VPg

VPg, short for genome-linked protein, is a small viral protein covalently bound to the 5' end of the RNA genome in several groups of positive-sense RNA viruses. It is characteristic of virus families such as Picornaviridae (for example poliovirus and rhinovirus), Caliciviridae, Astroviridae, and many plant viruses in the family Potyviridae. In these viruses the 5' terminal nucleotide of the RNA is linked to a specific residue in VPg by a phosphodiester bond, and the VPg molecule is subsequently used as a primer for RNA replication. During genome replication, VPg is uridylylated to form a VPg-pUpU primer, which is extended by the viral RNA-dependent RNA polymerase to synthesize new genomes. The exact mechanism and cofactors can vary among virus families.

In addition to its role in replication, VPg can influence translation. Some VPg-containing plant viruses recruit host translation factors to initiate translation of their RNA in a cap-independent manner, effectively substituting for the cellular 5' cap structure. The dual roles of VPg as primer for replication and as an adaptor for translation reflect the compact, multifunctional strategies of positive-sense RNA viruses.

The VPg region is encoded within the viral polyprotein and is released in a way that aligns with the replication cycle; its presence at the genome terminus is a diagnostic hallmark of these viruses and a focus of research into antiviral strategies.