Polyprotein

Lily Mottle virus genome map . The genome of this virus contains only one gene, for a polyprotein. This is cut into the following components by proteases at the points marked by wedges: P1 ( protease 1 ), HC ( helper component ), P3 ( protease 3 ), CI ( cylindrical inclusions protein ), VPg ( genomic virus protein ), Pro ( protease ), NI ( nuclear inclusions protein ), CP ( capsid / coat protein ). The interfaces of the proteases are shown as wedges.

A polyprotein is an intermediate stage that occurs during the multiplication of some viruses , from which the individual virus proteins are only released by splitting off. Polyproteins are found particularly in those viruses whose genes are arranged in a single open reading frame ( monocistronic mRNA ). During translation , a very long polyprotein is produced without the reading of individual gene segments being interrupted by a stop codon . In most cases, individual proteins are split off by proteases during the synthesis of the polyprotein (cotranslational processing). This cleavage can be carried out by viral proteases (partly autoproteolytic in trans ) or cellular proteases (for example the signal peptidase ). Polyproteins can also be important in the formation of capsids . In picornaviruses (VP1, VP2, VP3), the three subunits are assembled as a polyprotein, and only then are they split into the individual proteins.

Polyproteins are typical of RNA viruses with a single-stranded RNA as genome, for example picornaviruses, togaviruses and flaviviruses . The synthesis of individual virus proteins cannot be controlled or regulated via the detour of a polyprotein. There are always equimolar amounts of the proteins resulting from the polyprotein. However, since structural proteins (envelope proteins and capsid proteins) are necessary in larger quantities than functional proteins (e.g. polymerases ) during the virus replication , the deposition of structural proteins that are not required leads to clusters or deposits of these proteins. These become visible as viroplasm or inclusion bodies during the histological examination of the cells.

The cleavage of the polyprotein by proteases is used in the therapy of some viral infections through the use of specific inhibitors in order to interrupt virus replication at this point. These drugs are called protease inhibitors .

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• SJ Flint, LW Enquist, VR Racaniello, and AM Skalka: Principles of Virology. Molecular Biology, Pathogenesis, and Control of Animal Viruses. 2nd edition, ASM-Press Washington DC 2004, pp. 394-398 and 457f ISBN 1-55581-259-7