This envelope-like membrane glycoprotein is responsible for ligand-independent activation of the erythropoietin receptor EPOR leading to the abnormally rapid proliferation of erythroid precursor cells. In the first stage of Friend disease, constitutive activation of EPOR by gp55 causes uncontrolled, polyclonal proliferation of infected erythroblasts, leading to polycythemia (massive increase in the number of mature red cells). Host susceptibility to SSFV-induced erythroblastosis depends on the expression of the truncated isoform of MST1R receptor tyrosine kinase (MST1R isoform sf-Stk). Interaction with SSFV gp 55 results in constitutive tyrosine phosphorylation and activation of MST1R isoform sf-Stk (By similarity). Homooligomer. Forms heterooligomers with mouse EPOR, probably via their respective transmembrane domains. Forms covalent heterodimers with mouse MST1R isoform sf-Stk, probably via disulfide bonds (By similarity). The envelope-like membrane glycoprotein gp55 is defective in its transport to the cell surface and remains associated predominantly with the rough endoplasmic reticulum (RER) membrane. It is almost not incorporated into virions. Host cell surface expression appears to be a prerequisite for its leukemogenicity (By similarity). Compared to other gammaretroviruses which possess 2 envelope proteins (gp70 and p15E), gp55 corresponds to a gp70-p15E fusion protein with a deletion of a portion of p15E. It is encoded by the defective env gene of the virus. The Friend murine leukemia virus complex induces a rapid and fatal erythroleukemia in adult mice. It is the replication-defective spleen focus-forming virus (SFFV) contained in this complex that causes foci of proliferating erythroid cells in spleens of infected mice. The second component is a replication competent Friend murine leukemia virus (F-MuLV) that serves as a helper virus for SFFV.