Interaction Interface between 5-HT3A serotonin receptor and chaperone RIC-3

Serotonin or 5-hydroxytryptamine receptors type 3 (5-HT3) belong to the family of pentameric ligand-gated ion channels (pLGICs), which also includes other neurotransmitter-gated ion channels such as nicotinic acetylcholine receptors (nAChRs). pLGICs have been long-standing therapeutic targets for psychiatric disorders such as anxiety, schizophrenia, and addiction, and neurological diseases like Alzheimers and Parkinsons disease. Due to structural conservation and significant sequence similarities of pLGICs extracellular and transmembrane domains across the more than 40 subunits found in humans, clinical trials for drug candidates targeting these two domains have been largely hampered by undesired effects mediated by off-subunit modulation. With the present study, we explore the interaction interface of the 5-HT3A intracellular domain (ICD) with the resistance to inhibitors of choline esterase (RIC-3) protein. Previously, we have shown that RIC-3 directly interacts with the ICD of 5-HT3A subunits. Using a sequential deletion approach, we identified the L1-MX segment of the ICD fused to maltose-binding protein as sufficient for the interaction. For the present study, synthetic L1-MX-based peptides, Ala-scanning, and a pull-down assay identified positions W347, R349, and L353 as critical for binding to RIC-3. In complementary studies with full-length 5-HT3A subunits, the identified Ala substitutions reduced the modulation of functional surface expression by co-expression of RIC-3. Additionally, we found and characterized a duplication of the binding motif at the transition between the ICD MA-helix and transmembrane segment M4. Analogous Ala substitutions at W447, R449, and L454 disrupt MAM4-peptide RIC-3 interactions and reduce modulation of functional surface expression. In summary we identify two binding sites for RIC-3 with a shared duplicated motif in 5-HT3A subunits, one in the MX-helix and one at the MAM4-helix transition.