Processing of the Semliki Forest virus structural polyprotein: role of the capsid protease

Biosynthesis, membrane translocation, and surface expression of Sindbis virus E1 glycoprotein

Sindbis virus glycoproteins PE2 (precursor of E2) and E1 are coded in this order by the same monocistronic mRNA, cotranslationally inserted in the endoplasmic reticulum membrane and then transported to the cell surface where the progeny virus is released by budding. In the virion, three E1 plus three E2 molecules form hexameric spike complexes. Previous work (S. Bonatti, G. Migliaccio, G. Blobel, and P. Walter (1984), Eur. J. Biochem. 140, 499-502) revealed a single signal sequence for cotranslational translocation located at the aminoterminus of PE2. We have generated progressive deletions of the coding region upstream of E1 and have engineered the resulting cDNAs in plasmids suitable for in vitro transcription/translation and in vivo expression. The results we obtained with this approach show that (i) internal signal sequence(s) are present upstream of E1, and this protein may be generated and inserted in the endoplasmic reticulum membrane independently of the PE2 signal sequence; and (ii) E1 is efficiently transported to the plasma membrane in the absence of PE2/E2; exit from the endoplasmic reticulum of E1 takes place with almost the same timing in the presence or in the absence of PE2.

Processing the nonstructural polyproteins of sindbis virus: nonstructural proteinase is in the C-terminal half of nsP2 and functions both in cis and in trans

The processing of the Sindbis virus nonstructural polyprotein translated in vitro has been studied. When Sindbis virus genomic RNA was translated in a reticulocyte lysate, polyprotein P123 was cleaved efficiently to produce nsP1, nsP2, and nsP3. Inhibition of this processing by anti-nsP2 antibodies, but not by antibodies specific for nsP1, nsP3, or nsP4, suggested that the viral proteinase was present in nsP2. To localize the proteolytic activity more precisely, deletions were made in a full-length cDNA clone of Sindbis virus, and RNA was transcribed from these constructs with SP6 RNA polymerase and translated in vitro. Although virtually all of the nsP1, nsP3, and nsP4 sequences could be deleted without affecting processing, deletions in the N-terminal half of nsP2 led to aberrant processing, and deletions in the C-terminal half abolished proteolysis. However, inactive polyproteins containing the nsP2 deletions could be processed by exogenously supplied proteins translated from virion RNA, demonstrating that cleavage was virus specific and not due to a protease present in the reticulocyte lysate and that the deleted polyproteins still served as substrates for the enzyme. From these results and from experiments in which processing was studied at increasingly higher dilution, we have concluded the following: (i) the viral nonstructural proteinase is located in the C-terminal half of nsP2; (ii) in the P123 precursor the cleavage between nsP2 and nsP3 occurs efficiently as a bimolecular reaction (in trans) to remove nsP3, while the bond between nsP1 and nsP2 is cleaved inefficiently, but detectably, in trans, but no autoproteolysis of P123 was detected; (iii) once nsP3 has been removed, the bond between nsP1 and nsP2 in the P12 precursor is cleaved efficiently by autoproteolysis (in cis). This mode of processing leads to a slow rate of cleavage, particularly early in infection, suggesting that the polyproteins might play roles in virus RNA replication distinct from those of the cleaved products. A hypothesis is presented that the proteinase is a thiol protease related to papain.

Sindbis virus core protein crystals

The core protein of Sindbis virus has been crystallized. Three different crystal forms have been observed. They diffract variously from 2.5 A to 3.5 A resolution.

Semliki Forest virus capsid protein acts as a pleiotropic regulator of host cellular protein synthesis

The Semliki Forest virus capsid (C) protein was introduced into various target cells by electroporation-, liposome-, and erythrocyte-ghost-mediated delivery. Data are presented which show that the incorporated C protein is biologically active and, at low concentrations (10(3) to 10(4) molecules per cell), markedly induces host cellular protein synthesis (average value, up to 90%). On the other hand, high concentrations (10(5) to 10(6) molecules per cell) led to a significant inhibition (average value, up to 60%). The cellular response to C protein was found to be identical in P3X63Ag8 suspension cells, CV-1 cells, and GpBind4 cells. Following electroporation-mediated delivery of C-protein molecules, both induction and repression of cellular protein synthesis were immediate, whereas with liposome-mediated delivery these events were delayed by about 1 h. Maximum stimulation and repression occurred between 0 and 1 h after delivery of C protein and decreased thereafter to reach control values at about 4 h. The analysis of the proteins synthesized suggests that low amounts of microinjected C protein are responsible for the induction of classes with specific Mrs, whereas high amounts lead to an inhibition of overall protein synthesis.

Influenza C virus esterase: analysis of catalytic site, inhibition, and possible function

The active site serine of the acetylesterase of influenza C virus was localized to amino acid 71 of the hemagglutinin-esterase protein by affinity labeling with 3H-labeled diisopropylfluorophosphate. This serine and the adjacent amino acids (Phe-Gly-Asp-Ser) are part of a consensus sequence motif found in serine hydrolases. Since comparative analysis failed to reveal esterase sequence similarities with other serine hydrolases, we suggest that this viral enzyme is a serine hydrolase constituting a new family of serine esterases. Furthermore, we found that the influenza C virus esterase was inhibited by isocoumarin derivatives, with 3,4-dichloroisocoumarin being the most potent inhibitor. Addition of this compound prevented elution of influenza C virus from erythrocytes and inhibited virus infectivity, possibly through inhibition of virus entry into cells.

Expression of Sindbis virus 26S cDNA in Spodoptera frugiperda (Sf9) cells, using a baculovirus expression vector

To study protein processing in an insect Spodoptera frugiperda (fall armyworm; Sf9) cell line, a 26S cDNA encoding the sequence of Sindbis virus structural proteins (capsid protein, of 30 kilodaltons [kDa]; p62 [the precursor of E3 and E2], of 62 kDa; a 6-kDa peptide; and the E1 protein, of 56 kDa) was inserted into the genome of Autographa californica nuclear polyhedrosis virus (AcNPV) adjacent to the polyhedrin promoter. By immunoblot analysis with antisera directed against whole Sindbis virus and the individual structural proteins (capsid, E2, and E1), we have shown that polypeptides similar in size and antigenicity to those synthesized in Sindbis virus-infected BHK cells are expressed in Sf9 cells infected with the recombinant baculovirus Ac373-SV26. By pulse-chase labeling in the presence or absence of tunicamycin, by endo-beta-N-acetylglucosaminidase H (endo-H) treatment of the recombinant glycoproteins, and by N-terminal sequence analysis of the E1 envelope glycoprotein, we have further shown that the 26S transcription translation unit of Sindbis virus, although normally encoded by nonnuclear RNA, is expressed and proteolytically cleaved similarly, if not identically, in Sf9 cells as compared with BHK cells when a baculovirus expression vector is used.

Sequence of the region coding for virion proteins C and E2 and the carboxy terminus of the nonstructural proteins of rubella virus: comparison with alphaviruses

The sequence of the 3' 4508 nucleotides (nt) of the genomic RNA of the Therien strain of rubella virus (RV) was determined for cDNA clones. The sequence contains a 3189-nt open reading frame (ORF) which codes for the structural proteins C, E2 and E1. C is predicted to have a length of 300 amino acids (aa). The N-terminal half of the C protein is highly basic and hydrophilic in nature, and is putatively the region of the protein which interacts with the virion RNA. At the C terminus of the C protein is a stretch of 20 hydrophobic aa which also serves as the signal sequence for E2, indicating that the cleavage of C from the polyprotein precursor may be catalyzed by signalase in the lumen of the endoplasmic reticulum. E2 is 282 aa in length and contains four potential N-linked glycosylation sites and a putative transmembrane domain near its C terminus. The sequence of E1 has been previously described [Frey et al., Virology 154 (1986) 228-232]. No homology could be detected between the amino acid sequence of the RV structural proteins and the amino acid sequence of the alphavirus structural proteins. From the position of a region of 30 nt in the RV genomic sequence which exhibited significant homology with the sequence in the alphavirus genome at which subgenomic RNA synthesis is initiated, the RV subgenomic RNA is predicted to be 3346 nt in length and the nontranslated region from the 5' end of the subgenomic RNA to the structural protein ORF is predicted to be 98 nt. In a different translation frame beginning at the 5' end of the RV nt sequence reported here is a 1407 nt ORF which is the C terminal region of the nonstructural protein ORF. This ORF overlaps the structural protein ORF by 149 nt. A low level of homology could be detected between the predicted amino acid sequence of the C-terminus of the RV nonstructural protein ORF and the replicase proteins of several positive RNA viruses of animals and plants, including nsp4 of the alphaviruses, the protein encoded by the C-terminal region of the alphavirus nonstructural ORF. However, the overall homology between RV and the alphaviruses in this region of the genome was only 18%, indicating that these two genera of the Togavirus family are only distantly related. Intriguingly, there is a 2844-nt ORF present in the negative polarity orientation of the RV sequence which could encode a 928-aa polyprotein.