CYTOKINE RECEPTOR-INDEPENDENT, CONSTITUTIVELY ACTIVE VARIANTS OF STAT5

STAT (signal transducers and activators of transcription) proteins are dual function proteins, which participate in cytokine-mediated signal transduction events at the cell. surface and transcriptional regulati on in the nucleus, We have exploited insights into the activation mech anism of STAT factors to derive constitutively active variants, Chimer ic genes encoding fusion proteins of STATE and the kinase domain of JA K2 have been derived, The functional properties of the fusion proteins have been investigated in transiently transfected COS cells or in HeL a cells stably transfected with STAT5-JAK2 gene constructs regulated b y a tetracycline-sensitive promoter, The STAT5-JAK2 proteins exhibit t yrosine kinase activity and are phosphorylated on tyrosine, The molecu les are activated through an intramolecular or a cross-phosphorylation reaction and exhibit constitutive, STAT5-specific DNA binding activit y, The transactivation potentials of three constitutively activated ST AT5-JAK2 variants comprising different transactivation domains (TADs) derived from STATE, STAT6, and VP16 were compared. The chimeric molecu le containing the STAT5 TAD had no or only a very low, the molecule wi th the STAT6 TAD a medium, and the molecule with the VP16 TAD a very h igh transactivation potential, Transcription from STATE-responsive gen e promoter regions of the beta-casein, oncostatin RI, and the cytokine -inducible Src homology 2 domain-containing protein genes was observed , These chimeric STAT molecules allow the study of the function of STA TE independent of cytokine receptors and the activation of other signa l transduction pathways.