Design of Conditionally Active STATs: Insights into STAT Activation and Gene Regulatory Function

Abstract

The STAT (signal transducer and activator of transcription) signaling pathway is activated by a large number of cytokines and growth factors. We sought to design a conditionally active STAT that could not only provide insight into basic questions about STAT function but also serve as a powerful tool to determine the precise biological role of STATs. To this end, we have developed a conditionally active STAT by fusing STATs with the ligand-binding domain of the estrogen receptor (ER). We have demonstrated that the resulting STAT-ER chimeras are estrogen-inducible transcription factors that retain the functional and biochemical characteristics of the cognate wild-type STATs. In addition, these tools have allowed us to evaluate separately the contribution of tyrosine phosphorylation and dimerization to STAT function. We have for the first time provided experimental data supporting the model that the only apparent role of STAT tyrosine phosphorylation is to drive dimerization, as dimerization alone is sufficient to unmask a latent STAT nuclear localization sequence and induce nuclear translocation, sequence-specific DNA binding, and transcriptional activity.

ACKNOWLEDGMENTS

We thank J. Miner and P. Lamb for helpful discussions. We also gratefully acknowledge C. Lowe, E. Delorme, G. Stark, I. Kerr, J. Darnell, J. Ihle, K. Marschke, and E. Allegretto for useful reagents.

ADDENDUM IN PROOF

A recent paper by Kamogawa et al. (J. Immunol.161:1074–1077, 1998) reported the construction of a Stat6-ER chimera similar to one of the constructs described here. Kamogawa et al. obtained results consistent with ours in that they demonstrated a three- to fourfold induction of a Stat6-responsive luciferase reporter and up-regulation of CD23 surface expression by Stat6-ER in their system in response to tamoxifen.