The AIDS epidemic remains a worldwide emergency and the molecular mechanisms leading to the disease are not fully understood. Nef is a gene product of primate lentiviruses crucial for virus replication in vivo and for AIDS progression. The mechanisms supporting its pathogenic role remain undefined. Among other activities, Nef enhances intrinsic virus infectivity, a feature well conserved and under selective pressure during disease progression. While the significance of this activity in vivo is unknown, a recent finding from the applicant (Pizzato M. et al, PNAS, 104, 2007) shows that it requires the interaction of Nef with the cellular GTPase dynamin 2. In addition, preliminary results show that glycogag, encoded by gamma-retroviruses, exerts a similar enhancing activity on HIV infectivity, suggesting that this is a fundamental property of unrelated retroviruses. The aim of the research is to establish in vivo the role of the Nef-induced enhancement of HIV-1 infectivity, and to evaluate the importance of the interaction with dynamin 2. A recently developed humanized mouse will be used as model of HIV infection in vivo. Because Nef exerts several functions, the specific role of the enhancement of infectivity will be studied by separating this from other activities, following four objectives: 1) To establish in vivo the phenotype of HIV-1 carrying Nef mutations which selectively impair specific activities. 2) To study the requirement of Nef in vivo using a CD4-independent HIV whose infectivity cannot be affected by the Nef-induced CD4 downregulation activity. 3) To assess the role of glycogag-induced enhancement of infectivity for HIV replication and pathogenesis in vivo. 4) To study the importance in vivo of the Nef surface interacting with dynamin 2. This research will establish the significance of the Nef-induced enhancement of infectivity in vivo and will indicate whether the Nef-dynamin 2 interaction could be suitable as an antiretroviral target.