Syk activation during FcγR-mediated phagocytosis involves Syk palmitoylation and desulfenylation
The non-receptor Spleen tyrosine kinase Syk acts downstream of several receptors of the immune system such as the Fc{gamma}R. Syk is composed of a kinase domain and two SH2 domains that interact with the bi- phosphorylated ITAMs motifs of the Fc{gamma}R upon phagocytosis. This results in the activation of Syk by auto- phosphorylation, triggering phosphorylation of several downstream targets in a process that will culminate in F-actin polymerization and phagocytosis of the IgG-opsonized target. We found that Syk is S-acylated upon phagocytosis by macrophages. Palmitoylation is performed on a single Syk-Cys by the protein S-acyl transferase DHHC5 that specifically associates with Syk upon phagocytosis. Syk palmitoylation is required for Syk localization to the phagocytic cup, Syk phosphorylation/activation, Cdc42 recruitment to the cup, F-actin polymerization and phagocytosis. We also observed that another Syk-Cys residue is modified by sulfenylation. Mutation of the sulfenylated Cys that belongs to a redox-motif inactivated the Syk catalytic activity and phagocytosis. We found that Syk desulfenylation occurs during phagocytosis. Molecular dynamics studies indicated that desulfenylation increased the mobility and exposure of a loop within the Syk interdomain B, likely facilitating phosphorylation of key Syk-Tyr residues by upstream effectors such as Src kinases. We thus propose an original updated model for Syk activation during Fc{gamma}R-mediated phagocytosis that involves both Syk palmitoylation and desulfenylation.