TRANSCRIPTION FACTORS STAT3 AND STAT5B ARE PRESENT IN RAT-LIVER NUCLEI LATE IN AN ACUTE-PHASE RESPONSE AND BIND INTERLEUKIN-6 RESPONSE ELEMENTS

Proteins binding at the interleukin-6 response element of the rat alph a(2) macroglobulin gene were purified by a combination of chromatograp hic procedures including binding site specific DNA-affinity chromatogr aphy as the principal step. Three polypeptides of 92, 91, and 86 kDa w ere enriched approximately 6,300-fold from nuclei of rat Livers excise d 12 h after the induction of an experimental acute phase response, Am ino acid sequence analysis identified the 86- and 91-kDa species as tw o forms of the transcription factor Stat3 and the 92-kDa species as th e factor Stat5b. This identification was confirmed by gel mobility shi ft-supershift experiments using specific antisera for Stat3 and Stat5. Unexpectedly, activated Stat5 was also detected in the nuclei of untr eated control rats. cDNA clones representing Stat3 and two isoforms of Stat5b mere isolated from a cDNA Library prepared with mRNA from rat livers excised at the peak of an experimental acute phase response, Fu ll-length Stat5b, predicted from cDNA, consisted of 786 amino acids, w hile the variant Stat5b Delta 40C lacked 41 amino acids at the COOH te rminus, The amino acid sequence of rat Stat5b showed 26.7% overall ide ntity with rat Stat3, 87.3% with sheep Stat5a, 92.5% with murine Stat5 a, and 98.7% with murine Stat5b.