Dephosphorylation of phosphotyrosine on STAT1 dimers requires extensive spatial reorientation of the monomers facilitated by the N-terminal domain.

We report experiments that infer a radical reorientation of tyrosine-phosphorylated parallel STAT1 dimers to an antiparallel form. Such a change in structure allows easy access to a phosphatase. With differentially epitope-tagged molecules, we show that the two monomers of a dimer remain together during dephosphorylation although they most likely undergo spatial reorientation. Extensive single amino acid mutagenesis within crystallographically established domains, manipulation of amino acids in an unstructured tether that connects the N-terminal domain (ND) to the core of the protein, and the demonstration that overexpressed ND can facilitate dephosphorylation of a core molecule lacking an ND all support this model: When the tyrosine-phosphorylated STAT1 disengages from DNA, the ND dimerizes and somehow assists in freeing the reciprocal pY-SH2 binding between the monomers of the dimer while ND ND dimerization persists. The core of the monomers rotate allowing reciprocal association of the coiled:coil and DNA-binding domains to present pY at the two ends of an antiparallel dimer for ready dephosphorylation.