FcRIn

FcRIn is a hypothetical intracellular adaptor protein that has been proposed to participate in signaling downstream of Fc receptors in immune cells. In the proposed model, FcRIn acts as a molecular bridge connecting activated Fc receptors to intracellular kinases and adaptor networks, thereby shaping responses such as phagocytosis and cytokine production. The precise molecular identity of FcRIn remains to be established, and different research groups have reported candidate proteins with varying domain architectures that could fulfill the proposed function.

Putative features of FcRIn include modular domains such as SH2-like motifs or other phosphotyrosine-binding regions that enable recruitment of kinases (for example, Syk) and adaptor proteins to FcR signaling complexes. These domains would allow FcRIn to recognize phosphorylated motifs on activated Fc receptors and assemble signaling platforms that control downstream responses.

Expression and interactions: Evidence for FcRIn comes mainly from in vitro studies and animal models. It is suggested to be expressed in myeloid lineages such as monocytes, macrophages, and neutrophils, with potential inducibility by inflammatory stimuli. Speculative models propose interactions with ITAM-bearing Fcγ receptors, SH2-domain signaling proteins, and kinases like Syk or Lyn.

Clinical relevance and research status: Because FcRIn is not yet definitively characterized, its physiological significance remains uncertain. If confirmed, it could represent a target to modulate Fc receptor–mediated responses in autoimmune disease, infection, and cancer immunotherapy. Current challenges include lack of consensus on identity, redundancy with other adaptors, and the need for in vivo validation. See also: Fc receptors, Syk kinase, ITAM, adaptor proteins in immune signaling.