Lentiviruses, including HIV-1, have transmembrane envelope (Env) glycoproteins with cytoplasmic tails that are quite long compared with those of other retroviruses. However, mainly because of the lack of biochemical studies performed in cell types that are targets for HIV-1 infection, no clear consensus exists regarding the function of the long lentiviral Env cytoplasmic tail in virus replication. In this report, we characterize the biological and biochemical properties of an HIV-1 mutant lacking the gp41 cytoplasmic tail. We find that the gp41 cytoplasmic tail is necessary for the efficient establishment of a productive, spreading infection in the majority of T cell lines tested, peripheral blood mononuclear cells, and monocyte-derived macrophages. Biochemical studies using a highlevel, transient HIV-1 expression system based on pseudotyping with the vesicular stomatitis virus glycoprotein demonstrate that in HeLa and MT-4 cells, mutant Env incorporation into virions is reduced only 3-fold relative to wild type. In contrast, gp120 levels in virions produced from a number of other T cell lines and primary macrophages are reduced more than 10-fold by the gp41 truncation. The Env incorporation defect imposed by the cytoplasmic tail truncation is not the result of increased shedding of gp120 from virions or reduced cell-surface Env expression. These results demonstrate that in the majority of T cell lines, and in primary cell types that serve as natural targets for HIV-1 infection in vivo, the gp41 cytoplasmic tail is essential for efficient Env incorporation into virions. The HIV-1 envelope (Env) glycoprotein is synthesized as a precursor, gp160, that is proteolytically cleaved by a cellular protease during transport to the cell surface (1, 2). Cleavage of gp160 produces the two components of the mature Env glycoprotein complex: the surface Env glycoprotein gp120 and the transmembrane Env glycoprotein gp41. After its arrival at the plasma membrane, the gp120͞gp41 complex is incorporated into nascent, budding virus particles by a mechanism that remains poorly understood.Lentiviruses, including HIV-1, encode transmembrane glycoproteins with cytoplasmic tails (CTs) that are quite long compared with those of other retroviruses. For example, the CT of the HIV-1 and HIV-2 transmembrane glycoproteins are generally around 150 residues in length; in contrast, those of the avian and murine oncoretroviruses are typically 20-30 aa long. Although the role of the long lentiviral Env CT has been the focus of a number of studies, no clear consensus has emerged regarding the function of this unique domain. Some studies observed that deletions of more than 19 (3) or 43 (4) residues from the gp41 C terminus blocked virus infectivity; this phenotype was attributed in part to reduced Env incorporation into virions. Another study (5) observed that the gp41 CT played no major role in Env incorporation, but was required for virus infectivity. Wilk et al. (6) reported that a mutant lacking essentially all of the gp41 CT efficiently estab...

Several members of the chemokine receptor family have been shown to function in association with CD4 to permit human immunodeficiency virus type 1 (HIV-1) entry and infection. The CXC chemokine receptor CXCR4/fusin is a receptor for pre–B cell growth stimulating factor (PBSF)/stromal cell–derived factor 1 (SDF-1) and serves as a coreceptor for the entry of T cell line–tropic HIV-1 strains. Thus, the development of CXCR4 antagonists or agonists may be useful in the treatment of HIV-1 infection. T22 ([Tyr5,12,Lys7]-polyphemusin II) is a synthesized peptide that consists of 18 amino acid residues and an analogue of polyphemusin II isolated from the hemocyte debris of American horseshoe crabs (Limulus polyphemus). T22 was found to specifically inhibit the ability of T cell line–tropic HIV-1 to induce cell fusion and infect the cell lines transfected with CXCR4 and CD4 or peripheral blood mononuclear cells. In addition, T22 inhibited Ca2+ mobilization induced by pre–B cell growth stimulating factor (PBSF)/SDF-1 stimulation through CXCR4. Thus, T22 is a small molecule CXCR4 inhibitor that blocks T cell line–tropic HIV-1 entry into target cells.