The glycosylated gag protein of MuLV is a determinant of neuroinvasiveness: analysis of second site revertants of a mutant MuLV lacking expression of this protein

Neuroinvasiveness is a property of all neurovirulent murine retroviruses, although the factors which facilitate infection of the CNS are not understood. We previously showed that mutant MuLV which lack expression of an accessory protein, glycosylated gag, had lost neurovirulence, indicating that this protein may be involved in promoting CNS infection. The mutations were located in the "Kozak" consensus sequence of the initiation codon for this protein. Here it is shown that shortly after inoculation of mice with one of these mutant viruses, revertants emerged which had regained expression of glycosylated gag and had also regained the neuroinvasiveness and neurovirulence exhibited by the wild-type virus. The phenotypic revertants retained the mutations in the "Kozak" consensus sequence but exhibited a G-->A mutation 12 codons downstream from the mutated start site, creating a new initiation codon and a glycosylated gag protein, which was truncated at its N-terminus. Using antibodies specific for glycosylated gag it is shown that the frequency of splenic infectious centers expressing revertant virus increased progressively during the 2 months following inoculation of mutant virus until > or = 50% of the virus-producing cells in the spleen expressed revertant virus. In contrast, although phenotypic revertants were detectable at low frequency after cell-free passage in vitro in M. dunni fibroblasts, there was no evidence for selection. These results indicate that glycosylated gag facilitates virus spread within the spleen and to extra-splenic sites, such as the CNS, and suggest that the protein may function through its interaction with the host.