Identification of the kinase that activates a nonmetazoan STAT gives insights into the evolution of phosphotyrosine-SH2 domain signaling.

SH2 domains are integral to many animal signaling pathways. By interacting with specific phosphotyrosine residues, they provide regulatable protein-protein interaction domains. Dictyostelium is the only nonmetazoan with functionally characterized SH2 domains, but the cognate tyrosine kinases are unknown. There are no orthologs of the animal tyrosine kinases, but there are very many tyrosine kinase-like kinases (TKLs), a group of kinases which, despite their family name, are classified mainly as serine-threonine kinases. are transcription factors that dimerize via phosphotyrosine-SH2 domain interactions. is activated by phosphorylation on Tyr922 when cells are exposed to the prestalk inducer differentiation inducing factor (DIF-1), a chlorinated hexaphenone. We show that in a null mutant for Pyk2, a tyrosine-specific TKL, exposure to DIF-1 does not activate Conversely, overexpression of Pyk2 causes constitutive duanyu1813c activation. Pyk2 phosphorylates duanyu1813c on Tyr922 in vitro and complexes with duanyu1813c both in vitro and in vivo. This demonstration that a TKL directly activates a has significant implications for understanding the evolutionary origins of SH2 domain-phosphotyrosine signaling. It also has mechanistic implications. Our previous work suggested that a predicted constitutive duanyu1813c tyrosine kinase activity is counterbalanced in vivo by the DIF-1-regulated activity of PTP3, a Tyr922 phosphatase. Here we show that the complex is formed constitutively by an interaction between the duanyu1813c SH2 domain and phosphotyrosine residues on Pyk2 that are generated by autophosphorylation. Also, as predicted, Pyk2 is constitutively active as a duanyu1813c kinase. This observation provides further evidence for this highly atypical, possibly ancestral, duanyu1813 regulation mechanism.

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