Transforming Activity of Retroviral Genomes Encoding Gag-Axl Fusion Proteins

Retroviral genomes encoding a portion of the Moloney murine leukemia virus Gag protein fused to portions of the murine axl cDNA were constructed so as to mimic naturally occurring transforming viruses. Virus MA1 retained 5 amino acids of the extracellular domain and the complete transmembrane and intracellular domains of Axl; virus MA2 retained only the intracellular Axl sequences beginning 33 amino acids downstream of the transmembrane region. Although both viruses could transform NIH 3T3 cells, they induced different morphological changes. MA1 transformants became elongated and assumed a cross-hatched pattern, while MA2 transformants were round and very refractile and grew to high density. Gag-Axl and Glyco-Gag-Axl proteins were detected in both types of transformed cells and were predominantly localized to the cytoplasmic compartment. When cell-free v-axl virus supernatants were introduced into wild-type BALB/c neonates, Rag-2-deficient mice, or c-myc transgenic mice, they did not cause tumors in a 3-month period. However, MA2-transformed NIH 3T3 cells, but not MA1 or control cells, could establish sarcomas by subcutaneous or intraperitoneal injection into BALB/c neonates. These results show that the transforming potential of the axl gene can be activated by truncation of the extracellular domain of the receptor and fusion of the remaining sequence to the gag gene.