Genetic and Biochemical Analyses of Receptor and Cofactor Determinants for T-Cell-Tropic Feline Leukemia Virus Infection

Abstract

Entry by retroviruses is mediated through interactions between the viral envelope glycoprotein and the host cell receptor(s). We recently identified two host cell proteins, FeLIX and Pit1, that are necessary for infection by cytopathic, T-cell-tropic feline leukemia viruses (FeLV-T). Pit1 is a classic multiple transmembrane protein used as a receptor by several other simple retroviruses, including subgroup B FeLV (FeLV-B), and FeLIX is a secreted cellular protein expressed from endogenous FeLV-related sequences (enFeLV). FeLIX is nearly identical to FeLV-B envelope sequences that encode the N-terminal half of the viral surface unit (SU), because these FeLV-B sequences are acquired by recombination with enFeLV. FeLV-B SUs can functionally substitute for FeLIX in mediating FeLV-T infection. Both of these enFeLV-derived cofactors can efficiently facilitate FeLV-T infection only of cells expressing Pit1, not of cells expressing the related transport protein Pit2. We therefore have used chimeric Pit1/Pit2 receptors to map the determinants for cofactor binding and FeLV-T infection. Three distinct determinants appear to be required for cofactor-dependent infection by FeLV-T. We also found that Pit1 sequences within these same domains were required for binding by FeLIX to the Pit receptor. In contrast, these determinants were not all required for receptor binding by the FeLV-B SU cofactors used in this study. These data indicate that cofactor binding is not sufficient for FeLV-T infection and suggest that there may be a direct interaction between FeLV-T and the Pit1 receptor.Retroviral entry requires a specific interaction between the viral envelope glycoprotein and a cell surface receptor. The envelope protein is synthesized as a precursor protein that is cleaved into surface (SU) and transmembrane (TM) subunits by a cellular protease. The TM anchors the SU to the viral membrane and plays an important role in fusion between the viral and host cell membranes. The SU contains the receptor binding domain (RBD) and is therefore the major viral determinant for cell tropism. Binding of the SU to the receptor triggers structural rearrangements within the envelope glycoprotein that activate the fusion peptide within the TM subunit. Host-range and receptor binding studies have mapped the murine leukemia virus (MLV) RBD to the N terminus of the SU (8-11, 15, 16, 35, 39, 42), and structural studies suggest that the variable regions (VRA and VRB) within the RBD are organized into three disulfide bonded loops (21). The major receptor binding determinants of the feline leukemia virus (FeLV) envelope have also been localized to the N terminus of SU (4, 12, 29, 50-52). For viruses such as MLV and most FeLVs, it is thought that only one receptor is required for both binding and activation of the fusion machinery.While the role of the SU in receptor binding has long been recognized, recent data indicate that it may also participate in postbinding events in viral entry (5,30,31,59). For example, mutation of an N-terminal histidine in MLV...

Human T Cell Leukemia Virus Envelope Binding and Virus Entry Are Mediated by Distinct Domains of the Glucose Transporter GLUT1

“…Mapping of viral entry or binding determinants on gammaretrovirus receptors has also made use of chimeric molecules (10,(42)(43)(44)(45), but the presence of receptor determinants that distinctively control Env binding and post-binding viral entry has not been reported. In contrast, binding and postbinding entry determinants located within the viral Env have been characterized for an increasing number of gammaretroviruses (22).…”

“…From the context of the cellular Env receptors, little is known concerning potential post-binding determinants and receptor conformational changes that follow SU binding (9,10). In the case of HIV and other related lentiviruses, it has been shown that post-binding events involve the recruitment of additional molecules, co-receptors, that belong to the seven membrane-spanning chemokine receptor family (11).…”

A soluble envelope protein of endogenous retrovirus (FeLIX) present in serum of domestic cats mediates infection of a pathogenic variant of feline leukemia virus

“…FeLIX shares high amino acid identity with the SU envelope protein of FeLV subgroup B (FeLV-B) and binds to Pit1, a Na + -dependent phosphate symporter, which functions as an FeLV-B receptor (Takeuchi et al, 1992). Although the FeLIXmediated infection process requires Pit1, FeLV-T RBD seems not to bind to Pit1 directly (Lauring et al, 2002). Furthermore, it was reported that infection by FeLV-T deleted of its RBD could be rescued by soluble cofactors, suggesting that FeLV-T does not require its cognate receptor but can use alternative receptors recognized by the soluble cofactors (Barnett et al, 2003).…”

Structure and Mechanism of a Coreceptor for Infection by a Pathogenic Feline Retrovirus

“…The initial studies of defective mutant viruses indicated that virus binding to receptor is a prerequisite for transactivation (4,29,30). It has been proposed that Pit1 is the receptor for FeLV-T, as well as FeLIX (26). If true, the hamster isoform of Pit1 receptor must bind to the FeLV-T RBD during Fr-RBD/ mCAT-dependent transactivation.…”