Topography of plasma membrane microdomains and its consequences for mast cell signaling

Thy‐1 (CD90) is a glycoprotein bound to the plasma membrane by a GPI anchor. Aggregation of Thy‐1 in mast cells and basophils induces activation events independent of the expression of Fcϵ receptor I (FcϵRI). Although we and others have previously suggested that plasma membrane microdomains called lipid rafts are implicated in both Thy‐1 and FcϵRI signaling, properties of these microdomains are still poorly understood. In this study we used rat basophilic leukemia cells and their transfectants expressing both endogenous Thy‐1.1 and exogenous Thy‐1.2 genes and analyzed topography of the Thy‐1 isoforms and Thy‐1‐induced signaling events. Light microscopy showed that both Thy‐1 isoforms were in the plasma membrane distributed randomly and independently. Electron microscopy on isolated membrane sheets and fluorescence resonance energy transfer analysis indicated cross‐talk between Thy‐1 isoforms and between Thy‐1 and FcϵRI. This cross‐talk was dependent on actin filaments. Thy‐1 aggregates colocalized with two transmembrane adaptor proteins, non‐T cell activation linker (NTAL) and linker for activation of T cells (LAT), which had been shown to inhabit different membrane microdomains. Thy‐1 aggregation led to tyrosine phosphorylation of these two adaptors. The combined data indicate that aggregated GPI‐anchored proteins can attract different membrane proteins in different clusters and thus can trigger different signaling pathways.