Abelsonmurineleukemiavirus (Ab-MLV) transforms cells by expressing the v-Abl oncoprotein (21). Despite the presence of a strong viral oncogene, pre-B-cell transformation in vivo and in vitro is a multistep process (10, 18, 32, 33). When bone marrow populations are infected, pre-B cells undergo an initial proliferative phase called primary transformation, which is fol- lowed by a period of apoptosis and erratic growth called crisis. Some clones of primary transformants succumb to crisis, and others emerge as fully malignant cell lines (18, 26, 27). About half of the cell lines that survive acquire a mutation that ren- ders the p53 tumor suppressor nonfunctional (26), and most of the others down-modulate the p19Arf protein (18), a protein that leads to p53 activation through effects on Mdm2 (reviewed in reference 24). Consistent with the important role of the p53 pathway in the crisis phase of transformation, pre-B cells from animals lacking functional p53 or the Ink4a/Arf locus that en- codes p19Arf bypass crisis (18, 27).
Mutations affecting MA or the majority of Gag sequences decrease transformation. In contrast to mutations affecting the p12 portion of the v-Abl protein, loss of MA sequences re- duced NIH 3T3 cell transformation by more than 200-fold (Table 1). Despite this marked reduction in transformation, both the ⌬ 35–81 mutant and the ⌬ 82–133 mutant gave rise to a small number of transformants; recovery of the viral se- quences from these cells and PCR analysis revealed that the original deletions were still present. These rare transformants could have arisen because of compensating mutations else- where in the virus or through the effects of complementary mutations present in a minority of NIH 3T3 cells. However, no changes that altered protein size, such as those observed by Prywes and coworkers (18, 19) that affected large portions of the carboxyl terminus, were detected (data not shown). No transformants were recovered from NIH 3T3 cells infected with the ⌬ 35–236 mutant, a mutant that encodes a v-Abl pro- FIG. 2. Ab-MLV mutants express v-Abl proteins that retain kinase