The general aim of this grant proposal is to analyze in any detail the “interactome” of the HIV-1 Vpu protein and thus to elucidate at the molecular level the cellular mechanisms by which Vpu modifies the identity and physiology of the plasma membrane of the host cell in order to facilitate viral dissemination and persistence. The chronic and persistent viral replication that characterizes HIV-1 infection reflects the complex interaction between the host defenses and certain viral factors which primary function is to facilitate the evasion of such immune responses. To infect a new host, replicate on it for many years and spread to new individuals, HIV-1 must bypass not only the innate defenses, which include the so-called “restrictive antiviral factors”, but also those antigen-specific, adaptive humoral and cellular ones. To date, a number of restriction factors that actively act against HIV-1 have been identified, including APOBEC3G, TRIM5α, cyclophilin A, tetherin/BST-2, SAMHD1 and SERINC3/5. HIV-1 has evolved a variety of mechanisms to evade these cellular factors by either acquiring mutations in the viral proteins susceptible to their activity or by encoding specific proteins that neutralize them. The latter viral factors, known as “accessory proteins”, facilitate not only the inactivation of the host innate but also the adaptive immunity by promoting the formation of ternary complexes between the viral protein, the antiviral restriction factor (or other target protein, such as the HIV-1 receptor CD4) and one or more host cell cofactors. Thus, the HIV-1 Vif, Vpr, Vpu and Nef genes encode proteins that act as molecular adapters connecting specific cellular targets with proteolytic pathways or alternative intracellular trafficking routes. In this scientific plan, we will focus on one of the most attractive proteins from a clinical and pharmacological perspective: Vpu. Because this viral protein greatly contributes to the virulence of HIV-1, the identification of targets and cellular cofactors to which Vpu is associated could lead to the screening of natural and synthetic compounds that eventually allow the discovery of molecules with potential therapeutic properties. In addition, the structural bases of the ternary complexes between Vpu, its cellular targets and those cofactors with which it associates could allow the rational design of new antiretroviral drugs. More importantly, Vpu is responsible for the HIV-1 evasion of the host antiviral immune responses by promoting CD4 degradation, which leads to a dramatic decrease in the levels of macrophages and CD4+ T lymphocytes in those patients in whom chronic and persistent HIV-1 infection has progressed to AIDS (Acquired Immune Deficiency Syndrome). We reason that inhibiting the already known multiple functions of Vpu and those that remain to be discovered could enhance the innate and adaptive immunity of the host in order to facilitate the virus elimination and the resolution of the HIV-1 infection.