Infection by viruses in the Togaviridae family causes significant worldwide morbidity and mortality. There is currently no specific treatment for patients infected with any virus from this family. Viruses in the Alphavirus genus cause diseases ranging from fever, rash and arthritis to fatal encephalitis. Several members of this genus have been identified as possible agents of bioterrorism. Our preliminary studies indicate that the rat zinc-finger antiviral protein (ZAP), which was originally discovered as a host protein that inhibits replication of retroviruses, potently inhibits replication of multiple members of the Alphavirus genus. Sindbis virus (SIN), Ross River virus, Semliki Forest virus and Venezuelan equine encephalitis virus replication were all inhibited to varying extents. The long-term objectives of this project are to elucidate, using virological, molecular and cell biological approaches, the mechanism(s) by which ZAP inhibits replication of alphaviruses. Investigations using SIN, which has been studied extensively and is dramatically inhibited by ZAP, show that replication is blocked after entry and at or before translation of the incoming plus strand genomic viral RNA. Experiments outlined in this proposal will determine the relationship between ZAP levels and SIN replication, and will investigate the mechanism by which ZAP blocks SIN RNA translation. We will characterize the interaction between ZAP and the SIN RNA, as well as the interaction, if any, between ZAP and other host or viral factors. Understanding the mechanism of ZAP's inhibition of alphavirus replication may lead to the development of new therapeutic or preventative agents for illness due to alphavirus infection.