important role for mouse mtv-vSAgs and Vbeta16 T cell stimulation in the development of GC-derived murine B cell lymphomas.

Cloning of an infectious milk-borne mouse mammary tumor virus (MMTV) DNA from a mammary tumor that developed in an endogenous MMTV-free wild mouse

Molecular characterization of infectious mouse mammary tumor viruses (MMTVs) has been hampered due to the problem of cloning a full-length exogenous virus into a plasmid. The present report describes our strategy for obtaining a full-length clone of an exogenous MMTV from a mouse mammary tumor that arose spontaneously in a wild Chinese mouse free of endogenous MMTV and shows that the cloned virus (JYG-MMTV) is expressed in rat RBA cells. Four-week-old C58/J x CBA/CaJ female mice, free of both endogenous and exogenous MMTVs, were injected with virus-secreting RBA cells. The progeny of these mice were bred, and their offspring were tested for the presence of MMTV. These third-generation mice were found to actively produce MMTV that was shed in their milk and transmitted to their offspring. The virus was detected not only in the mammary glands of these young mice, but also in their spleens and bone marrow. These results suggest that our plasmid-cloned exogenous JYG-MMTV is infectious. This virus can now be used effectively in manipulating the various genes of JYG-MMTV and other MMTV strains to understand their structure/function relationships.

Phorbol diester-inducible, cyclosporine-suppressible transcription from a novel promoter within the mouse mammary tumor virus env gene

The mouse T-cell lymphoma cell line EL4.E1 constitutively synthesizes mouse mammary tumor virus (MMTV) transcripts encoding either the entire proviral genome or segments of it. In addition to these conventional mRNAs, however, an mRNA of about 1 kilobase accumulates after induction of these cells with phorbol myristate acetate (PMA). The accumulation of this transcript is strongly inhibited by the immunosuppressive agent cyclosporin A. Its pattern of induction by PMA and suppression by cyclosporin A is thus the same as seen for several lymphokine mRNAs in these cells, including interleukin-2 and granulocyte-macrophage colony-stimulating factor. The short MMTV transcript is the most abundant PMA-induced transcript in EL4.E1 cells, but was not found in a series of other leukocyte tumor cell lines. It is initiated from a novel promoter within the env gene, and a segment of 1,161 nucleotides is then spliced out. The major part of the transcript is a copy of the long terminal repeat (LTR) of MMTV. The MMTV proviral genomes in these cells, and the short transcript, contain a 491-nucleotide deletion in the LTR compared with the normal MMTV provirus. The resulting open reading frame could encode a protein of molecular weight 22,800, which is a likely candidate for an LTR-related protein with a similar molecular weight recently described in this system (J. Racevskis, J. Virol. 58:441-449, 1986).

Regulation of endogenous murine mammary tumor virus expression in C57BL mouse lactating mammary glands: transcription of functional mRNA with a block at the translational level

Expression of endogenous murine mammary tumor viruses (MuMTVs) in various mouse strains is regulated in different ways, and in the absence of exogenous MuMTV, this regulation influences the incidence of spontaneous mammary tumors. Two mouse strains with low mammary tumor incidence, BALB/c and C57BL, control endogenous MuMTV expression at different stages. Neither of the strains had any detectable MuMTV polypeptides in its lactating mammary glands (LMG). However, in C57BL LMG, substantial amounts of MuMTV RNA were present, whereas very little viral RNA was detected in BALB/c LMG. By determining MuMTV RNA levels in LMG of hybrids and backcrosses of BALB/c and C57BL mice, we found that there are three unlinked, independently segregating genetic loci in C57BL mice that are responsible for the presence of moderately high amounts of MuMTV RNA in LMG. The viral RNA in C57BL LMG was processed and transported to the cytoplasm where it was found to cosediment with EDTA-sensitive polysomes. No viral proteins were detected in run-off reactions that permit completion of nascent polypeptide synthesis with polysomes from C57BL LMG, and sensitive radioimmunoassays failed to detect any MuMTV proteins in these tissues. In contrast, MuMTV mRNA purified from C57BL LMG did direct the synthesis of both gag and env MuMTV polypeptides when added to a heterologous rabbit reticulocyte lysate cell-free translation system. We propose that MuMTV mRNA in C57BL LMG, for unknown reasons, is blocked at the translational level.

Structural analysis of a 1.7-kilobase mouse mammary tumor virus-specific RNA

We have detected a mouse mammary tumor virus (MMTV)-specific 1.7-kilobase (kb) polyadenylated RNA in mammary glands of several mouse strains. In BALB/c mice, it is the only MMTV-specific RNA species present. C3H and GR mammary glands and tumors contain, in addition, 3.8- and 7.8-kb MMTV RNAs. Nuclease S1 analysis was performed to map 1.7-kb polyadenylated RNA. It contains predominantly long terminal repeat (LTR) sequences. The 5' end maps approximately 134 nucleotides upstream from the 3' end of the LTR. Colinearity with complete proviral DNA continues to a site about 153 nucleotides downstream from the left (5') LTR. No sequences from the middle part of proviral DNA were found. Colinearity with proviral DNA is resumed 72 nucleotides upstream from the right (3') LTR. The nucleotide sequence in this area is TTCCAGT, which is a splice acceptor consensus sequence. The anatomy of 1.7-kb RNA indicates that it may serve as a messenger for the 36,700-dalton protein encoded by the LTRs of MMTV.

In vivo modification of retroviral gag gene-encoded polyproteins by myristic acid

It has recently been shown by mass spectral analysis (Henderson et al., Proc. Natl. Acad. Sci. U.S.A. 80:339-343, 1983) that the p15gag protein of murine leukemia viruses contains a novel post-translational modification, an amino-terminal myristyl (tetradecanoyl) amide. In this report we show that p15gag is the only structural protein to contain this fatty acid. In addition, the gag precursor polyproteins of type B, C, and D retroviruses have been examined for the presence of myristic acid by metabolic labeling and immunoprecipitation studies. In a panel of mammalian type C retroviruses we found that the precursor polyprotein Pr65gag homologs, but not the glycosylated forms (gPr80gag homologs), were specifically labeled after a 5-min incubation of infected cells with [3H]myristic acid. The gag precursor polyprotein was also labeled in mouse mammary tumor virus and in Mason-Pfizer monkey virus, but Pr76gag of Rous sarcoma virus failed to incorporate [3H]myristate. Under similar conditions, [3H]palmitate was not found to be incorporated into any viral gag proteins. Thus, myristylation appears to be a common feature of mammalian type B, C, and D retroviruses but not of avian retroviruses.

Transcription of mouse mammary tumor virus: identification of a candidate mRNA for the long terminal repeat gene product

We have examined an assortment of preneoplastic and neoplastic mouse mammary tissues for the presence of an mRNA which could encode the putative long terminal repeat gene product of mouse mammary tumor virus. We report here the detection of a novel mouse mammary tumor virus-specific, polyadenylic acid-containing transcript in certain preneoplastic and neoplastic mammary tissue of BALB/c mice. The molecule is 1.6 kilobases in length and contains sequences from the transcriptional leader and the U3 region of the proviral DNA. The upstream terminus of the 3' information lies 75 to 80 nucleotides from the beginning of the long terminal repeat open reading frame, in close proximity to a consensus splice acceptor in the DNA. The transcript was detected in hormonally or chemically induced neoplastic, preneoplastic, and lactating mammary tissue of BALB/c mice, but not in preneoplastic or tumor tissue induced by exogenous viruses in any strain of mice examined. This implies that the RNA we observed is transcribed from an endogenous provirus template.

Effect of hormones on the expression of proviral genes Mtv-2 and Mtv-3 in mouse mammary gland

We have investigated the expression of the Mtv-2 and Mtv-3 proviral genes in mouse mammary glands by examining the effect of hormones on levels of mammary tumor virus (MTV) proteins p27 (gag) and gp52 (env) in mouse mammary explants. We also investigated the effect of the hormones on DNA synthesis in the explants. The mammary glands were derived from inbred GR and 020 mice, and from the respective congenic mouse strains GR/Mtv-2- and 020/Mtv-2+. The addition of insulin to the culture medium caused increases in p27 and gp52 levels in GR and 020/Mtv-2+ glands; a further increase in the viral proteins was obtained by also adding dexamethasone. Prolactin in combination with progesterone enhanced p27 and gp52 levels, but to a lesser extent than did dexamethasone. Dexamethasone caused a slight but significant increase in p27 protein in mammary explants from GR/Mtv-2- mice. Our data indicate that the Mtv-2 locus and the Mtv-3 locus in mouse mammary gland are under separate glucocorticoid control, and that Mtv-2 expression is also stimulated by the prolactin and progesterone combination. Whereas dexamethasone enhances MTV protein levels in mouse mammary explants, it inhibits DNA synthesis in the explants.

Processing and amino acid sequence analysis of the mouse mammary tumor virus env gene product

The envelope proteins of mouse mammary tumor virus (MMTV) are synthesized from a subgenomic 24S mRNA as a 75,000-dalton glycosylated precursor polyprotein which is eventually processed to the mature glycoproteins gp52 and gp36. In vivo synthesis of this env precursor in the presence of the core glycosylation inhibitor tunicamycin yielded a precursor of approximately 61,000 daltons (P61env). However, a 67,000-dalton protein (P67env) was obtained from cell-free translation with the MMTV 24S mRNA as the template. To determine whether the portion of the protein cleaved from P67env to give P61env was removed from the NH2-terminal end of P67env and as such would represent a leader sequence, the NH2-terminal amino acid sequence of the terminal peptide gp52 was determined. Glutamic acid, and not methionine, was found to be the amino-terminal residue of gp52, indicating that the cleaved portion was derived from the NH2-terminal end of P67env. The NH2-terminal amino acid sequences of gp52's from endogenous and exogenous C3H MMTVs were determined though 46 residues and found to be identical. However, amino acid composition and type-specific gp52 radioimmunoassays from MMTVs grown in heterologous cells indicated primary structure differences between gp52's of the two viruses. The nucleic acid sequence of cloned MMTV DNA fragments (J. Majors and H. E. Varmus, personal communication) in conjunction with the NH2-terminal sequence of gp52 allowed localization of the env gene in the MMTV genome. Nucleotides coding for the NH2 terminus of gp52 begin approximately 0.8 kilobase to the 3' side of the single EcoRI cleavage site. Localization of the env gene at that point agrees with the proposed gene order -gag-pol-env- and also allows sufficient coding potential for the glycoprotein precursor without extending into the long terminal repeat.

Dexamethasone induction of the intracellular RNAs of mouse mammary tumor virus

We have studied the kinetics of dexamethasone induction of mouse mammary tumor virus (MMTV) RNAs and proteins in virus-infected rat XC cells and GR mouse mammary tumor cells. A detectable increase in viral RNA in infected XC cells was present within 10 min after hormone addition, and half-maximal induction was achieved in less than 2 h. The increase in viral RNA concentration was apparent first in nuclear RNA and later in the cytoplasm. Within the first 15 min of induction, only genome-sized RNA (35S, 7.8 kilobases) was present in augmented amounts, whereas the major subgenomic RNA (24S, 3.8 kilobases) did not appear until at least 30 to 60 min postinduction. The sequential appearance of these RNAs, the probable mRNA's for the gag and env proteins, paralleled the order of appearance of the gag and env proteins, respectively, after hormone treatment. An additional species of viral RNA (20S, 2.5 kilobases) was detected during these induction experiments, but the role of this RNA is not known. Both subgenomic RNAs contain sequences derived from both the 5' and 3' termini of genomic RNA and are presumably spliced. After dexamethasone induction of infected XC cells, we detected two smaller env-related proteins which were not found in full hormone induction. The functional role of these smaller proteins is not known. A previously reported smaller species of RNA (13S, 1.0 kilobase) did not appear to be induced and was shown to be cellular rather than viral in origin. In the fully induced infected XC and GR mammary tumor cells, the only viral RNAs present were the 35S and 24S RNAs. In addition, mammary tumors contained only these two viral RNAs. Thus, tumor cells appear to contain only the viral RNAs which direct the synthesis of the gag, pol, and env proteins of the virion.

Quantitative of murine mammary tumor virus-related RNA in mammary tissues of low- and high-mammary-tumor-incidence mouse strains

Lactating mammary glands and hormonally induced mammary tumors of BALB/c mice from three geographically separated breeding colonies were examined by molecular hybridization, using murine mammary tumor virus (MuMTV) cDNA representing the entire viral genome to determine the amount of MuMTV-related RNA expressed in these tissues. The RNA extracted from these tissues by the classical sodium dodecyl sulfate-pronase, phenol-chloroform procedure (method 1) contained barely detectable levels of MuMTV-related sequences. In contrast, both normal lactating mammary glands and hormonally induced mammary tumors of these mice were found to contain approximately one to two copies of the MuMTV genome per cell by using a new procedure in which the RNA was extracted with guanidine derivatives (method 2). No significant differences in the MuMTV-related RNA content of the BALB/c mammary tissues were observed regardless of their colony of origin. Our results suggest that expression of MuMTV RNA does not change in BALB/c mammary glands during transformation to a malignant state and that MuMTV expression does not play a role in tumorigenesis in these mice. In view of the increased recovery of MuMTV-related RNA from BALB/c mice with method 2, we compared the level of MuMTV RNA expression in lactating mammary glands and mammary tumors of other mouse strains, including C57BL/6 and RIII, using both extraction methods. Yields of MuMTV-related RNA from mammary tissues increased by as much as 35- to 40-fold, using method 2 as compared with method 1. Therefore method 2, involving guanidine derivatives, appears to be method of choice for MuMTV-related RNA extraction from the mammary tissues of certain strains of mice, particularly those expressing relatively low levels of MuMTV RNA.

Selection of rat hepatoma cells defective in hormone-regulated production of mouse mammary tumor virus RNA

We have been studying the mechanism of glucocorticoid hormone action by using mouse mammary tumor virus (MMTV)-infected rat hepatoma cells as a model system. J2.17, a clonal cell line that contains one MMTV provirus, induces tyrosine aminotransferase (TyrATase; L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5), viral RNA, and the cell surface viral glycoprotein gp52 in response to dexamethasone. Using a fluorescence-activated cell sorter and a rabbit antiserum directed against gp52, we selected a cell population that displays a reduced hormone-mediated increase in cell surface gp52. Fourteen clones of this population were assayed for induction of viral gp52 and RNA and of cellular TyrATase. The results of these assays revealed that the clones display a variety of responses to hormone. One clone has retained wild-type responses of both TyrATase and gp52. Six clones exhibit coordinately reduced or abolished responses of both markers. Seven clones show normal induction of TyrATase but reduced or undetectable induction of gp52. These latter clones exhibit reduced production of MMTV RNA and thus may represent a unique class of variants defective in the regulation of MMTV gene expression.

Purification and translation of murine mammary tumor virus mRNA's

We have studied the functions of the intracellular RNAs of mouse mammary tumor virus (MMTV) by purification and translation in vitro. Two major size classes of MMTV RNA, 35S and 24S RNA, were isolated from MMTV-infected rat (XC) cells and cultured mammary tumor cells by preparative hybridization of whole cell or polyadenylated RNA to cloned MMTV DNA covalently bound to chemically activated paper disks (diazobenzyloxymethyl paper). Genomic-length (35S) RNA was prepared free of 24S RNA by rate zonal sedimentation in sucrose gradients. Experiments using [3H]uridine-labeled cellular RNA indicated that the preparative annealing method was highly specific and capable of effecting a 300-fold enrichment for viral RNA; the recovered RNA appeared to be intact under denaturing conditions and directed synthesis of full-length gag and env polypeptides in vitro. The products of in vitro translation were identified by gel mobility, immunoprecipitation tests with antisera against gag and env products, and partial digestion with Staphylococcus V8 protease. The 35S RNA species directed synthesis of several gag-related polypeptides, including three previously reported in extracts of infected cells; 24S RNA directed synthesis of two polypeptides closely related to env proteins from infected cells. Therefore, 35S RNA includes mRNA's for gag and gag-pol, whereas 24S RNA is the mRNA for env. These results help establish the position of env on the physical map of the MMTV genome and bear upon the coding potential of the genome.

Hormone-responsive expression of an endogenous proviral gene of mouse mammary tumor virus after molecular cloning and gene transfer into cultured cells

A recombinant lambda phage containing mouse mammary tumor virus (MMTV) proviral DNA was isolated from a gene library constructed from GR mouse liver DNA. Restriction enzyme analyses reveal that the cloned molecule contains a copy of one of the GR endogenous MMTV proviruses flanked on both sides by 2--3 kb of mouse genomic DNA. In this report we have examined the expression of the cloned MMTV provirus after cotransfection with the herpes thymidine kinase (TK; ATP:thymidine 5'-phosphotransferase,, EC 2.7.1.21) gene and integration into mouse LTK- cells. Nine individual TK+ transformants were selected, and all were found to contain MMTV-transfected DNA. One of the TK+ transformants was chosen for further study. Total poly(A)-containing RNA was isolated from the cells, and liquid hybridization analyses with MMTV cDNA showed that it contained 0.02% MMTV-specific RNA. The sizes of the MMTV-specific species were determined and found to correspond to the 35S and 24S mRNAs synthesized in MMTV-infected cells. Glucocorticoid hormones have been shown to increase the concentration of MMTV RNA in virus-infected cultured cells. Therefore, we tested the effect of dexamethasone on the concentration of MMTV-specific RNA in cells transfected with the MMTV proviral DNA. The amount of MMTV-specific poly(A)-containing RNA found in the cells grown in the presence of hormone was 0.17%. Therefore, dexamethasone causes an 8-fold increase in the amount of MMTV-specific RNA in mouse cells containing several copies of a cloned and transfected MMTV proviral gene.

Synthesis of murine mammary tumor viral proteins in vitro

The coding potential of murine mammary tumor viral genomic RNA was investigated by in vitro translation of various size classes of RNAs isolated from the virions. The major products of translation of full-size 35S polyadenylylated virion RNA were gag-related polyproteins of 75,000, 105,000, and 180,000 daltons (P75, P105, and P180, respectively). Studies on the kinetics of translation of these three proteins established that they were synthesized independently and that the smaller proteins were not post-translational cleavage products of the larger proteins. Tryptic peptide mapping showed that almost all of the P75 sequences were contained within P105 and almost all of the P105 sequences were contained within P180. The syntheses of all three proteins were inhibited by m7GTP, indicating that they were translated from capped mRNA's. Although a 24S polyadenylylated RNA had been identified as the intracellular mRNA for env precursor polyprotein, no such protein could be translated from the 24S polyadenylylated RNA isolated from the virions. However, translation of a 14S size class of polyadenylylated virion yielded four prominent proteins of about 36,000, 23,000, 21,000, and 20,000 daltons. These proteins were unrelated to murine mammary tumor viral structural proteins, as suggested from tryptic peptide mapping and immunoprecipitation data. They might be the products of an as-yet-unidentified gene located near the 3' terminus of the murine mammary tumor viral genomic RNA.

Protein-coding potential of mouse mammary tumor virus genome RNA as examined by in vitro translation

The protein-coding capacity of the mouse mammary tumor virus genome has been examined by in vitro translation of genome length and polyadenylated subgenomic fragments of viral RNA. Intact genome RNA of about 35S programmed synthesis of the Pr77gag, Pr110gag and Pr160gag/pol precursors seen in infected cells in vivo. Polyadenylated RNA fragments of 18 to 28S encoded products whose tryptic peptide maps resembled those of the nonglycosylated precursor to the envelope glycoproteins, confirming the gene order 5'-gag-pol-env-3'. Translation of polyadenylated RNA fragments smaller than 18S yielded a series of related proteins whose peptide maps bore no resemblance to any of the virion structural proteins. Thus, a region of the mouse mammary tumor virus genome distal to the env gene appears to have an open reading frame sufficient to encode at least 36,000 daltons of protein as of yet unknown function.

Localization of a gene for expression of mouse mammary tumor virus antigens in the GR/Mtv-2- mouse strain

The GR/Mtv-2- mouse strain is congenic to the GR strain but lacks the Mtv-2 gene for high amounts of mouse mammary tumor virus (MMTV) virion particles in the milk and early mammary tumors. With a sensitive competition radioimmunoassay for individual viral proteins of MMTV, substantial amounts of the gag proteins p27 and p10 could still be detected in extracts of the mammary glands of GR/Mtv-2- mice, but essentially no viral envelope antigens. The genetic transmission of the MMTV gag expression in the GR/Mtv-2- strain was investigated. In a cross with the virus-negative BALB/c strain, the MMTV p27 expression behaved as a dominant feature. Double backcross analysis proved that the p27 expression was governed by a single gene located on chromosome 11, cloe to the Es-3 locus. The gene was thereby not allelic to any of the previously described MMTV induction genes, Mtv-1 and Mtv-2, and is therefore called Mtv-3. It is concluded that the total MMTV expression in the GR strain is under control of two separate loci, Mtv-2 on chromosome 18, inducing high levels of complete virus particles and also early mammary tumors; and Mtv-3 on chromosome 11, coding for partial MMTV expression.

Analysis of secondary modifications of mouse mammary tumor virus proteins by two-dimensional gel electrophoresis

The structural proteins of mouse mammary tumor virus (MMTV) were analyzed by two-dimensional electrophoresis on isoelectric focusing and sodium dodecyl sulfate gels. Many of the viral proteins displayed heterogeneity in charge due to variable contents of carbohydrates (in particular, sialic acid) and phosphate residues. Neuraminidase treatment of the virions influenced the isoelectric pattern of the envelope glycoproteins. The glycoproteins of an MMTV variant which was attenuated by replication in feline kidney cells had different isoelectric points. This suggested that the acquisition of an altered carbohydrate configuration had changed the host range of the virus. The major MMTV structural core protein, p27, consisted of two species, which had identical iodinated tryptic peptide compositions but differed in phosphate contents. Another MMTV phosphoprotein, p21, was separated into four different phosphorylated species. Phosphorylation of p21 could be performed in vitro by the MMTV virion-associated protein kinase. This enzyme also has a high affinity for MMTV p30 as a substrate. Possible functions of this enzyme are discussed.

Structure and processing of the mouse mammary tumor virus glycoprotein precursor pr73env

The polyprotein precursor to the envelope glycoproteins of mouse mammary tumor virus was investigated by using subcellular fractionation procedures, pactomycin mapping techniques, tunicamycin inhibition of glycosylation, and endo-beta-N-acetyl glucosaminidase H-catalyzed removal of glycosylated residues in order to characterize the biosynthesis and processing of the precursor. The results suggest that the precursor (Pr73env) is synthesized on the rough endoplasmic reticulum as a transmembrane protein, with the carboxyl terminus remaining on the cytoplasmic side. The apoprotein as an estimated molecular weight of 60,000 and acquires five core oligosaccharide units during synthesis. Cleavage of the precursor precedes the secondary glycosylation steps and therefore probably occurs before transport to the plasma membrane. However, a minor population of Pr73env containing complex oligosaccharides was also found in the plasma membrane. The order of the glycoproteins in the precursor, as determined by pactomycin mapping, in NH2-gp52-gp36-COOH.