WILD-TYPE PAX3 PROTEIN AND THE PAX3-FKHR FUSION PROTEIN OF ALVEOLAR RHABDOMYOSARCOMA CONTAIN POTENT, STRUCTURALLY DISTINCT TRANSCRIPTIONAL ACTIVATION DOMAINS

Alveolar rhabdomyosarcoma (ARMS) is characterized cytogenetically by a t(2;13)(q35;q14) chromosomal translocation involving two transcriptio n factor genes: PAX3 and FKHR. ARMS cells express a PAX3-FKHR fusion p rotein containing the complete N-terminal, DNA-binding domain of PAX3 and the C-terminus of FKHR. Recently we demonstrated that PAX3-FKHR is a more potent transcriptional activator than PAX3 despite impaired bi nding to canonical PAX3 binding sites. Therefore, we propose that the gene fusion results in snitching of PAX3 and FKHR transactivation doma ins with distinct structure, potency or function. To compare the PAX3 and putative PAX3-FKHR transactivation domains, we fused C-terminal te st fragments to the heterologous GAL4 DNA-binding domain and tested ac tivation of a reporter gene co-transfected into four cell types. GAL4- PAX3 and GAL4-PAX3-FKHR were found to be potent activators exhibiting different concentration-dependent transactivation profiles and distinc t structural motifs. Deletion mapping demonstrated essential acidic an d/or serine/threonine-rich domains in the extreme 3' ends of their res pective coding regions and positive modifying elements in adjacent 5' sequences. These data demonstrate that PAX3 and PAX3-FKHR contain stru cturally distinct transcriptional activation domains and suggest that a consequent difference in function is important for oncogenesis.