Regulation of Syk by phosphorylation on serine in the linker insert.

The Syk protein-tyrosine kinase is phosphorylated on multiple tyrosines after the aggregation of the B cell antigen receptor. However, metabolic labeling experiments indicate that Syk is inducibly phosphorylated to an even greater extent on serine after receptor ligation. A combination of phosphopeptide mapping and mass spectrometric analyses indicates that serine 291 is a major site of phosphorylation. Serine 291 lies within a 23-amino acid insert located within the linker B region that distinguishes Syk from SykB and Zap-70. The phosphorylation of serine-291 by protein kinase C enhances the ability of Syk to couple the antigen receptor to the activation of the transcription factors NFAT and Elk-1. Protein interaction studies indicate a role for the phosphorylated linker insert in promoting an interaction between Syk and the chaperone protein, prohibitin.