FcεRI clusters coupling with signaling condensates and membrane raft domains in mast cell activation

T cell receptor (TCR) binding to foreign peptides presented by MHC (major histocompatibility complex) triggers T cell activation. Our recent work shows that this T cell activation is promoted by coupling membrane domains with protein condensates containing T cell signal transduction proteins LAT, Grb2, and Sos1. To test whether these observations are generalizable to other cellular systems, we evaluated the coupling of protein condensates to membrane domains in mast cells. Mast cells share many components in common with T cell activation, specifically, LAT, Grb2, and Sos1 are required for mast cell activation downstream of FcεRI clustering induced by multivalent antigens binding IgE. First, using giant plasma membrane vesicles (GPMVs) derived from a model mast cell line (RBL-2H3), we found that FcεRI partitions into the liquid disorder phase prior to antigen stimulation. The addition of a model antigen crosslinks the IgE-FcεRI and changes its partitioning, leading to enrichment in the relatively ordered (lipid raft) phase. Thus, antigen-induced clustering induces FcεRI raft affinity. Concurrently, TIRF imaging showed the formation of LAT+Grb2 condensates after RBL activation, which colocalized with activated FcεRI. These condensates recruited distinct membrane environments, demonstrating the potential coupling of raft domains with FcεRI and protein condensates. Our results indicate that raft coupling with signaling condensates may be a general mechanism for immune cell signaling regulation.