STING signaling modulation by COPII cargo recognition.

Stimulator of interferon genes (STING) activation requires coat protein complex II (COPII)-mediated endoplasmic reticulum (ER) exit, but the mechanism remains elusive. Here, we identify EEΦxΦ ((339)EEVTV(343) in human STING) as the ER-exit motif recognized by SEC24 homolog C (SEC24C). Using AlphaFold3, we present a predicted structure of SEC24C binding to a STING dimer, revealing the EEΦxΦ motif in a previously structurally unresolved region. Mutations in this motif or the SEC24C cargo-binding site disrupt STING trafficking and signaling. Our findings support a STING oligomerization and avidity threshold model that explains regulated ER exit. The EEΦxΦ motif is conserved in vertebrate STING homologs and is sufficient to mediate ER exit of unrelated proteins. Interestingly, the STING ER-exit motif is suboptimal compared with known SEC24C cargos, which is crucial for preventing immune overactivation. An engineered "super-ER-exit" STING is constitutively active and induces potent antitumor immunity. Tandem repeats of this motif competitively inhibit endogenous STING signaling. Collectively, this study elucidates the STING-ER-exit mechanism and presents strategies for modulating STING signaling.