The secondary structures of and the interactions between the intracellular domains (the three loops and the C-terminal tail) of the mouse-derived prostaglandin E2 receptor EP3 beta subtype were investigated using peptides mimicking the domains. The N-termini of the peptides were palmitoylated to anchor on unilamellar vesicles composed of phosphatidylserine, enriched in the cytoplasmic leaflet of mammalian plasma membranes. Circular dichroism spectroscopy revealed that the peptides corresponding to the intracellular third loop (D) and the C-terminus (C-term) assumed beta-sheet and associated alpha-helical structures, respectively. A structural change was observed when i3 was mixed with C-term, indicating an interaction between them. Fluorescence experiments showed that D suppressed the self-association of C-term, confirming the interaction. These results demonstrate for the first time specific interaction between the intracellular third loop and the C-terminus. A model is proposed for the activation of the receptor. (C) 2008 Elsevier Inc. All rights reserved.