Diagram of how a virus capsid can be constructed using multiple copies of just two protein molecules
A viral protein is a protein encoded by a virus genome. Viral proteins are extremely diverse and comprise enzymatic functions that do not exist in today's cellular organism, for example reverse-transcriptase or RNA-dependent RNA polymerase. This pool of unique enzymatic activities plays a role in evolution of cellular organisms. The syncytin is a retrovirus protein that have been captured in mammals genome to allow membrane fusion in placental morphogenesis.
Viruses encode for two kind of proteins: the replication cycle proteins are conserved within virus family and comprise structural and replication proteins. These classes of proteins can be identified by knockout experiment leading to impaired virus replication cycle in cell culture. The host-modulation proteins vary much between each species because they are closely adapted to each host; their function is to counteract innate and acquired antiviral defenses, and to modulate host cell metabolism. These kinds of proteins have often been called viral regulatory and accessory proteins because in cell culture their function can be disrupted without much effect on virus replication. It is only in the natural host context that their function makes sense.
A different classification commonly used is structural proteins vs non-structural proteins. Structural proteins are present in the virion, including a capsid protecting the viral genome for most viruses (just as the polyomavirus capsid protein (VP1)), and membrane proteins for enveloped viruses.Viral nonstructural proteins are all proteins encoded by the virus but not found in the virion. This naming is not much used nowadays, because it relies on a negative result that can prove wrong in the long term.