IgA Fc receptor (FcαR) cross-linking recruits tyrosine kinases, phosphoinositide kinases and serine/threonine kinases to glycolipid rafts

Abstract

The human IgA Fc receptor (FcαR, CD89) triggers several important physiological functions, including phagocytosis, NADPH oxidase activation and antigen presentation. Efforts are underway to delineate FcαR signal-transduction pathways that control these functions. In a previous study, we demonstrated that cross-linking of FcαR increased its partitioning into membrane glycolipid rafts and was accompanied by γ-chain-dependent recruitment and phosphorylation of the tyrosine kinases Lck/Yes-related novel protein tyrosine kinase (Lyn) and Bruton's tyrosine kinase (Btk). Here we have performed a more extensive characterization of signalling effectors recruited to rafts on FcαR cross-linking. We demonstrate that in addition to tyrosine kinases Lyn and Btk, FcαR cross-linking also recruits B-lymphocyte kinase (Blk) and spleen tyrosine kinase (Syk) to rafts. We show recruitment of phosphoinositide kinases, including 3-phosphoinositide 3-kinase and phospholipase Cγ2, and serine/threonine kinases such as protein kinase C (PKC) α, PKC∊, and protein kinase B (PKB) α. This suggests that lipid rafts serve as sites for FcαR-triggered recruitment of multiple classes of signalling effectors. We further demonstrate that tyrosine kinases and PKCα have a sustained association with rafts, whereas phosphoinositide 3-kinase and its downstream effectors have a transient association with rafts. This is consistent with temporally regulated divergence of FcαR signalling pathways in rafts. Furthermore, we suggest the spatial separation of signalling effectors by transport of phosphoinositide 3-kinase, phosphoinositide-dependent kinase 1, PKBα and PKC∊ to endocytic compartments containing internalized FcαR.