COMPONENTS OF A STAT RECOGNITION CODE - EVIDENCE FOR 2 LAYERS OF MOLECULAR SELECTIVITY

Latent and activated forms of Stat1 and Stat6 have been expressed and purified, enabling biochemical experiments relating to their functiona l activities. Stat1 bound to a phosphotyrosine peptide derived from th e IFN gamma receptor with a K-D of 50 nM, whereas State bound to an IL -4 receptor peptide with a K-D Of 300 nM. Stat-receptor peptide intera ctions were specific and dependent upon tyrosine phosphorylation. Acti vated forms of Stat1 and Stat6 were used to select their optimal DNA b inding sites. Stat1 selected a recognition site having dyad half-sites separated by 3 bp. Stat6 selected a recognition site composed of the same dyad half-sites, yet separated by 4 bp. Chimeric Stat1-Stat6 reco mbinants were expressed, purified, and assayed for receptor coupling a nd DNA binding specificity. Such studies led to the identification of polypeptide domains that specify these activities. These observations provide a framework for understanding how different cytokines elicit d istinctive patterns of gene expression.