Complete sequence of the Rous sarcoma virus env gene: identification of structural and functional regions of its product

E Hunter, E Hill, M Hardwick, A Bhown, D E Schwartz, R Tizard
1983 Journal of Virology  
The amino-terminal amino acid sequences of gp85 and gp37, the envelope glycoproteins of Rous sarcoma virus (RSV), were determined. Alignment of these sequences with the amino acid sequence predicted from the complete nucleotide sequence of the Prague strain of RSV, subgroup C (PR-C), has allowed us to delineate the env gene-coding region of this virus. The coding sequences for gp85 and gp37 have been placed in an open reading frame that extends from nucleotide 5045 to nucleotide 6862 and
more » ... ide 6862 and predict sizes of 341 amino acids (36,962 molecular weight) for gp85 and 198 amino acids (21,566 molecular weight) for gp37. Carbohydrate makes a significant contribution to the observed molecular weights of these polypeptides-the amino acid sequence contains 14 potential glycosylation sites (Asn-X-Ser/Thr) in gp85 and two in gp37. Experiments aimed at estimating the number of carbohydrate side chains yielded results consistent with most or all of these sites being occupied. Although an initiation codon is located early (codon 4) in the open reading frame, it is likely that splicing yields an mRNA on which translation initiates at the same AUG as that of the gag gene to produce a nascent polypeptide in which gp85 is preceded by a 62-amino-acid-long leader peptide. This leader contains the hydrophobic sequence (signal sequence) necessary for translocation across the endoplasmic reticulum and is completely removed from the env gene product during translation. The polyprotein precursor, pr95env is cleaved to gp85 and gp37 at the carboxyl side of the basic sequence:-Arg-Arg-Lys-Arg-. gp85 is attached through a disulphide linkage to gp37, and although the positions of the cysteines involved in this linkage are not known, the presence of a 27-amino-acid-long hydrophobic region at the carboxy-terminus of gp37 is consistent with its role as a membrane anchor for the viral glycoprotein complex. The location of host range variable regions with respect to the possible tertiary structure of the complex is discussed. The env gene of Rous sarcoma virus (RSV) codes for two glycosylated polypeptides, gp85 and gp37, that are inserted into the lipid bilayer of the virion. The two glycoproteins are linked via a disulphide bond to yield the so-called VGP complex of the virion (50), which in electron micrographs can be observed as a knobbed spike; gp85 represents the knob and gp37 the spike (3, 9, 10) . Both polypeptides are susceptible to protease digestion which generates noninfectious, "bald" particles (68). The viral glycoproteins function in viral attachment to and penetration of the host cell where they interact with specific surface receptors coded for by the target cell (16, 60, 82) . The sequence encoding the host range determinants appears to reside within gp85 (33, 38), the mole-cule to which neutralizing antibodies are primarily elicited in animals (81). The mRNA for the envelope glycoproteins is a spliced molecule that retains the 5' terminal sequences of the genomic RNA but which has lost over 4,000 nucleotides of the gag and pol sequences (32, 54, 86) . This RNA is translated on membrane-bound polyribosomes (51, 66) to yield a glycosylated polyprotein precursor, Pr95env, that is later cleaved proteolytically to the two virion structural proteins (14, 22, 31, 56) . Pactamycin mapping experiments indicate the order NH2-gp85-gp37-COOH on Pr95env (41, 74) . This precursor is highly glycosylated, since in the presence of the glycosylation inhibitor tunicamycin (TM), the in vivo translation product of the env gene is a molecule of approximate-920 on May 8, 2020 by guest
doi:10.1128/jvi.46.3.920-936.1983 fatcat:vxqjiqeqgnfyzepxxaqoqdeflq