THE RECEPTOR-BINDING SITE OF FELINE LEUKEMIA-VIRUS SURFACE GLYCOPROTEIN IS DISTINCT FROM THE SITE INVOLVED IN VIRUS NEUTRALIZATION

The external surface glycoprotein (SU) of feline leukemia virus (FeLV) contains sites which define the viral subgroup and induce virus-neutr alizing antibodies. The subgroup phenotypic determinants have been loc ated to a small variable region, VR1, towards the amino terminus of SU . The sites which function as neutralizing epitopes in vivo are unknow n. Recombinant SU proteins were produced by using baculoviruses that c ontained sequences encoding the SUs of FeLV subgroup A (FeLV-A), FeLV- C, and two chimeric FeLVs (FeLV-215 and FeLV-VC) in which the VR1 doma in of FeLV-A had been replaced by the corresponding regions of FeLV-C isolates. The recombinant glycoproteins, designated Bgp70-A, -C, -215, and -VC, respectively, were similar to their wild-type counterparts i n several immunoblots and inhibited infection of susceptible cell line s in a subgroup-specific manner. Thus, Bgp70-A interfered with infecti on by FeLV-A, whereas Bgp70-C, -VC and -215 did not. Conversely, Bgp70 -C, Bgp70-C, and -215 blocked infection with FeLV-C, while Bgp70-A had no effect, These results indicate that the site on SU which binds to the FeLV cell surface receptor was presented in the recombinant glycop roteins. It was also found that the recombinant proteins were able to bind naturally occurring neutralizing antibodies, Bgp70-A, -VC, and -2 15 interfered with the action of anti-FeLV-A neutralizing antibodies, whereas Bgp70-C did not. Furthermore, Bgp70-C interfered with the acti on of anti-FeLV-C neutralizing antibodies, while the other proteins di d not. These results indicate that the neutralizing epitope(s) of FeLV SU lies outside the subgroup-determining VR1 domain.