Spatial Raft Coalescence Represents an Initial Step in FcγR Signaling

Abstract

Characterization of lipid rafts as separated membrane microdomains consist of heterogeneous proteins suggesting that lateral assembly of rafts after Ag receptor cross-linking represents the earliest signal generating process. In line with the concept, cross-linked Ag receptors have been shown to associate with detergent-insoluble raft fraction without the aid of Src family kinases. However, it has not been established whether spatial raft coalescence could also precede Src family kinase activation. In this study, we showed that spatial raft coalescence after low-affinity FcgammaR cross-linking in RAW264.7 macrophages is independent of Src family kinase activity. The lateral raft assembly was found to be ascribed to the action of ligand-binding subunits, rather than to immunoreceptor tyrosine-based activation motif-bearing signal subunits, because monomeric murine FcgammaRIIb expressed in rat basophilic leukemia cells successfully induced spatial raft reorganization after cross-linking. We also showed that extracellular and transmembrane region of FcgammaRIIb is sufficient for raft stabilization. Moreover, this receptor fragment triggers rapid calcium mobilization and linker for activation of T cells phosphorylation, in a manner sensitive to Src family kinase inhibition and to cholesterol depletion. Presence of immunoreceptor tyrosine-based inhibitory motif and addition of immunoreceptor tyrosine-based activation motif to the receptor fragment abolished and enhanced the responses, respectively, but did not affect raft stabilization. These findings support the concept that ligand-binding subunit is responsible for raft coalescence, and that this event triggers initial biochemical signaling.

Receptor-mediated endocytosis of immune complexes in rat liver sinusoidal endothelial cells is mediated by FcγRIIb2

“…9) and those of others suggesting that cross-linking of Fc␥RIIb alone (homotypic cross-linking) does not lead to immunoreceptor tyrosine-based inhibitory motif tyrosine phosphorylation 16,46,47 or Syk tyrosine phosphorylation. 48 The observation that lysosomal degradation of ICs internalized through Fc␥RIIb2, in contrast to Fc␥RIII-mediated IC degradation, does not require a functional Syk tyrosine kinase, 49 points also in this direction. Experimental evidence indicates that co-crosslinking of Fc␥RIIb2 and activating receptors such as B cell antigen receptor does not induce tyrosine phosphorylation of Fc␥RIIb2.…”

Role of B cell inhibitory receptor polymorphisms in systemic lupus erythematosus: a negative times a negative makes a positive

“…1). It is convincingly argued that this process precedes tyrosine phosphorylation signaling, and, moreover, is required for the de novo generation of signaling Suzuki et al 2000;Kono et al 2002;Pierce 2002). Since we have unveiled the critical roles of their transmembrane domains in the association of Fc receptor with lipid rafts (Kono et al 2002), we hypothesized that Ile232Thr substitution influences FccRIIb redistribution to lipid rafts.…”

The coordination of signaling during Fc receptor-mediated phagocytosis

“…DRM have been implicated in early stages of FcR activation. In particular, ligation or cross-linking FcR facilitates its localization to DRM [76,77], possibly through regulated generation of ceramide [78]. The consequences of FcR recruitment to DRM are not yet clear, but they appears to facilitate interaction of the receptor with early signaling kinases such as Lyn [76].…”

“…First, cholesterol-or ceramide-dependent clustering could facilitate initial kinase recruitment to ligated FcR [77,78]. Second, many cytoplasmic proteins contain structural elements-pleckstrin homology (PH); phox homology (PX); or Fab1p, YOTB, Vps27p, EEA1 (FYVE) domains-which bind to PIs and allow regulation of protein association with membranes [94 -96].…”