Implications of an antiparallel dimeric structure of nonphosphorylated STAT1 for the activation–inactivation cycle

doi:10.1073/pnas.0501063102

Implications of an antiparallel dimeric structure of nonphosphorylated STAT1 for the activation–inactivation cycle

^*Laboratory of Molecular Cell Biology, The Rockefeller University, 1230 York Avenue, New York, NY 10021; ^‡Division of Hematology-Oncology, Department of Medicine, University of California, Los Angeles, CA 90095; and ^¶Department of Biochemistry and Molecular Biology, University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030

Contributed by James E. Darnell, Jr., February 8, 2005

Abstract

IFN-γ treatment of cells leads to tyrosine phosphorylation of signal transducer and activator of transcription (STAT) 1 followed by dimerization through a reciprocal Src homology 2–phosphotyrosine interaction near the –COOH end of each monomer, forming a parallel structure that accumulates in the nucleus to drive transcription. Prompt dephosphorylation and return to the cytoplasm completes the activation–inactivation cycle. Nonphosphorylated STATs dimerize, and a previously described interface between N-terminal domain (ND) dimers has been implicated in this dimerization. A new crystal structure of nonphosphorylated STAT1 containing the ND dimer has two possible configurations for the body of STAT1, one of which is antiparallel. In this antiparallel structure, the Src homology 2 domains are at opposite ends of the dimer, with the coiled:coil domain of one monomer interacting reciprocally with the DNA-binding domain of its partner. Here, we find that mutations in either the coiled:coil/DNA-binding domain interface or the ND dimer interface block dimerization of nonphosphorylated molecules and cause a resistance to dephosphorylation in vivo and resistance to a tyrosine phosphatase in vitro. We conclude that a parallel STAT1 phosphodimer not bound to DNA most likely undergoes a conformational rearrangement (parallel to antiparallel) to present the phosphotyrosine efficiently for dephosphorylation.

Footnotes

↵ ∥ To whom correspondence may be addressed. E-mail: darnell{at}rockefeller.edu or kshuai{at}mednet.ucla.edu.
↵ † M.Z., M.A.H., and K.T. contributed equally to this work.
↵ § Present address: Zentaris GmbH, 60314 Frankfurt, Germany.
Author contributions: M.Z., M.A.H., K.T., O.S., B.L., J.t.H., Z.R., and X.M. performed research; X.C., K.S., and J.E.D. designed research; K.S. and J.E.D. analyzed data; and J.E.D. wrote the paper.
Abbreviations: STAT, signal transducer and activator of transcription; SH2, Src homology 2; ND, N-terminal domain; CC, coiled:coil; DBD, DNA-binding domain; pY, phosphotyrosine.