A Membrane-associated, Carbohydrate-modified Form of the V-abl Protein That Cannot Be Phosphorylated in Vivo or in Vitro

Abelson murine leukemia virus encodes a transforming protein which contains tyrosine kinase activity and is phosphorylated in vivo and in vitro. We found that P160 and P160-derived virus strains expressed an additional, altered v-abl protein which could not be phosphorylated. The altered v-abl protein (L-v-abl) differed from the phosphorylated form (K-v-abl) in that it was glycosylated and localized exclusively to the membrane fraction. Tunicamycin inhibition of N-linked carbohydrate addition did not restore phosphorylation. It did, however, reveal that L-v-abl had additional sequences relative to K-v-abl. The coding sequences required for this region and for the expression of L-v-abl were identified by replacing sequences in the P120 virus genome, which did not express L-v-abl, with sequences from the P160 virus genome. The necessary sequences were localized to the Moloney murine leukemia virus-derived gag gene. Comparison between the in vitro altered P120 and wild-type P120 virus strains indicated that expression of L-v-abl did not increase the efficiency of lymphoid transformation. Although the biological role of L-v-abl is not clear, our analyses have revealed that a specific amino terminal gag sequence can prevent v-abl from acting as a kinase substrate and can alter the cellular localization and modification of v-abl. These properties distinguish L-v-abl from previously reported v-abl proteins.