The 5-HT_2A receptor is of particular clinical interest due to its suspected involvement in hallucinogenic and antipsychotic drug activity, thus implicating it as a potential therapeutic target for the treatment of various neuropsychiatric disorders (Dean, 2003). It is capable of signalling via various pathways, including the recruitment of the small G-protein ADP-Ribosylation Factor (ARF) and subsequent activation of phospholipase D (PLD) (Robertson et al, 2003). The process by which receptor-associated ARF may induce PLD activity is as yet unknown, however, recent experiments have isolated potential binding sites for both ARF 1 and PLD within the carboxy-terminal tail of the 5-HT_2A receptor. Glutathione S-Transferase (GST)-fusion proteins of 5-HT_2A receptor domains of interest were generated and immobilised on glutathione-Sepharose beads as a bait for interacting proteins, which were identified by subsequent Western blot. ARF 1 and to a much lesser extent ARF 6 bound selectively to the carboxy-terminal domain of the receptor. Further GST-fusion protein studies showed that residues 376-384, positioned at the junction between transmembrane domain 7 and the carboxy-terminal domain of the receptor were necessary for maximal ARF binding, implicating not only the conserved NPxxY motif but also part of the adjacent i4 loop in ARF binding. In COS7 cells expressing the 5-HT_2A receptor, co-transfection of wild type PLD1 facilitated, whilst negative mutant PLD1 inhibited, 5-HT-induced PLD responses without affecting PLC responses. These effects were not reproduced by PLD2 constructs. In addition, co-immunoprecipitation experiments have demonstrated that PLD1 specifically interacts with the 5-HT_2A receptor. Subsequent GST-fusion protein studies have further isolated the position of PLD1 binding, again within the carboxy-terminal domain of the 5-HT_2A receptor, but in this case, within residues K385 – V471, in a distal region distinct from the ARF 1 binding site. Further co-immunoprecipitation experiments with the M₃ muscarinic receptor have shown that distal regions of its carboxy-terminal tail also represent a specific PLD binding site but in this case both PLD 1 and PLD 2 are ligands. It has also been reported that the mu opioid receptor binds PLD2 (Koch et al, 2003). Comparison of potential PLD binding sites within these receptors may point to residues involved in PLD binding.