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Human Immunodeficiency Virus type 1 (HIV-1) assembly is a multistep process mediated by the viral precursor polyprotein Gag (Pr55Gag). Matrix (MA), which constitutes the N-terminal domain of Pr55Gag, is essential for membrane binding and targeting of Gag to the plasma membrane (PM). MA has a bipartite signal for membrane binding: a myristate moiety at the N-terminus and amino acid residues 17-31 that form a highly basic region (HBR) on the surface of MA. The N-terminal myristate is normally sequestered into the MA globular domain, and a structural change exposes myristate, thereby enhancing membrane binding. The HBR on the other hand is thought to bind acidic lipids. Previous results from the lab suggest that a PM-specific acidic lipid, phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P2], is important for Gag localization to the PM. In this thesis, I have shown that Gag interacts specifically with PI(4,5)P2 and that this interaction is important for efficient membrane binding of Gag. To elucidate the molecular mechanisms by which Gag-PI(4,5)P2 interaction is regulated, site-directed mutagenesis was performed on the MA HBR. Using this approach, we identified three lysines that facilitate membrane binding by interacting with PI(4,5)P2. Strikingly, mutations in two other lysines in the MA HBR enhance PI(4,5)P2-independent membrane binding by exposing myristate. Thus, MA HBR has opposing roles in membrane binding. Notably, another major finding of this thesis is that RNA also negatively regulates membrane binding of Gag. In the absence but not in the presence of PI(4,5)P2, RNA bound to the MA HBR abolishes membrane binding of Gag. Overall, the results from this thesis suggest that the MA HBR regulates membrane binding both positively by binding to PI(4,5)P2 and negatively through myristate sequestration and RNA binding. This regulation ensures that Gag is targeted specifically to the PM, where it likely interacts with other viral and cellular molecules for efficient virus assembly and release.