Hormone-conditional transformation by fusion proteins of c-Abl and its transforming variants.

Fusion of the hormone binding domain (HBD) of steroid receptors to transcription factors renders them hormone-dependent. We show here that an SH3-deleted, oncogenic variant of the Abl tyrosine kinase becomes hormone-dependent for transformation by fusion to the estrogen receptor (ER) HBD, extending the phenomenon to tyrosine kinases. Surprisingly, fusion of the HBD to the normal, non-transforming c-Abl (IV) protein activated transforming activity in a hormone-dependent fashion. In the presence of hormone, the c-Abl:ER fusion protein was transforming, cytoplasmic and tyrosine phosphorylated, whereas it was non-transforming, nuclear and hypophosphorylated without hormone. We have examined the kinetics of activation of the c-Abl:ER protein and found that protein synthesis is required both for kinase activation and for redistribution of the c-Abl:ER protein from the nucleus to the cytoplasm. We suggest that the activation of c-Abl could be due to HBD-mediated dimerization and/or to the ability to overexpress conditionally the normally toxic c-Abl protein. This novel approach may be applicable to a wide variety of proteins, particularly when activating mutations or physiological inducers are unknown or when the protein is toxic to cells.