ROLE OF VARIABLE-REGION-A AND VARIABLE-REGION-B IN RECEPTOR-BINDING DOMAIN OF AMPHOTROPIC MURINE LEUKEMIA-VIRUS ENVELOPE PROTEIN

For the amphotropic murine leukemia virus (MuLV), a 208-amino-acid ami no-terminal fragment of the surface unit (SU) of the envelope glycopro tein is sufficient to bind to its receptor, Pit2, Within this binding domain, two hypervariable regions, VRA and VRB, have been proposed to be important for receptor rc:cognition. In order to specifically locat e residues that are important for the interaction with Pit2, we genera ted a number of site-specific mutations in both VRA and VRB and analyz ed the resulting envelope proteins when expressed on retroviral vector s, Concurrently, we substituted portions of the amphotropic SU with ho mologous regions from the polytropic MuLV envelope protein. The amphot ropic SU was unaffected by most of the point mutations we introduced. In addition, the deletion of eight residues in a region of VRA that wa s previously suggested to be essential for Pit2 utilization only decre ased titer on NIH 3T3 cells by 1 order of magnitude. Although the repl acement of the amino-terminal two-thirds of VRA with the polytropic se quence abolished receptor binding, smaller nonoverlapping substitution s did not affect the function of the protein. We were not able to iden tify a single critical receptor contact point within VRA and we sugges t that the amphotropic receptor binding domain probably makes multiple contacts with the receptor and that the loss of some of these contact s carl be tolerated.