The Gross
cell surface antigen (GCSA), associated with expression of endogenous Gross-type murine leukemia virus (G-MuLV) in tissues of mice, is defined by the cytotoxic reaction of a C57BL/6 antiserum, anti-AKR spontaneous
leukemia K36, with cells of the Gross virus-induced C57BL/6
leukemia, Emale symbolG2. Sequential
lactoperoxidase-catalyzed radioiodination of Emale symbolG2 cells,
Nonidet P-40 lysis, precipitation with anti-K36 serum, and
sodium dodecyl sulfate-
polyacrylamide gel electrophoresis identified molecules with properties of
polyproteins encoded by the gag region of the viral genome. These cell surface species could also be labeled by in vitro culturing of Emale symbolG2 with radioactive
glucosamine. The viral specificity of these molecules and their participation in the GCSA typing system were established as follows. (i) Absorption of anti-K36 serum with GCSA(+), but not GCSA(-),
leukemias led to a marked decrease in precipitation of these
proteins. (ii) The same Emale symbolG2
cell surface proteins were also precipitated by
antisera against the MuLV virion
proteins p30 and p15. (iii) Anti-K36 was shown to possess
antibodies against Gross virus p30 and p15. (iv) "Clearing" the Emale symbolG2 lysate of molecules reactive with anti-p30 or anti-p15 sera removed molecules reactive with anti-K36 serum. (v) Absorption of anti-K36 serum with disrupted G-MuLV virions or with Gross p30 or p15 removed GCSA cytotoxic
antibodies; partial absorption was achieved with disrupted Rauscher-MuLV (R-MuLV) or with R-MuLV p30, and no absorption was found with R-MuLV p15. These data show that Emale symbolG2 cells express, on their surfaces, MuLV core
polyproteins that apparently can be glycosylated and on which the determinants of GCSA are located.