PATHOGENICITY OF A MUTANT FRIEND SPLEEN FOCUS-FORMING VIRUS ENCODING AN ENV-LIKE MEMBRANE GLYCOPROTEIN (GP55) WITH SUBSTITUTION BY A XENOTROPIC MURINE LEUKEMIA-VIRUS ENV GP70 SEQUENCE

Friend spleen focus-forming virus (F-SFFV) is a replication-defective acutely leukemogenic mouse retrovirus and encodes an envelope protein (Env)-like membrane glycoprotein (gp55) in its defective env gene, whi ch is responsible for the early stage of the viral leukemogenesis. Gp5 5 is a modified Env protein and contains a polytropic mink cell focus- inducing (MCF) murine leukemia virus (MuLV) Env gp70-derived sequence in its amino-terminal region. To evaluate the possibility that the pre sumed binding of gp55 to an MCF MuLV receptor protein has some role in leukemogenesis, we examined the biological activities of a mutant gp5 5 (XE gp55), which has a xenotropic MuLV Env gp70 amino-terminal regio n. XE gp55 displayed almost the same biological activities as the wild -type gp55, excluding the above possibility.