Nonstructural proteins

The majority of nonstructural proteins appear only within infected cells, although a few may be carried within the virion (or help in its formation, such as the scaffolding proteins described above) but they are not part of its architecture. The number and function of nonstructural proteins varies enormously from one virus family to another, depending on the complexity of the viral genome and the replication cycle. Many are enzymes with functions critical to virus replication (e.g. the reverse transcriptase, protease, and integrase of HIV, the thymidine kinase and DNA polymerase of HSV). Because they are different to cellular enzymes (even those with equivalent functions) they are key targets for antiviral drugs. Many HSV tegument proteins) have  regulatory roles  during the  phases of virus gene  transcription and  others form part of nucleic acid synthesis complexes (e.g. DNA helicase, DNA binding proteins) for genome replication. The shut-off of host cell macromolecular synthesis is usually the function of viral nonstructural proteins and  allows the cellular machinery to be devoted exclusively to virus replication; this  is of course a major cause of cell death during virus infection.

There are a number of virus proteins that can act to transform infected cells (i.e. to affect unscheduled mitosis). One such example is the family Papillomaviridae that promotes infected cell mitosis and hence leads to the formation of warts or in some cases tumors (as do the Polyomaviridae and some members of the Retroviridae). Some of these proteins act directly to stimulate mitosis (these are oncogenic proteins encoded by viral oncogenes), others activate cellular proto-oncogenes, encouraging production of the cell’s own mitotic stimulators, and yet others act to block cellular tumor suppressors, proteins that normally safeguard the cell against unwanted cell division. Nonstructural proteins have been identified with roles involving the rearrangement of the cellular cytoskeleton, with  inhibition of apoptosis (programmed cell death, a form  of resistance to infection) and the suppression or evasion of immune responses, transmission between hosts, and  with long-term (possibly lifelong) infection of a host  with a virus.