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The term viral protein refers to both the products of the genome of a virus and any host proteins incorporated into the viral particle. Viral proteins are grouped according to their functions, and groups of viral proteins include structural proteins , nonstructural proteins , regulatory proteins , and accessory proteins. [ 1 ]
It is the first step of viral replication. Some viruses attach to the cell membrane of the host cell and inject its DNA or RNA into the host to initiate infection. Attachment to a host cell is often achieved by a virus attachment protein that extends from the protein shell (), of a virus.
Each type of protein is a specialist that usually only performs one function, so if a cell needs to do something new, it must make a new protein. Viruses force the cell to make new proteins that the cell does not need, but are needed for the virus to reproduce. Protein synthesis consists of two major steps: transcription and translation. [34]
Replication involves the synthesis of viral messenger RNA (mRNA) from "early" genes (with exceptions for positive-sense RNA viruses), viral protein synthesis, possible assembly of viral proteins, then viral genome replication mediated by early or regulatory protein expression. This may be followed, for complex viruses with larger genomes, by ...
The viral genome is composed of RNA, which enters the cell through receptor-mediated endocytosis. From there, the RNA acts as a template for complementary RNA synthesis. The complementary strand acts as a template for the production of new viral genomes that are packaged and released from the cell ready to infect more host cells.
The viral replication, protein synthesis and assembly require a considerable amount of energy, provided by large clusters of mitochondria at the periphery of viroplasms. The virus factory is often enclosed by a membrane derived from the rough endoplasmic reticulum or by cytoskeletal elements .
The classification of viral proteins as early proteins or late proteins depends on their relationship with genome replication.While many viruses (such as HIV) are described as expressing early and late proteins, this definition of these terms is commonly reserved for class I DNA viruses.
One of the proteins specified by the coronavirus genome is a non-structural protein, nsp14, that is a 3’-to-5’ exoribonuclease (ExoN). This protein resides in the protein complex nsp10-nsp14 that enhances replication fidelity by proofreading RNA synthesis, an activity critical for the virus life cycle. [10]