Components of a Stat recognition code: evidence for two layers of molecular selectivity.

Latent and activated forms of Stat1 and Stat6 have been expressed and purified, enabling biochemical experiments relating to their functional activities. Stat1 bound to a phosphotyrosine peptide derived from the IFN gamma receptor with a KD of 50 nM, whereas Stat6 bound to an IL-4 receptor peptide with a KD of 300 nM. Stat-receptor peptide interactions were specific and dependent upon tyrosine phosphorylation. Activated forms of Stat1 and Stat6 were used to select their optimal DNA binding 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 recombinants were expressed, purified, and assayed for receptor coupling and 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 distinctive patterns of gene expression.