Purification of immature cores of mouse mammary tumor virus and immunolocalization of protein domains

Distinct Particle Morphologies Revealed through Comparative Parallel Analyses of Retrovirus-Like Particles

The Gag protein is the main retroviral structural protein, and its expression alone is usually sufficient for production of virus-like particles (VLPs). In this study, we sought to investigate-in parallel comparative analyses-Gag cellular distribution, VLP size, and basic morphological features using Gag expression constructs (Gag or Gag-YFP, where YFP is yellow fluorescent protein) created from all representative retroviral genera: Alpharetrovirus, Betaretrovirus, Deltaretrovirus, Epsilonretrovirus, Gammaretrovirus, Lentivirus, and Spumavirus. We analyzed Gag cellular distribution by confocal microscopy, VLP budding by thin-section transmission electron microscopy (TEM), and general morphological features of the VLPs by cryogenic transmission electron microscopy (cryo-TEM). Punctate Gag was observed near the plasma membrane for all Gag constructs tested except for the representative Beta- and Epsilonretrovirus Gag proteins. This is the first report of Epsilonretrovirus Gag localizing to the nucleus of HeLa cells. While VLPs were not produced by the representative Beta- and Epsilonretrovirus Gag proteins, the other Gag proteins produced VLPs as confirmed by TEM, and morphological differences were observed by cryo-TEM. In particular, we observed Deltaretrovirus-like particles with flat regions of electron density that did not follow viral membrane curvature, Lentivirus-like particles with a narrow range and consistent electron density, suggesting a tightly packed Gag lattice, and Spumavirus-like particles with large envelope protein spikes and no visible electron density associated with a Gag lattice. Taken together, these parallel comparative analyses demonstrate for the first time the distinct morphological features that exist among retrovirus-like particles. Investigation of these differences will provide greater insights into the retroviral assembly pathway.

IMPORTANCE

Comparative analysis among retroviruses has been critically important in enhancing our understanding of retroviral replication and pathogenesis, including that of important human pathogens such as human T-cell leukemia virus type 1 (HTLV-1) and HIV-1. In this study, parallel comparative analyses have been used to study Gag expression and virus-like particle morphology among representative retroviruses in the known retroviral genera. Distinct differences were observed, which enhances current knowledge of the retroviral assembly pathway.

The noncanonical Gag domains p8 and n are critical for assembly and release of mouse mammary tumor virus

The mouse mammary tumor virus (MMTV) Gag contains the unique domains pp21, p3, p8, and n. We investigated the contribution of these domains to particle assembly and found that the region spanning the p8 and n domains is critical for shape determination and assembly. Deletion of pp21 and p3 reduced the number of released particles, but deletion of the n domain resulted in frequent formation of aberrant particles, while deletion of p8 severely impaired assembly. Further investigation of p8 revealed that both the basic and the proline-rich motifs within p8 contribute to MMTV assembly.

Cell-free assays for HIV-1 uncoating

Uncoating is an essential step in the retrovirus life cycle about which little is known. Uncoating is defined as the specific dissociation of the capsid shell from the viral core in the host cell cytoplasm. In this chapter, biochemical assays for studying HIV-1 uncoating in vitro are described. These techniques have proven useful for characterizing HIV-1 mutants that exhibit defects in the uncoating step of infection.

Localization of self-interacting domains within betaretrovirus Gag polyproteins

The Betaretrovirus genus is characterized by the ability to preassemble immature capsids within the cytoplasm. For Mason-Pfizer monkey virus (M-PMV) this ability depends in part upon the unique Internal Scaffold Domain (ISD) within the p12 region of Gag. In this study, we have further characterized the ability of M-PMV p12 to promote Gag-Gag interaction and have examined the Gag polyprotein of the related mouse mammary tumor virus (MMTV) to potentially identify a region with equivalent function. Using the yeast two-hybrid system, we confirmed that both Gag polyproteins strongly interact, primarily through the CA-NC regions, but also through additional domains N-terminal to CA. For M-PMV, this auxiliary interaction domain was p12. For MMTV, no single strongly self-interacting protein was identified. Instead, MMTV Gag appears to utilize the weak contributions of several protein domains to support the main interaction of its CA-NC. Our findings suggest that, in addition to the canonical NC "I-domain" interaction, MMTV Gag self-association results from the concerted action of multiple regions of the polyprotein while M-PMV Gag relies mainly on its p12 domain.

Biochemical and structural analysis of isolated mature cores of human immunodeficiency virus type 1

Mature human immunodeficiency virus type 1 (HIV-1) particles contain a cone-shaped core structure consisting of the internal ribonucleoprotein complex encased in a proteinaceous shell derived from the viral capsid protein. Because of their very low stability after membrane removal, HIV-1 cores have not been purified in quantities sufficient for structural and biochemical analysis. Based on our in vitro assembly experiments, we have developed a novel method for isolation of intact mature HIV-1 cores. Concentrated virus suspensions were briefly treated with nonionic detergent and immediately centrifuged in a microcentrifuge for short periods of time. The resuspended pellet was subsequently analyzed by negative-stain and thin-section electron microscopy and by immunoelectron microscopy. Abundant cone-shaped cores as well as tubular and aberrant structures were observed. Stereo images showed that core structures preserved their three-dimensional architecture and exhibited a regular substructure. Detailed analysis of 155 cores revealed an average length of ca. 103 nm, an average diameter at the base of ca. 52 nm, and an average angle of 21.3 degrees. There was significant variability in all parameters, indicating that HIV cores are not homogeneous. Immunoblot analysis of core preparations allowed semiquantitative estimation of the relative amounts of viral and cellular proteins inside the HIV-1 core, yielding a model for the topology of various proteins inside the virion.

In vitro assembly of virus-like particles with Rous sarcoma virus Gag deletion mutants: identification of the p10 domain as a morphological determinant in the formation of spherical particles

Retroviruses are unusual in that expression of a single protein, Gag, leads to budding of virus-like particles into the extracellular space. We have developed conditions under which virus-like particles are formed spontaneously in vitro from fragments of Rous sarcoma virus (RSV) Gag protein purified after expression in Escherichia coli. The CA-NC fragment of Gag was shown previously to assemble into hollow cylinders (S. Campbell and V. M. Vogt, J. Virol. 69:6487-6497, 1995). We have now extended these studies to larger Gag proteins. In every case examined, assembly into regular structures required RNA. A nearly full-length Gag missing only the C-terminal PR domain, as well as similar proteins missing in addition the N-terminal half of MA, the C-terminal half of MA, the entire MA sequence, or the entire p2 sequence, all assembled into spherical particles resembling RSV in size. By contrast, proteins missing p10 assembled into cylindrical particles like those formed by CA-NC alone. Thin section electron microscopy showed that each of these Gag proteins formed in the expressing E. coli cells particles similar in shape to those seen in vitro. We conclude from these results that neither the sequences required for membrane binding in vivo, near the N terminus of Gag, nor the sequences required for a late step in budding, in the p2 portion of Gag, are essential for formation of virus-like particles in this system. Furthermore, we postulate the existence of a shape-determining sequence in p10, which provides or facilitates interactions required for the growing particle to be constrained to a spherical shape.

Protein composition and morphology of human foamy virus intracellular cores and extracellular particles

Characterization of human foamy virus (HFV) gag-encoded precursors and the search for a Gag-Pol polyprotein and mature proteins derived from proteolytic processing were carried out in HFV-infected cells and with purified preassembled cores and extracellular virus by Western blotting and radioimmunoprecipitation using antisera against synthetic peptides corresponding to putative Gag and protease proteins. Precursor proteins, Pr78gag/74gag and Pr135pol, were found in the nucleus of epithelial and fibroblast cells 3-4 days after HFV infection. Kinetic analysis of HFV Pr78gag and Pr74gag indicated that Pr78gag is a precursor to Pr74gag. South-Western blot analysis indicated that Pr78gag and Pr74gag have properties associated with nucleic acid binding protein although they lack the typical zinc-finger motifs found in retroviral nucleocapsid proteins. Western blot analyses of preassembled HFV cores isolated from the cytoplasm of infected cells and purified by sucrose gradient centrifugation demonstrated the presence of Pr78gag/74gag and Pr135pol, but no proteolytically processed Gag proteins were observed. The majority of extracellular HFV particles were found to have pentagon-shaped cores, as observed intracellularly, and are believed to be the immature extracellular form of the virus. The highest concentration of extracellular particles, estimated by EM, Western blot, and reverse transcriptase assays were found in sucrose gradient fractions having a density of 1.21-1.24 g/cm3. Western blot analysis revealed that Pr78gag/74gag and Pr135pol were the major viral proteins associated with these extracellular particles, as only small amounts of putative proteolytically cleaved capsid (p32) were observed. Our results support the notion that Pol is translated independent of Gag in HFV-infected cells.

Intracellular transport of the murine leukemia virus during acute infection of NIH 3T3 cells: nuclear import of nucleocapsid protein and integrase

The entry and intracellular transport of Moloney-murine leukemia virions inside mouse NIH 3T3 cells have been followed by electron microscopy techniques. Five viral proteins--matrix (MA, p15), capsid (CA, p30), nucleocapsid (NC, p10), integrase (IN), and the envelope glycoprotein (SU, gp70)--were located by immunolabeling using gold probes. After entering the cells, viral particles were frequently detected inside cytoplasmic vesicles of variable size. Their viral envelope was apparently lost during intracytoplasmic transport. When the unenveloped viral cores reached the nuclear membrane or its vicinity, they were disrupted. Two of the immunolabeled proteins, NC and IN, were detected entering the nucleus of non-dividing cells, where both were targeted to the nucleolus. However, MA and CA were found only in the cytoplasm. NC is a nucleic acid-binding protein which contains potential nuclear localization signals. We suggest that NC could enter the nucleus as part of a nucleoprotein complex, associated with IN, and possibly, also with viral DNA.

Membrane protein molecules of transmissible gastroenteritis coronavirus also expose the carboxy-terminal region on the external surface of the virion

The binding domains of four monoclonal antibodies (MAbs) specific for the M protein of the PUR46-MAD strain of transmissible gastroenteritis coronavirus (TGEV) have been located in the 46 carboxy-terminal amino acids of the protein by studying the binding of MAbs to recombinant M protein fragments. Immunoelectron microscopy using these MAbs demonstrated that in a significant proportion of the M protein molecules, the carboxy terminus is exposed on the external surface both in purified viruses and in nascent TGEV virions that recently exited infected swine testis cells. The same MAbs specifically neutralized the infectivity of the PUR46-MAD strain, indicating that the C-terminal domain of M protein is exposed on infectious viruses. This topology of TGEV M protein probably coexists with the structure currently described for the M protein of coronaviruses, which consists of an exposed amino terminus and an intravirion carboxy-terminal domain. The presence of a detectable number of M protein molecules with their carboxy termini exposed on the surface of the virion has relevance for viral function, since it has been shown that the carboxy terminus of M protein is immunodominant and that antibodies specific for this domain both neutralize TGEV and mediate the complement-dependent lysis of TGEV-infected cells.

Efficient in vivo and in vitro assembly of retroviral capsids from Gag precursor proteins expressed in bacteria

The capsid precursor protein (Gag) of Mason-Pfizer monkey virus, the prototype type D retrovirus, has been expressed to high levels in bacteria under the control of the phage T7 promoter. Electron microscopic studies of induced cells revealed the assembly of capsid-like structures within inclusion bodies that formed at the poles of the cells 6 h after induction with isopropyl-beta-D-thiogalactopyranoside (IPTG). The inclusion bodies and enclosed capsid-like structures were solubilized completely in 8 M urea, but following renaturation, we observed assembly in vitro of capsid-like structures that demonstrated apparent icosahedral symmetry. These results demonstrate for the first time that retroviral capsid precursors have the propensity to self-assemble in vitro and point to new approaches for the analysis of retroviral assembly and structure.