Tyrosines in the carboxy-terminal region regulate Syk function (86.18)

Abstract

Abstract The Syk tyrosine kinase family has an essential role in immunoreceptor tyrosine-based activation motif (ITAM) signaling. The binding of Syk to tyrosine phosphorylated ITAMs in the subunits of FcϵRI results in a conformational change, with an increase in the enzymatic activity of Syk. This conformational change exposes the COOH terminal region of Syk which has three conserved Tyr residues (Y623, Y624, Y625 of rat Syk). To understand the role of these residues in signaling, wild-type and mutant Syk with these three Tyr mutated to Phe were expressed in Syk-deficient mast cells. There was decreased FcϵRI induced degranulation, together with reduced phosphorylation of MAP kinases p38 and p42/44 ERK. There was a parallel decrease in NFAT and NFκB activation with the mutated Syk. In non-stimulated cells, there was increased tyrosine phosphorylation of the mutated Syk, however, this still increased after FcϵRI activation. In vitro mutated Syk had decreased ITAM-binding and reduced kinase activity. In testing Syk mutated singly at each one of the tyrosines, Y623F had minimal effect while Y624F but specially Y625F had the most reduction in these assays. Also, tyrosines Y624 and Y625 controlled Syk capacity for autophosphorylation. Therefore these tyrosines in the tail region of Syk regulate its kinase activity and their phosphorylation is important for Syk function in mast cell FcϵRI-mediated signaling.