VPX OF SIMIAN IMMUNODEFICIENCY VIRUS IS LOCALIZED PRIMARILY OUTSIDE THE VIRUS CORE IN MATURE VIRIONS

Human immunodeficiency virus type 2 and the related simian immunodefic iency virus (SIV) contain a unique regulatory gene, vpx. The Vpx prote in is packaged in mature virions and is required for efficient viral r eplication in peripheral blood lymphocytes and macrophages. To study t he localization of Vpx in mature virions, conical and bar-shaped core structures of SIV from macaques (SIV(mac)) were purified. The SIV(mac) core has a density of approximately 1.25 g/cm3, compared with 1.16 g/ cm3 for an intact virion. The relative proportions of major capsid pro tein (p27) and reverse transcriptase activity were similar for intact virions and core structures. The majority of matrix protein (p14) was removed from the purified core structure, suggesting its association w ith the viral membrane. Similarly, most of the Vpx protein was absent from the purified core structure. This result suggests that as with th e matrix protein, the majority of Vpx proteins are localized outside t he virus core. The localization of Vpx suggests that it may be involve d in virus entry such as penetration or uncoating.