Stomatin (STOM) is a monotopic integral membrane protein found in all classes of life that has been shown to regulate members of the acid-sensing ion channel (ASIC) family. However, the mechanism by which STOM alters ASIC function is not known. Using chimeric channels, we combined patch-clamp electrophysiology and FRET to search for regions of ASIC3 critical for binding to and regulation by STOM. With this approach, we found that regulation requires two distinct sites on ASIC3: the distal C-terminus and the first transmembrane domain (TM1). The C-terminal site is critical for formation of the STOM–ASIC3 complex, while TM1 is required only for the regulatory effect. We then looked at the mechanism of STOM-dependent regulation of ASIC3 and found that STOM does not alter surface expression of ASIC3 or shift the pH dependence of channel activation. However, a point mutation (Q269G) that prevents channel desensitization also prevents STOM regulation, suggesting that STOM may alter ASIC3 currents by stabilizing the desensitized state of the channel. Based on these findings, we propose a model whereby STOM is anchored to the channel via a site on the distal C-terminus and stabilizes the desensitized state of the channel via an interaction with TM1.

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