Gag-onc fusion protein

The gag-onc fusion protein (also written as Gag-v-Onc, with "v" indicating that the Onc sequence resides in a viral genome) is a general term for a fusion protein formed from a group-specific antigen ('gag') gene and that of an oncogene ('onc'), a gene that plays a role in the development of a cancer. Onc is a generic placeholder for a given specific oncogene, such as C-jun. (In the case of a fusion with C-jun, the resulting "gag-jun" protein is known alternatively as p65).

Gag genes are part of a general architecture for retroviruses, viruses that replicate through reverse transcription, where the gag region of the genome encodes proteins that constitute the matrix, capsid and nucleocapsid of the mature virus particles. Like in HIV's replication cycle, these proteins are needed for viral budding from the host cell's plasma membrane, where the fully formed virions leave the cell to infect other cells.

As a specific case, a Gag-v-Onc fusion protein from the Rous sarcoma virus is useful in illustrating the dual role that the fusion protein plays in both the viral and host cellular life cycle. For example, the viral gene Src (as in "sarcoma") is not necessary for viral reproduction, but does affect virulence. Due to evidence of conserved homology between the v-Src gene and its host (animal) genomes, and its non-essential status for viral reproduction, the v-Src gene is likely to have been acquired from a host genome and altered by subsequent mutations. These subsequent mutations are responsible for the oncogenic capabilities of the virus, as the normal (host) version of the Src gene, c-Src promotes survival, angiogenesis, proliferation and invasion pathways. These native pathways are disrupted in the presence of the mutant Src gene (v-Src) such that oncogenesis becomes more likely for the infected host cells, since the v-Src gene is translated into a functionally distinct version of its host counterpart.

...
Wikipedia