Role of the transmembrane sequence of spleen focus-forming virus gp55 in erythroleukemogenesis

The membrane glycoprotein encoded by the env gene of either the polycythemi a- or anemia-inducing spleen focus-forming virus (SFFVp or SFFVa, respectiv ely) is responsible for the induction of erythroleukemia in mice. It has be en shown that the SFFVp glycoprotein, gp55, interacts with the erythropoiet in receptor (EPO-R) and promotes EPO-independent proliferation of an EPO-R- expressing hematopoietic cell line, Ba/F3 (Li et al., Nature 343:762, 1990) . We show here that when residues within the transmembrane (TM) sequence of an SFFVp gp55 are altered based on the sequences of the anemia-inducing gp 55s by a methionine-to-isoleucine (M-I) substitution, a di-leucine deletion (dLL), or both, the resulting mutants display an attenuated phenotype that resembles an SFFVa: they induce milder erythroproliferative disease withou t polycythemia in vivo and are unable to promote EPO-independent cell proli feration in vitro. The dLL mutation directly interferes with EPO-R binding by decreasing the affinity of gp55 for the receptor. On the other hand, the M-l mutation hampers the full mitogenic activation of EPO-R while having n o effect on receptor binding and asserts a dominant negative effect over th e wild-type SFFVp gp55. Two other sequence changes within the TM sequence d id not affect the biological activities of the SFFVp gp55. These results in dicate that the TM sequence of the SFFV env glycoprotein plays a prominent role in SFFV-induced erythroleukemogenesis through its influence on the mit ogenic activation of EPO-R. (C) 1998 Academic Press.