DELETION OF THE CARBOXYL-TERMINAL TRANSACTIVATION DOMAIN OF MGF-STAT5RESULTS IN SUSTAINED DNA-BINDING AND A DOMINANT-NEGATIVE PHENOTYPE

The Stat (signal transducer and activator of transcription) factors tr ansmit cytokine, growth factor, and hormone responses, Seven members o f the Stat gene family are known, MGF-Stat5a has been discovered as a mediator of the prolactin response in mammary epithelial cells. Two cl osely related variants of Stat5, Stat5a and Stat5b, are encoded by dis tinct genes. We examined the functional properties of the carboxyl ter mini of these molecules, Wild-type Stat5a (794 amino acids) and the ca rboxyl-terminal deletion mutant Stat5a Delta 772 supported prolactin-i nduced transcription of a beta-casein promoter-reporter construct in C OS7 cells; Stat5a Delta 750 did not, Upon prolactin activation, tyrosi ne phosphorylation and the specificity of DNA binding were indistingui shable among the three Stat5a variants, Tyrosine dephosphorylation and the downregulation of the DNA-binding activity were delayed in the St at5a Delta 750 mutant, The carboxyl-terminal transactivation domain of Stat5a, amino acids 722 to 794, can be conferred to the DNA-binding d omain of the yeast transcription factor GAL4. Coexpression of Stat5a o r Stat5b and of the carboxyl-terminal deletion mutants resulted in the suppression of transcriptional induction in COS or Ba/F3 cells. We pr opose that Stat5a Delta 750 and Stat5b Delta 754 are lacking functiona l transactivation domains and exert their dominant negative effects by blocking the DNA-binding site in Stat5-responsive gene promoters.