Gag-derived but not abl-derived determinants are exposed on the surface of Abelson virus-transformed cells.

The organization of the transforming protein encoded by Abelson murine leukemia virus (A-MuLV) in transformed lymphoid and fibroblast cells was examined using immunofluorescent analysis. Antibodies specific for v-abl were capable of detecting cytoplasmic Abelson protein molecules in fixed cells, but none were able to stain the surface of live A-MuLV transformed cells. However, a series of monoclonal antibodies selected for the ability to bind to the surface of A-MuLV-transformed cells did stain live cells. These antibodies were shown to react with a determinant within the helper virus-derived p15 sequences that are present at the amino terminus of the Abelson protein, indicating that gag-derived determinants are exposed on the surface of transformed cells. The inability of a p12-specific monoclonal antibody to stain live cells indicates that only a small portion of the amino terminal sequences are exposed. Examination of the ability of these antibodies to react with Abelson protein encoded by a series of gag deletion mutants suggests that the determinant recognized by these antibodies lies between amino acids 38 and 114 of p15.

Monoclonal antibodies specific for v-abl- and c-abl-encoded molecules.

Monoclonal antibodies specific for regions of the transforming protein of Abelson murine leukemia virus were prepared. Antibodies directed against the kinase domain inhibited the autophosphorylation of v-abl proteins, and all of the antibodies reacted with the products of the murine and human c-abl loci.

Only site-directed antibodies reactive with the highly conserved src-homologous region of the v-abl protein neutralize kinase activity.

Antisera specific for six regions of the v- abl protein were used to serologically characterize the Abelson murine leukemia virus tyrosine kinase. Chemically synthesized peptides corresponding to the predicted v- abl protein sequence and larger regions of the v- abl protein expressed as fusion proteins in bacteria were used as immunogens. The specificity of each antiserum was confirmed by immunoprecipitation analysis with defined deletion mutants of Abelson murine leukemia virus. Several of these v- abl -specific antisera display much higher titers and avidities than serum harvested from mice bearing Abelson murine leukemia virus-induced tumors, previously the only source of anti- abl -specific serum. Two antisera were found to block the in vitro autophosphorylation of the v- abl protein as well as its ability to phosphorylate a peptide substrate. Examination of the sites against which the kinase-blocking antisera were prepared revealed that both are in close proximity to the in vivo sites of tyrosine phosphorylation, which fall within the region of high homology with v-src and other tyrosine kinases. Antisera directed against other regions of v- abl did not inhibit kinase activity.