DIFFERENTIAL REGULATION OF OXIDATIVE AND OSMOTIC-STRESS INDUCED SYK ACTIVATION BY BOTH AUTOPHOSPHORYLATION AND SH2 DOMAINS

Syk, a nonreceptor protein-tyrosine kinase, is activated by both oxida tive and osmotic stress and plays different roles in the transduction of stress signals. In this study, the regulation of oxidative and osmo tic stress induced Syk activation was investigated utilizing Syk-negat ive DT40 cells, expressing various Syk mutants. Phosphorylation of Y51 8Y519 was demonstrated to be required for both oxidative and osmotic s tress induced Syk activation. Syk activation by these two types of str ess stimuli was a combination of both autophosphorylation and the acti vities of additional tyrosine kinases. Oxidative stress induced Syk ty rosine phosphorylation was almost completely attributed to autophospho rylation, whereas other tyrosine kinases were largely responsible for osmotic stress induced Syk tyrosine phosphorylation. Moreover, the Src homology 2 (SH2) domains of Syk differentially regulated Syk activati on. Both mSH2(N) Syk and mSH2(C) Syk, in which the phosphotyrosine-dep endent binding motif within the SH2 domains contained point mutations, showed a significantly higher activity than that observed in wild-typ e Syk, following osmotic stress treatment. In comparison, in response to oxidative stress, only mSH2(N) Syk demonstrated a stronger activati on than wild-type Syk. Therefore, differential activation and regulati on of Syk may give an insight into the distinctive functions of Syk in oxidative and osmotic stress signaling.