The classification of viral

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

as early proteins or

late proteins A late protein is a viral protein that is formed after replication of the virus. One example is VP4 from simian virus 40 (SV40). In Human papillomaviruses In Human papillomavirus (HPV), two late proteins are involved in capsid formation: a maj ...

depends on their relationship with

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...

replication. While many viruses (such as

HIV The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune ...

) are described as expressing early and

late Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

proteins, this definition of these terms is commonly reserved for class I DNA viruses. (HIV has two stages of protein expression but these are not as a result of two stages of transcription surrounding replication but by the production of the Rev protein which is required for the export of the transcripts of the second set of proteins transcribed form the

cell nucleus The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...

.) Early proteins are those produced following entry into the host cell but prior to replication. The expression of early genes, commonly encoding non-structural proteins, initiates replication of the genome and expression of late genes. In some, simpler viruses, this pattern of expression is clearly defined, while in those with more complex genomes, such as the herpesviruses, these expression periods overlap.

Example

An example of early gene expression is the expression of the small, middle and

large T antigen The large tumor antigen (also called the large T-antigen and abbreviated LTag or LT) is a protein encoded in the genomes of polyomaviruses, which are small double-stranded DNA viruses. LTag is expressed early in the infectious cycle and is essent ...

encoded by the polyomavirus. The middle T antigen is not required for replication and it acts to enhance transcription by binding host proteins which interact with the late promoter. On the other hand, the large T antigen is required and it acts to initiate replication directly. It binds the viral origin of replication and recruits

DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create ...

and s/s DNA-binding protein such that once its concentration is great enough it blocks the transcription of early genes and initiates genome replication. It also acts to cause the entry of the host cell into S phase.

References

{{Viral proteins Viral protein class