Identification of envelope determinants of feline leukemia virus subgroup B that permit infection and gene transfer to cells expressing human Pit1 orPit2

The retroviral vector systems that are in common use for gene therapy are d esigned to infect cells expressing either of two widely expressed phosphate transporter proteins, Pit1 or Pit2. Subgroup B feline leukemia viruses (Fe LV-Bs) use the gibbon ape leukemia virus receptor, Pit1, as a receptor for entry. Our previous studies showed that some chimeric envelope proteins enc oding portions of FeLV-B could also enter cells by using a related receptor protein, Pit2, which serves as the amphotropic murine leukemia virus recep tor (S. Boomer, M. Eiden, C. C. Burns, and J. Overbaugh, J. Virol. 71:8116- 8123, 1997). Here we show that an arginine at position 73 within variable r egion A (VRA) of the FeLV-B envelope surface unit (SU) is necessary for vir al entry into cells via the human Pit2 receptor. However, C-terminal SU seq uences have a dominant effect in determining human Pit2 entry, even though this portion of the protein is outside known receptor binding domains. This suggests that a combination of specific VRA sequences and C-terminal seque nces may influence interactions between FeLV-B SU and the human Pit2 recept or. Binding studies suggest that the C-terminal sequences may affect a post binding step in viral entry via the Pit2 receptor, although in all cases, b inding of FeLV-B SU to human Pit2 was weak. In contrast, neither the argini ne 73 nor specific C-terminal sequences are required for efficient binding or infection with Pit1.Taken together, these data suggest that different re sidues in SU may interact with these two receptors. The specific FeLV-Bs de scribed here, which can enter cells using either human Pit receptor, may be useful as envelope pseudotypes for viruses used in gene therapy.