Members of the 14-3-3 protein family may represent a more common class of Syk ligands as these adaptors are ubiquitously expressed and implicated in a plethora of signaling cascades. A direct docking site for 14-3-3γ is provided by the prominently detected Syk phosphosite, serine 297 within the linker insert. BCR-induced phosphorylation of serine 297 attenuated inducible membrane anchoring and concomitant tyrosine phosphorylation of Syk. Consequently, BCR-proximal signal chains such as mobilization of the Ca2+ second messenger were inhibited. Loss of this negative feedback loop, for example, upon exclusive expression of the short Syk isoform, which lacks find more the linker insert region,

may promote cellular hyperactivation

and contribute to the oncogenic potential of Syk. Our SILAC-based mass-spectrometric approach allowed us to not only identify a total of 32 Syk phosphosites but also quantify 16 individual sites and hence to monitor their BCR-induced phosphorylation kinetics. Three classes of Syk phosphosites could be distinguished. Early and late acceptor sites undergo rapid or delayed phosphorylation, respectively, while downregulated sites undergo inducible dephosphorylation. The majority of phosphorylations, i.e. 47%, occurred on tyrosine residues with a very rapid kinetics. The dominance of phosphotyrosines is remarkable check details as this amino acid represents only 2% of the cellular phosphoamino acid pool in eukaryotic cells while the average distribution of phosphoserine and phosphothreonine is about 86 and 12%, respectively 43. The high proportion of phosphorylations on tyrosine however is consistent with the key role of this modification for Syk activation

7. In fact, the highest fold increase was observed for a doubly phosphorylated peptide encompassing Y348 and Y352 in interdomain B. These residues and the corresponding sites in ZAP70 have been shown to mediate autoinhibition of the kinase domain until they become phosphorylated 44, 45. Our data provide further evidence that the inhibition of catalytic activity in resting cells is similar between Syk and ZAP70. For signal-induced feedback inhibition, Syk utilizes ioxilan serine 297 in the linker insert of interdomain B. Our SILAC-based interactome analysis revealed 14-3-3 adaptor proteins as candidate ligands of phospho-S297 because the amino acid sequence environment perfectly matches the consensus mode 1 binding motif for this class of phosphoserine/threonine-binding proteins 42. Indeed, 14-3-3γ co-immunoprecipitated with wild-type but not S297A mutant Syk and Far Western blotting showed that this specific interaction is direct. Quantitative reverse interactome analysis confirmed that interaction and revealed increased association of the S297A variant with ubiquitin and BCR signaling subunits.