COOPERATIVE TRANSCRIPTIONAL ACTIVITY OF JUN AND STAT3-BETA, A SHORT-FORM OF STAT3

To identify proteins that regulate the transcriptional activity of c-J un, we have used the yeast two-hybrid screen to detect mammalian polyp eptides that might interact functionally with the N-terminal segment o f c-Jun, a known regulatory region, Among the proteins identified is a short form of Stat3 (designated Stat3 beta), Stat3 beta is missing th e 55 C-terminal amino acid residues of the long form (Stat3 alpha) and has 7 additional amino acid residues at its C terminus, In the absenc e of added cytokines, expression of Stat3 beta (but not Stat3 alpha) i n transfected cells activated a promoter containing the interleukin 6 responsive element of the rat alpha(2)-macroglobulin gene; coexpressio n of Stat3 beta and c-Jun led to enhanced cooperative activation of th e promoter, Nuclear extracts of cells transfected with a Stat3 beta ex pression plasmid formed a complex with an oligonucleotide containing a Stat3 binding site, whereas extracts of cells transfected with a Stat 3 alpha plasmid did not, We conclude that there is a short form of Sta t3 (Stat3 beta), that Stat3 beta is transcriptionally active under con ditions where Stat3 alpha is not, and that Stat3 beta and c-Jun are ca pable of cooperative activation of certain promoters.