TRANSFORMING ACTIVITY OF RETROVIRAL GENOMES ENCODING GAG-AXL FUSION PROTEINS

Retroviral genomes encoding a portion of the Moloney murine leukemia v irus Gag protein fused to portions of the murine arl cDNA mere constru cted so as to mimic naturally occurring transforming viruses, Virus MA 1 retained 5 amino acids of the extracellular domain and the complete transmembrane and intracellular domains of Axl; virus MA2 retained onl y 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 transfor mants 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 tra nsformed cells and were predominantly localized to the cytoplasmic com partment. 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-tran sformed NIH 3T3 cells, but not MA1 or control cells, could establish s arcomas by subcutaneous or intraperitoneal injection into BALB/c neona tes. These results show that the transforming potential of the axl gen e can be activated by truncation of the extracellular domain of the re ceptor and fusion of the remaining sequence to the gag gene.