Despite great advances in AIDS diagnosis and treatment, the continuing AIDS epidemic demands continuing efforts to understand all aspects of HIV replication and to develop new methods for its inhibition. In pursuit of these goals, we have sought to define the activities and interactions of the HIV-1 structural (Gag) proteins, with a specific focus on the N-terminal matrix (MA) domain. The Gag proteins initially are synthesized as precursor Gag (PrGag) proteins that are myristoylated at the N-terminus of MA, and MA domains target PrGag delivery to plasma membrane (PM) virus assembly sites virtue of preferential binding to the signaling phospholipid phosphatidylinositol-4,5-bisphosphate (PI[4,5]P2). Evidence also indicates that MA-RNA binding helps chaperone PrGag proteins to assembly sites, and that virus membranes are enriched for lipid raft constituents such as cholesterol, sphingomyelin, and ceramide. In addition to its trafficking role, MA has been shown to influence the incorporation of wild type (WT) HIV-1 envelope (Env) glycoprotein trimers into virus particles. Previous investigations have shown that HIV-1 Env proteins that carry cytoplasmic tail deletions (CT) in their transmembrane (TM; gp41) domains can be incorporated into virions in a fashion that is cell type-dependent, but MA-independent. In contrast, MA mutations that impair WT Env incorporation into virions have been identified. Moreover, other MA mutations have been shown to suppress Env incorporation defects imposed either by MA mutations, or Env CT mutations. Such observations imply that there could be direct MA-CT interactions, but proof has been lacking. During the past funding period, we have made significant progress in understanding how MA and Env proteins interact. We have shown that MA directly binds to Env CTs, and that binding depends on MA trimerization. We have demonstrated that C-terminal amphipathic helices of HIV-1 Env CTs are involved in MA binding, and that MA-CT binding is blocked by MA-RNA binding. We have discovered lipid composition changes that perturb WT Env activity, and have obtained novel evidence of CT processing. Using this as a foundation, we propose the characterization of MA-CT interactions and the roles of these interactions in HIV-1 replication. In particular, we will define how Env proteins associate with MA trimers and lattices so as to determine how WT Env proteins become incorporated into virus particles; and we will examine how Gag and Env proteins collaborate in virus particles to perform their functions. Our results will help clarify how HIV Gag and Env proteins cooperate, and will foster the development of novel approaches to interfere with HIV-1 replication.