Robert L. Garcea

Seroepidemiology of human polyomaviruses.

In addition to the previously characterized viruses BK and JC, three new human polyomaviruses (Pys) have been recently identified: KIV, WUV, and Merkel Cell Py (MCV). Using an ELISA employing recombinant VP1 capsid proteins, we have determined the seroprevalence of KIV, WUV, and MCV, along with BKV and JCV, and the monkey viruses SV40 and LPV. Soluble VP1 proteins were used to assess crossreactivity between viruses. We found the seroprevalence (+/− 1%) in healthy adult blood donors (1501) was SV40 (9%), BKV (82%), JCV (39%), LPV (15%), KIV (55%), WUV (69%), MCV strain 350 (25%), and MCV strain 339 (42%). Competition assays detected no sero-crossreactivity between the VP1 proteins of LPV or MCV or between WUV and KIV. There was considerable sero-crossreactivity between SV40 and BKV, and to a lesser extent, between SV40 and JCV VP1 proteins. After correcting for crossreactivity, the SV40 seroprevalence was ∼2%. The seroprevalence in children under 21 years of age (n = 721) for all Pys was similar to that of the adult population, suggesting that primary exposure to these viruses likely occurs in childhood.

Structure of Small Virus-like Particles Assembled from the L1 Protein of Human Papillomavirus 16

The papillomavirus major late protein, L1, forms the pentameric assembly unit of the viral shell. Recombinant HPV16 L1 pentamers assemble in vitro into capsid-like structures, and truncation of ten N-terminal residues leads to a homogeneous preparation of 12-pentamer, icosahedral particles. X-ray crystallographic analysis of these particles at 3.5 A resolution shows that L1 closely resembles VP1 from polyomaviruses. Surface loops contain the sites of sequence variation among HPV types and the locations of dominant neutralizing epitopes. The ease with which small virus-like particles may be obtained from L1 expressed in E. coli makes them attractive candidate components of a papillomavirus vaccine. Their crystal structure also provides a starting point for future vaccine design.

DNA Sequences Similar to Those of Simian Virus 40 in Ependymomas and Choroid Plexus Tumors of Childhood

BACKGROUND Ependymomas and papillomas of the choroid plexus occur in early childhood. The ubiquitous human polyomaviruses, BK virus and JC virus, have been associated with the induction of these neoplasms in animal models. A related monkey polyomavirus, simian virus 40 (SV40), is highly tumorigenic in rodents and also induces choroid plexus papillomas. METHODS We tested the possibility that polyomaviruses were associated with these tumors in humans. Tumors from 31 children--20 with choroid plexus neoplasms and 11 with ependymomas--were evaluated for the presence of polyomavirus T-antigen gene sequences by means of amplification with the polymerase chain reaction. RESULTS Ten of the 20 choroid plexus tumors and 10 of the 11 ependymomas contained amplification products that preferentially hybridized to probes specific for SV40 viral DNA rather than BK or JC viral DNA. In two specimens, DNA sequencing demonstrated that the amplified sequence was identical to the sequence of that region of the SV40 gene. In three other specimens, amplification with SV40-specific primers revealed a 574-bp segment of the SV40 viral gene. In 7 of 11 tumors examined by immunohistochemical staining, viral T antigen was expressed in the nuclei of the neoplastic cells. CONCLUSIONS Half of the choroid plexus tumors and most of the ependymomas that we studied contained and expressed a segment of T-antigen gene related to SV40. These results suggest that SV40 or a closely related virus may have an etiologic role in the development of these neoplasms during childhood, as in animal models.

Self-assembly of purified polyomavirus capsid protein VP1

The polyomavirus major capsid protein VP1, purified after expression of the recombinant gene in E. coli, was isolated as oligomers resembling the dissociated capsomeres derived from viral capsids. Image analysis of low-dose electron micrographs demonstrates that these VP1 oligomers are exclusively pentamers. The purified VP1 pentamers associated to form capsid-like assemblies and polymorphic aggregates at high ionic strength. The capsid-like assemblies were stabilized at low ionic strength by the addition of calcium. Self-assembly of the unmodified, recombinant DNA-generated VP1 implies that the posttranslational charge modifications of VP1 and the minor virion protein components, VP2 and VP3, are not essential for capsid formation. The nonequivalently related subunits of the penta- and hexavalent capsomeres therefore must spontaneously switch their bonding specificity during assembly.

Polymorphism in the assembly of polyomavirus capsid protein VP1

Polyomavirus major capsid protein VP1, purified after expression of the recombinant gene in Escherichia coli, forms stable pentamers in low-ionic strength, neutral, or alkaline solutions. Electron microscopy showed that the pentamers, which correspond to viral capsomeres, can be self-assembled into a variety of polymorphic aggregates by lowering the pH, adding calcium, or raising the ionic strength. Some of the aggregates resembled the 500-A-diameter virus capsid, whereas other considerably larger or smaller capsids were also produced. The particular structures formed on transition to an environment favoring assembly depended on the pathway of the solvent changes as well as on the final conditions. Mass measurements from cryoelectron micrographs and image analysis of negatively stained specimens established that a distinctive 320-A-diameter particle consists of 24 close-packed pentamers arranged with octahedral symmetry. Comparison of this unexpected octahedral assembly with a 12-capsomere icosahedral aggregate and the 72-capsomere icosahedral virus capsid by computer graphics methods indicates that similar connections are made among trimers of pentamers in these shells of different size. The polymorphism in the assembly of VP1 pentamers can be related to the switching in bonding specificity required to build the virus capsid.

A cornucopia of human polyomaviruses

During the past 6 years, focused virus hunting has led to the discovery of nine new human polyomaviruses, including Merkel cell polyomavirus, which has been linked to Merkel cell carcinoma, a lethal skin cell cancer. The discovery of so many new and highly divergent human polyomaviruses raises key questions regarding their evolution, tropism, latency, reactivation, immune evasion and contribution to disease. This Review describes the similarities and differences among the new human polyomaviruses and discusses how these viruses might interact with their human host.

Taxonomical developments in the family Polyomaviridae

The Polyomaviridae Study Group of the International Committee on Taxonomy of Viruses (ICTV) has recommended several taxonomical revisions, as follows: The family Polyomaviridae, which is currently constituted as a single genus (Polyomavirus), will be comprised of three genera: two containing mammalian viruses and one containing avian viruses. The two mammalian genera will be designated Orthopolyomavirus and Wukipolyomavirus, and the avian genus will be named Avipolyomavirus. These genera will be created by the redistribution of species from the current single genus (Polyomavirus) and by the inclusion of several new species. In addition, the names of several species will be changed to reflect current usage.

Amino acid sequences that determine the nuclear localization of yeast histone 2B.

Histone-beta-galactosidase protein fusions were used to identify the domain of yeast histone 2B, which targets this protein to the nucleus. Amino acids 28 to 33 in H2B were required for nuclear localization of such fusion proteins and thus constitute a nuclear localization sequence. The amino acid sequence in this region (Gly-29 Lys Lys Arg Ser Lys Ala) is similar to the nuclear location signal in simian virus 40 large T antigen (Pro-126 Lys Lys Lys Arg Lys Val) (D. Kalderon, B.L. Roberts, W.D. Richardson, and A.E. Smith, Cell 39:499-509, 1984). A point mutation changing lysine 31 to methionine abolished nuclear localization of an H2B-beta-galactosidase fusion protein containing amino acids 1 to 33 of H2B. However, an H2B-beta-galactosidase fusion protein containing both this point mutation and the H2A interaction domain of H2B was nuclear localized. These results suggest that H2A and H2B may be cotransported to the nucleus as a heterodimer.

Immunization with a Pentameric L1 Fusion Protein Protects against Papillomavirus Infection

ABSTRACT The prophylactic papillomavirus vaccines currently in clinical trials are composed of viral L1 capsid protein that is synthesized in eukaryotic expression systems and purified in the form of virus-like particles (VLPs). To evaluate whether VLPs are necessary for effective vaccination, we expressed the L1 protein as a glutathioneS-transferase (GST) fusion protein in Escherichia coli and assayed its immunogenic activity in an established canine oral papillomavirus (COPV) model that previously validated the efficacy of VLP vaccines. The GST-COPV L1 fusion protein formed pentamers, but these capsomere-like structures did not assemble into VLPs. Despite the lack of VLP formation, the GST-COPV L1 protein retained its native conformation as determined by reactivity with conformation-specific anti-COPV antibodies. Most importantly, the GST-COPV L1 pentamers completely protected dogs from high-dose viral infection of their oral mucosa. L1 fusion proteins expressed in bacteria represent an economical alternative to VLPs as a human papillomavirus vaccine.

Chaperone-mediated in vitro assembly of Polyomavirus capsids

The polyomavirus coat protein viral protein 1 (VP1) has the intrinsic ability to self-assemble in vitro into polymorphic capsid-like structures on addition of calcium. In contrast, polyomavirus assembly in vivo is rigorously controlled, such that virions of uniform size are formed only in the cell nucleus. During viral infection, the 72 kDa cellular chaperone heat shock cognate protein (hsc70) binds VP1 posttranslation and colocalizes with VP1 to the nucleus, thereby suggesting a role for ≈70-kDa heat shock protein (hsp70) family chaperones in regulating the quality and location of capsid assembly. We found that, after expression of recombinant VP1 in Escherichia coli, the prokaryotic hsp70 chaperone DnaK copurified with the VP1 C-terminal domain that links pentamers in an assembled capsid. When stably bound to VP1, DnaK inhibited in vitro assembly induced by calcium. However, in the presence of ATP, the hsp70 chaperone system comprised of DnaK, DnaJ, and GrpE assembled VP1 into uniform capsids without requiring calcium. Chaperone-mediated assembly was similarly catalyzed by the eukaryotic hsc70 protein, in combination with the J-domain function of the simian virus 40 large T-antigen protein. Thus, polyomavirus capsid assembly can be recapitulated with high-fidelity in vitro using either prokaryotic or eukaryotic hsp70 chaperone systems, thereby supporting a role for cellular chaperones in the in vivo regulation of virion assembly.