Two Dictyostelium Tyrosine Kinase-Like kinases function in parallel, stress-induced STAT activation pathways

When Dictyostelium cells are hyper-osmotically stressed STATc is activated by tyrosine phosphorylation. Unusually, activation is regulated by serine phosphorylation and consequent inhibition of a tyrosine phosphatase: PTP3. The identity of the cognate tyrosine kinase is unknown and we show that two Tyrosine Kinase-Like (TKL) enzymes, Pyk2 and Pyk3, share this function; thus for stress-induced STATc activation, single null mutants are only marginally impaired but the double mutant is non-activatable. When cells are stressed Pyk2 and Pyk3 undergo increased auto-catalytic tyrosine phosphorylation. The site(s) that are generated bind the SH2 domain of STATc and then STATc becomes the target of further kinase action. The signaling pathways that activate Pyk2 and Pyk3 are only partially overlapping and there may be a structural basis for this difference because Pyk3 contains both a TKL domain and a pseudokinase domain. The latter functions, like the JH2 domain of metazoan JAKs, as a negative regulator of the kinase domain. The fact that two differently regulated kinases catalyse the same phosphorylation event may facilitate specific targeting because under stress Pyk3 and Pyk2 accumulate in different parts of the cell; Pyk3 moves from the cytosol to the cortex while Pyk2 accumulates in cytosolic granules that colocalise with PTP3.