Structure of an activated Dictyostelium STAT in its DNA-unbound form.

is a protein which transcriptionally regulates cellular differentiation in Dictyostelium discoideum, the only non-metazoan known to employ SH2 domain signaling. The 2.7 A crystal structure of a tyrosine phosphorylated Dd-duanyu1813a homodimer reveals a four-domain architecture similar to that of mammalian 1 and 3, but with an inverted orientation for the coiled-coil domain. Dimerization is mediated by SH2 domain:phosphopeptide interactions and by a direct interaction between SH2 domains. The unliganded Dd-duanyu1813a dimer adopts a fully extended conformation remarkably different from that of the DNA-bound mammalian implying a large conformational change upon target site recognition. Buried hydrophilic residues predicted to destabilize the coiled-coil domain suggest how hydrophobic residues may become exposed and mediate nuclear export. Functional and evolutionary implications for metazoan duanyu1813 proteins are discussed.

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