Polyomaviridae - Replication

Replication

Prior to genome replication, the processes of viral attachment, entry and uncoating occur. Cellular receptors for polyomaviruses are sialic acid residues of gangliosides. The attachment of polyomaviruses to host cells is mediated by viral protein 1 (VP1) via the sialic acid attachment region. This can be confirmed as anti-VP1 antibodies have been shown to prevent the binding of polyomavirus to host cells.

Polyomavirus virions are subsequently endocytosed and transported first to the endoplasmatic reticulum where a conformational change occurs revealing Vp2. Then by an unknown mechanism the virus is exported to the nucleus.

Polyomaviruses replicate in the nucleus of the host. They are able to utilise the host’s machinery because the genomic structure is homologous to that of the mammalian host. Moreover, the promoter sequence of Polyomavirus' promoter sequence is a strong attractant for the host's RNAP. Viral replication occurs in two distinct phases; early and late gene expression, separated by genome replication.

Early gene expression is responsible for the synthesis of non-structural proteins. Since Polyomaviruses rely on the host to control both the gene expression, the role of the non-structural proteins is to regulate the cellular mechanisms. Close to the N terminal end of polyomavirus genome are enhancer elements which induce activation and transcription of a molecule known as the T-antigen (see SV40 Large T-antigen). Early mRNA’s, encoding T-antigen are produced by host RNA polymerase II. T-antigen autoregulates early mRNA’s, subsequently leading to elevated levels of T-antigen. At high concentrations of T-antigen, early gene expression is repressed, triggering the late phase of viral infection to begin.

Genome replication acts to separate the early and late phase gene expression. The duplicated viral genome is synthesised and processed as if it were cellular DNA, exploiting the host’s machinery. As the daughter viral DNA are synthesised they associate with cellular nucleosomes to form structures that are often referred to as "minichromosomes". In this manner the DNA is packaged more efficiently.

Late gene expression synthesises the structural proteins, responsible for the viral particle composition. This occurs during and after genome replication. As with the early gene expression products, late gene expression generates an array of proteins as a result of alternative splicing.

Within each viral protein are 'nuclear localization signals' which cause the viral proteins to amass in the nucleus. Assembly of new virus particles consequently occurs within the nucleus of the host cell.

Release of newly synthesized polyomavirus particles exit the infected cell by one of two mechanisms. Firstly and less commonly, they are transported in cytoplasmic vacuoles to the plasma membrane, where budding occurs. More frequently, they are released when the cell lyses due to the cytotoxicity of virus particles present in the infected cell.