MUTATIONAL ANALYSIS OF ACUTE-PHASE RESPONSE FACTOR STAT3 ACTIVATION AND DIMERIZATION

Signal transducer and transcription (STAT) factors are activated by ty rosine phosphorylation in response to a variety of cytokines, growth f actors, and hormones. Tyrosine phosphorylation triggers dimerization a nd nuclear translocation of these transcription factors. In this study , the functional role of carboxy-terminal portions of the STAT family member acute-phase response factor/Stat3 in activation, dimerization, and transactivating potential was analyzed. We demonstrate that trunca tion of 55 carboxy-terminal amino acids causes constitutive activation of Stat3 in COS-7 cells, as is known for the Stat3 isoform Stat3 beta . By the use of deletion and point mutants, it is shown that both carb oxy- and amino-terminal portions of Stat3 are involved in this phenome non. Dimerization of Stat3 was blocked by point mutations affecting re sidues both in the vicinity of the tyrosine phosphorylation site (Y705 ) and more distant from this site, suggesting that multiple interactio ns are involved in dimer formation. Furthermore, by reporter gene assa ys we demonstrate that carboxy-terminally truncated Stat3 proteins are incapable of transactivating an interleukin-6-responsive promoter in COS-7 cells. In HepG2 hepatoma cells, however, these truncated Stat3 f orms transmit signals from the interleukin-6 signal transducer gp130 e qually well as does full-length Stat3. We conclude that, dependent on the cell type, different mechanisms allow Stat3 to regulate target gen e transcription either with or without involvement of its putative car boxy-terminal transactivation domain.