The synthesis and transport of SV40 structural proteins

Formation of covalently modified folding intermediates of simian virus 40 Vp1 in large T antigen-expressing cells

The folding and pentamer assembly of the simian virus 40 (SV40) major capsid protein Vp1, which take place in the infected cytoplasm, have been shown to progress through disulfide-bonded Vp1 folding intermediates. In this report, we further demonstrate the existence of another category of Vp1 folding or assembly intermediates: the nonreducible, covalently modified mdVp1s. These species were present in COS-7 cells that expressed a recombinant SV40 Vp1, Vp1ΔC, through plasmid transfection. The mdVp1s persisted under cell and lysate treatment and SDS-PAGE conditions that are expected to have suppressed the formation of artifactual disulfide cross-links. As shown through a pulse-chase analysis, the mdVp1s were derived from the newly synthesized Vp1ΔC in the same time frame as Vp1's folding and oligomerization. The apparent covalent modifications occurred in the cytoplasm within the core region of Vp1 and depended on the coexpression of the SV40 large T antigen (LT) in the cells. Analogous covalently modified species were found with the expression of recombinant polyomavirus Vp1s and human papillomavirus L1s in COS-7 cells. Furthermore, the mdVp1s formed multiprotein complexes with LT, Hsp70, and Hsp40, and a fraction of the largest mdVp1, md4, was disulfide linked to the unmodified Vp1ΔC. Both mdVp1 formation and most of the multiprotein complex formation were blocked by a Vp1 folding mutation, C87A-C254A. Our observations are consistent with a role for LT in facilitating the folding process of SV40 Vp1 by stimulating certain covalent modifications of Vp1 or by recruiting certain cellular proteins.

Association of simian virus 40 vp1 with 70-kilodalton heat shock proteins and viral tumor antigens

Proper folding of newly synthesized viral proteins in the cytoplasm is a prerequisite for the formation of infectious virions. The major capsid protein Vp1 of simian virus 40 forms a series of disulfide-linked intermediates during folding and capsid formation. In addition, we report here that Vp1 is associated with cellular chaperones (HSP70) and a cochaperone (Hsp40) which can be coimmunoprecipitated with Vp1. Studies in vitro demonstrated the ATP-dependent interaction of Vp1 and cellular chaperones. Interestingly, viral cochaperones LT and ST were essential for stable interaction of HSP70 with the core Vp1 pentamer Vp1 (22-303). LT and ST also coimmunoprecipitated with Vp1 in vivo. In addition to these identified (co)chaperones, stable, covalently modified forms of Vp1 were identified for a folding-defective double mutant, C49A-C87A, and may represent a "trapped" assembly intermediate. By a truncation of the carboxyl arm of Vp1 to prevent the Vp1 folding from proceeding beyond pentamers, we detected several apparently modified Vp1 species, some of which were absent in cells transfected with the folding-defective mutant DNA. These results suggest that transient covalent interactions with known or unknown cellular and viral proteins are important in the assembly process.

Formation of transitory intrachain and interchain disulfide bonds accompanies the folding and oligomerization of simian virus 40 Vp1 in the cytoplasm

Pentamer formation by Vp1, the major capsid protein of simian virus 40, requires an interdigitation of structural elements from the Vp1 monomers [Liddington, R. C., Yan, Y., Moulai, J., Sahli, R., Benjamin, T. L. & Harrison, S. C. (1991) Nature (London) 354, 278-284]. Our analyses reveal that disulfide-linked Vp1 homooligomers are present in the simian virus 40-infected cytoplasm and that they are derived from a 41-kDa monomeric intermediate containing an intrachain disulfide bond(s). The 41-kDa species, emerging within 5 min of pulse labeling with [(35)S]methionine, is converted into a 45-kDa, disulfide-free Vp1 monomer and disulfide-bonded dimers through pentamers. The covalent oligomer formation is blocked in the presence of a sulfhydryl-modifying reagent. We propose that there are two stages in this Vp1 disulfide bonding. First, the newly synthesized Vp1 monomers acquire intrachain bonds as they fold and begin to interact. Next, these bonds are replaced with intermolecular bonds as the monomers assemble into pentamers. This sequential appearance of transitory disulfide bonds is consistent with a role for sulfhydryl-disulfide redox reactions in the coordinate folding of Vp1 chains into pentamers. The cytoplasmic Vp1 does not colocalize with marker proteins of the endoplasmic reticulum. This paper demonstrates in vivo disulfide formations and exchanges coupled to the folding and oligomerization of a mammalian protein in the cytoplasm, outside the secretory pathway. Such disulfide dynamics may be a general phenomenon for other cysteine-bearing mammalian proteins that fold in the cytoplasm.

In vivo and in vitro association of hsc70 with polyomavirus capsid proteins

Members of the 70-kDa family of cellular stress proteins assit in protein folding by preventing inappropriate intra- and intermolecular interactions during normal protein synthesis and transport and when cells are exposed to a variety of environmental stresses. During infection of A31 mouse fibroblasts with polyomavirus, the constitutive form of hsp70, hsc70, coimmunoprecipitated with all three viral capsid proteins (VP1, VP2, and VP3). In addition, the subcellular location of hsc70 changed from cytoplasmic to nuclear late in polyomavirus infection, coincident with the nuclear localization of the viral capsid proteins. VP1 and VP2 expressed in Sf9 insect cells with recombinant baculovirus vectors also coimmunoprecipitated with an hsp70-like protein, and VP1 expressed in Escherichia coli coimmunoprecipitated with the hsp70 homolog DnaK. Capsid proteins expressed by in vitro translation coimmunoprecipitated with the hsc70 protein present in the reticulocyte translation extract. Therefore, the polyomavirus capsid proteins associate with hsc70 during virus infection as well as in recombinant protein expression systems. This association may play a role in preventing the premature assembly of capsids in the cytosol and/or in facilitating the nuclear transport of capsid protein complexes.

Expression of the polyomavirus minor capsid proteins VP2 and VP3 in Escherichia coli: in vitro interactions with recombinant VP1 capsomeres

The polyomavirus VP2 and VP3 capsid proteins were expressed in Escherichia coli. The majority of the expressed proteins were in an insoluble fraction, and they were extracted and initially purified in 8 M urea before renaturation. Soluble VP2 and VP3 were mixed with purified recombinant VP1 capsomeres, and their interactions were assayed by immunoprecipitation and ion-exchange chromatography. Coimmunoprecipitation could be demonstrated with antibodies to either VP1 or VP2/VP3. Mixing recombinant VP1 with VP2 and VP3 modified the recognition of VP1 by domain-specific antipeptide antibodies and altered the chromatographic behavior of the individual proteins. Similar results were observed when a truncated VP1 protein, delta NCOVP1, with 62 amino acids deleted from the carboxy terminus was mixed with VP2/VP3. After the mixing, equilibrium dissociation constants for their binding to either VP1 or delta NCOVP1 were determined to be 0.37 +/- 0.23 microM for VP2 and 0.18 +/- 0.21 microM for VP3. These studies demonstrate that the recombinant VP2 and VP3 proteins interact with VP1 to affect the biochemical properties of VP1 capsomeres and to change the epitope accessibility of VP1 pentamers. These changes may reflect conformational alterations in VP1 capsomeres which are necessary for viral genome encapsidation.

Functional complementation of nuclear targeting-defective mutants of simian virus 40 structural proteins

Structural proteins of simian virus 40 (SV40), Vp2 and Vp3 (Vp2/3) and Vp1, carry individual nuclear targeting signals, Vp3(198-206) (Vp2(316-324) and Vp1(1-8), respectively, which are encoded in different reading frames of an overlapping region of the genome. How signals coordinate nuclear targeting during virion morphogenesis was examined by using SV40 variants in which there is only one structural gene for Vp1 or Vp2/3, nuclear targeting-defective mutants thereof, Vp2/3(202T) and Vp1 delta N5, or nonoverlapping SV40 variants in which the genes for Vp1 and Vp2/3 are separated, and mutant derivatives of the gene carrying either one or both mutations. Nuclear targeting was assessed immunocytochemically following nuclear microinjection of the variant DNAs. When Vp2/3 and Vp1 mutants with defects in the nuclear targeting signals were expressed individually, the mutant proteins localized mostly to the cytoplasm. However, when mutant Vp2/3(202T) was coexpressed in the same cell along with wild-type Vp1, the mutant protein was effectively targeted to the nucleus. Likewise, the Vp1 delta N5 mutant protein was transported into the nucleus when wild-type Vp2/3 was expressed in the same cells. These results suggest that while Vp1 and Vp2/3 have independent nuclear targeting signals, additional signals, such as those defining protein-protein interactions, play a concerted role in nuclear localization along with the nuclear targeting signals of the individual proteins.

Import of simian virus 40 virions through nuclear pore complexes

How the DNA tumor virus, simian virus 40, reaches the nucleus is unknown. In this report we have tested the affinity of simian virus 40 toward the nucleus by microinjecting virion particles into the cytoplasm under conditions in which cell-surface-mediated viral infection was blocked. Subcellular localization of viral structural proteins Vp1, Vp2, and Vp3, large tumor antigen, and virion particles was followed immunocytochemically and ultrastructurally. Both virion particles and viral structural proteins localized in the nucleus within 1-2 hr after cytoplasmic injection and subsequently expressed large tumor antigen, which was detected in the nucleus as early as 3 hr after cytoplasmic injection. Vp1 and large tumor antigen nuclear accumulation, as well as virion nuclear entry, were blocked by wheat germ agglutinin and an anti-nucleoporin monoclonal antibody, mAb 414. Virion particles were visualized in the vicinity of nuclear pores and in the cytoplasm with this agent. We conclude that virion particles are karyophilic and enter through nuclear pores. This study suggests that virion structural proteins facilitate virion import into the nucleus and viral gene expression.

Two independent signals, a nuclear localization signal and a Vp1-interactive signal, reside within the carboxy-35 amino acids of SV40 Vp3

The carboxy-terminal 35 amino acids (numbering 199 to 234) of SV40 Vp3 are essential for the nuclear localization of the protein as well as for its interactions with Vp1. Here, we describe studies directed at the further mapping of these two functions. Deletion and site-directed mutants of Vp3 were created within both a eukaryotic transfection and an SP6 transcription vector which encode Vp3. The subcellular localization of mutant Vp3's was assayed by immunofluorescence microscopy following DNA transfections, and the Vp1-interactive determinant of Vp3 was mapped by a recently described eukaryotic in vitro translation/interaction system. We show that a plasmid-encoded wild-type Vp3, whose overlapping Vp1 coding segment has been removed by mutagenesis, continues to localize to the nucleus in the absence of any SV40 Vp1. Thus, Vp3 is capable of nuclear localization on its own. Modification of Lys-202 of Vp3 into Thr is sufficient to destroy the wild-type nuclear localization of the protein, but has no effect on its interactions with Vp1. Furthermore, deletion of the terminal 13 amino acids, 222 to 234, of Vp3 does not affect its wild-type nuclear localization, but is sufficient to destroy its interactions with Vp1. Thus, the Vp3 amino acids 199-221--specifically Lys-202--are important for its nuclear localization, while the Vp3 amino acids 222-234 play a role in its interactions with Vp1. Thus, the two functions, a Vp3 nuclear localization signal and a Vp1-interactive determinant, are spatially and functionally separable within the last 35 residues of Vp3 and are, hence, independent.

In vitro assay for protein-protein interaction: carboxyl-terminal 40 residues of simian virus 40 structural protein VP3 contain a determinant for interaction with VP1

Intermolecular interactions between polypeptide chains play essential roles in the functioning of proteins. We describe here an in vitro assay system for identifying and characterizing such interactions. Such interactions are difficult to study in vivo. We have translated synthetic, nonmethyl-capped RNAs in a cell-free protein-synthesizing system. The translation products were allowed to interact posttranslationally to form protein-protein complexes. The chemical nature of the protein interaction(s) was determined by coimmunoprecipitation of associating proteins, sedimentation through sucrose gradients, followed by NaDodSO4/polyacrylamide gel electrophoresis or by nonreducing NaDodSO4/polyacrylamide gel electrophoresis. The system has been utilized to show the self-assembly of monomeric VP1, the major structural protein of simian virus 40, into disulfide-linked pentamers and to show the noncovalent interaction of another structural protein, VP3, with VP1 at low monomer concentrations. Additionally, we show that the carboxyl-terminal 40 amino acids of VP3 are essential and sufficient for its interaction with VP1 in vitro. The in vitro assay system described here provides a method for identifying the domains involved in, and the molecular nature of, protein-protein interactions, which play an important role in such biological phenomena as replication, transcription, translation, transport, ligand binding, and assembly.

The carboxyl 35 amino acids of SV40 Vp3 are essential for its nuclear accumulation

To identify the moiety responsible for nuclear localization of the SV40 structural protein Vp3 in its natural environment, a transfection vector containing the entire coding regions of Vp2, Vp3, and agnoprotein, and one-third of the coding region of Vp1, was constructed. Several mutations were introduced into the plasmid and the subcellular distribution of Vp3 or mutant Vp3 was examined following DEAE-dextran-mediated DNA transfection into TC7 cells. Our study shows that Vp3 is synthesized and is transported into the nucleus in the absence of Vp2, agnoprotein, and intact Vp1. However, in the absence of its carboxyl-terminal 35 amino acids, the truncated Vp3 is limited to a cytoplasmic and perinuclear accumulation. Thus, the carboxyl 35 amino acids of Vp3 are required for its nuclear localization and may contain a nuclear accumulation signal.