SNARE mimicry by the CD225 domain of IFITM3 enables regulation of homotypic late endosome fusion.

The CD225/Dispanins superfamily consists of membrane proteins that regulate vesicular transport and membrane fusion events driving neurotransmission, glucose transport, and antiviral immunity. However, how the CD225 domain controls membrane trafficking was unknown. We reveal that the CD225 domain contains a SNARE-like motif that enables interaction with cellular SNARE fusogens. Proline rich transmembrane protein 2 (PRRT2) encodes a SNARE-like motif that enables interaction with neuronal SNARE proteins, and mutations therein disrupt SNARE binding and are linked to neurological disease. Another CD225 member, interferon-induced transmembrane protein 3 (IFITM3), protects cells against Influenza A virus infection. IFITM3 interacts with SNARE proteins that mediate late endosome-late endosome (homotypic) fusion and late endosome-lysosome (heterotypic) fusion. IFITM3 binds to syntaxin 7 (STX7) in cells and in vitro, and mutations that abrogate STX7 binding cause loss of antiviral activity against Influenza A virus. Mechanistically, IFITM3 disrupts assembly of the SNARE complex controlling homotypic fusion and accelerates the trafficking of endosomal cargo to lysosomes. Our results suggest that SNARE modulation plays a previously unrecognized role in the diverse functions performed by CD225 proteins.