SEQUENCE FLEXIBILITY IN THE POLYTROPIC ENV GP70-DERIVED REGION OF THEMEMBRANE GLYCOPROTEIN (GP55) OF FRIEND SPLEEN FOCUS-FORMING VIRUS AFFECTS ITS BIOLOGICAL-ACTIVITY

We previously reported (N, Watanabe, hi. Kishi, Y. Ikawa, and H, Amanu ma, J, Virol, 65:132-137, 1991) that the mutant Friend spleen focus-fo rming virus (F-SFFVMS), which encodes a mutant gp55 membrane glycoprot ein with an ecotropic env gp70 sequence, was nonpathogenic, Here we in jected the F-SFFVMS-Friend murine leukemia virus (F-MuLV) clone 57 com plex into newborn DBA/2 mice, We obtained four groups of pathogenic va riant F-SFFV complexes, each showing a different degree of pathogenici ty in adult mice and a different gp55 profile, Of these, group 1 varia nt F-SFFV was particularly interesting, because it was the most freque ntly obtained and because it produced doublet bands of gp55 (59 and 57 kDa), neither of which reacted with the nonecotropic gp70-specific mo noclonal antibody, and because its DNA intermediate did not hybridize with the nonecotropic env-specific probe, Cloning and DNA sequence ana lysis of the env region of one isolate of the group 1 variant F-SFFV r evealed that this virus consisted of two distinct F-SFFV genomes; one (clone 117) differed from the other (clone 118) due to the presence of a 39-bp in-frame deletion, Reconstitution to full length F-SFFV genom es and a pathogenicity assay showed that each reconstituted F-SFFV was pathogenic, with clone 117 showing a higher degree of pathogenicity t han clone 118, Both reconstituted F-SFFVs caused activation of the mou se erythropoietin receptor in the factor-independent cell proliferatio n assay, although much less efficiently than the wild-type polycythemi a-inducing isolate F-SFFVp. Clone 118 produced a gp55 of 59 kDa, while clone 117 produced one of 57 kDa, Clone 118 had a substitution by the F-MuLV clone 57 gp70 sequence, indicating that it was derived from th e F-SFFVMS env gene by a homologous recombination with the F-MuLV clon e 57 Env gene, The site of the 39-bp deletion in clone 117 corresponde d to the portion of the clone 118 sequence which was unique to the eco tropic env genes, These results indicated the importance for the biolo gical activity of gp55 of the sequences in the gp70 differential regio n, which are contained in both polytropic and ecotropic env genes.