Demonstration of a Direct Interaction between σ-1 Receptors and Acid-Sensing Ion Channels

The sigma-1 receptor is a widely expressed protein that interacts with a variety of ion channels, including the acid-sensing ion channel (ASIC) 1a. Here we used atomic force microscopy to determine the architecture of the ASIC1a/sigma-1 receptor complex. When isolated His(8)-tagged ASIC1a was imaged in complex with anti-His(6) antibodies, the angle between pairs of bound antibodies was 135 degrees, consistent with the known trimeric structure of the channel. When ASIC1a was coexpressed with FLAG/His(6)-tagged sigma-1 receptor, ASIC1a became decorated with small particles, and pairs of these particles bound at an angle of 131 degrees. When these complexes were incubated with anti-FLAG antibodies, pairs of antibodies bound at an angle of 134 degrees, confirming that the small particles were sigma-1 receptors. Of interest, we found that the sigma-1 receptor ligand haloperidol caused an similar to 50% reduction in ASIC1a/sigma-receptor binding, suggesting a way in which sigma-1 ligands might modulate channel properties. For the first time, to our knowledge, we have resolved the structure of a complex between the sigma-1 receptor and a target ion channel, and demonstrated that the stoichiometry of the interaction is 1 sigma-1 receptor/1 ASIC1a subunit.