v-mos proteins encoded by myeloproliferative sarcoma virus and its ts159 mutant

The myeloproliferative sarcoma virus (MPSV) v-mos protein was predicted to be identical in size to p39^c-mos because of an observed one-base deletion in the seventh codon of the env-mos open reading frame, which would allow translation to initiate at the methionine equivalent to codon 32 of the env-mos gene. On the basis of published results, p39^c-mos is known to have greatly reduced in vitro protein kinase activity compared with p37^env-mos encoded by Moloney murine sarcoma virus. Unexpectedly, the relative activity of the MPSV v-mos protein kinase was comparable to that of p37^env-mos. Consistent with this finding, the size of MPSV v-mos protein was found to be similar to the size of p37^env-mos. Moreover, the pattern and sizes of phosphorylated bands produced by autophosphorylation of the MPSV v-mos protein were similar to those of p37^env-mosg. These results were confirmed by in vitro transcription-translation of the MPSV v-mos gene. Resequencing portions of the MPSV mos gene failed to show the deletion within codon 7. Except for the codon 262 deletion, other mutations characteristic of MPSV and temperature-sensitive MPSV v-mos genes were confirmed. A glycine-to-arginine mutation at residue 338 of the MPSV env-mos sequence, previously shown to cause thermosensitivity of the mutant virus (termed ts159) transforming function, yielded a v-mos protein that had significantly reduced protein kinase activity in vitro. These findings indicate that MPSV, like other Moloney murine sarcoma virus strains, also encodes a functional env-mos protein.