Human immunodeficiency virus type-1 (HIV-1) initiates infection by direct fusion of the virus membrane with the plasma membrane of the target cell. This fusion event is a multi-step process mediated by the envelope (Env) surface subunit gp120 and the transmembrane subunit gp41 which anchors gp120 into the viral membrane. First the surface subunit gp120 binds to the primary receptor CD4. This interaction promotes actin cytoskeletal rearrangements in the target membrane that bring the chemokine coreceptor, CCR5 or CXCR4, into close proximity for binding and induces conformational changes in gp120 that allow it to couple to the coreceptor. Formation of the gp120-CD4-coreceptor complex triggers conformational changes in the transmembrane gp41 subunit that allows gp41 to insert into the target cell membrane, allowing lipid mixing or hemifusion, and pore formation. Previous studies from our lab have shown that reorganization of the actin cytoskeletal network is required for multiple steps in HIV-1 fusion. Additional data from our lab show that HIV-1 Env induced activation of Rac is critical for HIV-1-mediated membrane fusion. HIV-1 Env activates multiple signaling pathways that could potentially lead to Rac activation. In an effort to determine which signaling molecules were required both upstream and downstream of Rac activation, we utilized small interfering RNAs (siRNAs) and various small-molecule inhibitors to disrupt signaling pathways activated by HIV-1 Env through CD4 and coreceptors. Published data from our lab demonstrated that the Gαq pathway, activated by CCR5, is required for Env-mediated Rac activation and fusion. Further studies have shown that the downstream effectors of Rac that promote fusion are components of the Wave2 signaling complex and at least some of these signaling components are required at a posthemifusion step. Other studies from our lab and other labs, with mutant CD4 constructs, have shown that signaling through CD4 is also playing a role in membrane fusion, most likely at the step of forming the gp120-CD4-coreceptor complex. These results suggest a model in which gp120 signaling though CD4 promotes gp120 binding to and signaling through the coreceptor, which is necessary for full fusion and release of the viral nucleocapsid into the cell's cytosol.