1995;69:7410C7415. quail sTva-mIgG (32-, 324-, and 4,739-collapse, respectively) but did not alter their binding affinity for chicken sTva-mIgG. The ALV(A) mutants efficiently infected cells expressing the chicken Pyridoxal phosphate Tva receptor but were 2-fold (E149K), 10-fold (Y142N), and 600-fold (Y142N/E149K) less efficient at infecting cells expressing the quail Tva receptor. These mutations determine key determinants of the interaction between the ALV(A) glycoproteins and the Tva receptor. We also conclude from these results that, at least for the wild-type and variant ALV(A)s tested, the receptor binding affinity was directly related to illness effectiveness. The avian leukosis-sarcoma viruses (ALV) are useful for studying early events in retrovirus illness because they use distinct cellular receptors to gain access into cells. ALVs that infect chicken cells have been divided into six envelope subgroups, A through E and J [ALV(A) through ALV(E) and ALV(J)], based on sponsor range, interference patterns, and cross-reactivity with neutralizing antibodies (65). Some users of these envelope subgroups have also been classified as either noncytopathic [ALV(A), ALV(C), and ALV(E)] or cytopathic [ALV(B) and ALV(D)]. Illness of main avian fibroblasts by cytopathic ALVs usually causes a transient cytotoxicity that results in the death of 30 to 40% of the cells (66, 67). Replication of some subgroup ALV(B) and ALV(D) strains in DF-1 cells, a nontransformed cell collection derived from collection 0 chicken embryo fibroblasts (CEF), induces a transient cytotoxicity similar to the cytotoxicity observed in CEF (35, 56). Unexpectedly, some ALV(C) strains induce a transient cytotoxicity in DF-1 cells. DF-1 cells may be more sensitive to the cytotoxic effects of ALV(B), ALV(D), and ALV(C) than are CEF (35). However, not all ALV(B), ALV(D), ENX-1 and ALV(C) strains induce observable cytotoxicity. The degree of cytotoxicity in DF-1 cells and CEF appears to be related to the level of expression of the ALV envelope glycoproteins (56). The susceptibility of chicken cells to Pyridoxal phosphate illness by ALV(A) to ALV(E) Pyridoxal phosphate strains is determined by three genetic loci designated receptor settings susceptibility to ALV(A), the receptor settings susceptibility to ALV(C), and the receptor settings susceptibility to ALV(B), ALV(D), and ALV(E). Susceptibility or resistance to virus illness is definitely conferred by alleles at these loci (65). Several cell surface proteins have been identified as ALV receptors. Tva, the receptor for ALV(A), consists of sequences related to the ligand binding region of low-density lipoprotein receptors (LDLR) (7, 8, 69). TvbS3, a receptor for ALV(B) and ALV(D) (13, 60), TvbT, a receptor for ALV(E) (1), and TvbS1, a receptor for ALV(B), ALV(D), and ALV(E) (2), are all users of the tumor necrosis element receptor family. The subgroup C receptor has not yet been recognized. The ALV gene (Fig. ?(Fig.1A)1A) encodes a precursor polyprotein that assembles into a trimer in the endoplasmic reticulum (39). After transport to the Golgi, the polyproteins are glycosylated and consequently cleaved to produce two glycoproteins: the surface glycoprotein (SU), which contains the major domains that interact with the sponsor cell receptor, and the transmembrane glycoprotein (TM), which anchors SU to the cell membrane. The specific connection of SU with its receptor is definitely thought to result in a conformational switch in the envelope glycoprotein trimer necessary for the fusion peptide in TM to interact with the cell membrane and mediate the fusion of the viral and cellular membranes. The amino acid sequences of the ALV(A) to ALV(E) envelope glycoproteins are highly conserved except for five variable areas in SU (vr1, vr2, hr1, hr2, and vr3) (Fig. ?(Fig.1B)1B) (11, 12, 22). It should be noted, however, that there is some variance in these five areas in individual ALV subgroups (33). Open in a separate windowpane Pyridoxal phosphate FIG. 1 (A) Schematic representations of the ALV-based RCASBP replication-competent retroviral vector and the major domains of the envelope glycoproteins. The five regions of amino acid sequence variance (vr1, vr2, hr1, hr2, and vr3) recognized by comparing the sequences Pyridoxal phosphate of the surface glycoproteins (SU) of ALV(A) to ALV(E) will also be shown. (B) Assessment of the amino acid sequences of the two major SU variable domains, hr1 and hr2, of representative ALV(A) to ALV(E). The sequences were aligned with the ClustalW multiple-sequence alignment system of MacVector 6.5. Amino acids.