Abstract
Most DNA viruses replicate in the cell nucleus, although the specific sites of virion assembly are as yet poorly defined. Electron microscopy on freeze-substituted, plastic-embedded sections of murine polyomavirus (PyV)-infected 3T3 mouse fibroblasts or mouse embryonic fibroblasts (MEFs) revealed tubular structures in the nucleus adjacent to clusters of assembled virions, with virions apparently “shed” or “budding” from their ends. Promyelocytic leukemia nuclear bodies (PML-NBs) have been suggested as possible sites for viral replication of polyomaviruses (BKV and SV40), herpes simplex virus (HSV), and adenovirus (Ad). Immunohistochemistry and FISH demonstrated co-localization of the viral T-antigen (Tag), PyV DNA, and the host DNA repair protein MRE11, adjacent to the PML-NBs. In PML−/− MEFs the co-localization of MRE11, Tag, and PyV DNA remained unchanged, suggesting that the PML protein itself was not responsible for their association. Furthermore, PyV-infected PML−/− MEFs and PML−/− mice replicated wild-type levels of infectious virus. Therefore, although the PML protein may identify sites of PyV replication, neither the observed “virus factories” nor virus assembly were dependent on PML. The ultrastructure of the tubes suggests a new model for the encapsidation of small DNA viruses.
Highlights
Increasing evidence suggests that the assembly of many viruses occurs at specific intracellular sites, which have been termed ‘‘virus factories’’ [1,2]
The ultrastructure of the factories has been determined for a number of RNA viruses that assemble in the cytoplasm. Specific membrane compartments such as the ER appear to be co-opted and re-configured such that the surface of the membrane is used as a scaffold, where viral replication is spatially juxtaposed with capsid proteins delivered to these locations [3,4,5]
In infection full virions were somewhat dispersed throughout the volume of the nucleus and not yet arranged in clusters, they were closely associated with tubular structures (Figure 1A and inset)
Summary
Increasing evidence suggests that the assembly of many viruses occurs at specific intracellular sites, which have been termed ‘‘virus factories’’ [1,2]. These subcellular domains are gathering points for coordinating genome replication and capsid protein assembly into virions. Specific membrane compartments such as the ER appear to be co-opted and re-configured such that the surface of the membrane is used as a scaffold, where viral replication is spatially juxtaposed with capsid proteins delivered to these locations [3,4,5] Such scaffolds likely have a significant kinetic impact on virion assembly. Structural information concerning assembly sites for DNA viruses that replicate and assemble in the nucleus is less clear, perhaps due to the intrinsic complexity and dynamic nature of the nuclear architecture
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
