Homology-guided identification of a conserved motif linking the antiviral functions of IFITM3 to its oligomeric state

The interferon-inducible transmembrane (IFITM) proteins belong to the Dispanin/CD225 family and inhibit diverse virus infections. IFITM3 reduces membrane fusion between cells and virions through a poorly characterized mechanism. We identified a GxxxG motif in many CD225 proteins, including IFITM3 and proline rich transmembrane protein 2 (PRRT2). Mutation of PRRT2, a regulator of neurotransmitter release, at glycine-305 was previously linked to paroxysmal neurological disorders in humans. Here, we show that glycine-305 and the homologous site in IFITM3, glycine-95, drive protein oligomerization from within a GxxxG motif. Mutation of glycine-95 in IFITM3 disrupted its oligomerization and reduced its antiviral activity against Influenza A and HIV-1. The oligomerization-defective variant was used to reveal that IFITM3 promotes membrane rigidity in a glycine-95-dependent manner. Furthermore, a compound which counteracts virus inhibition by IFITM3, amphotericin B, prevented the IFITM3-mediated rigidification of membranes. Overall, these data suggest that IFITM3 oligomers inhibit virus-cell fusion by promoting membrane rigidity.

Revelation of a Novel Protein Translocon in Bacterial Plasma Membrane

Many proteins are translocated across biomembranes via protein translocons in targeting to their subcellular destinations. Hitherto, the SecYEG/Sec61 translocon, existing in prokaryotes and eukaryotes, represents the most intensively studied one. According to the current perception, both periplasmic and β-barrel outer membrane proteins (β-barrel OMPs) are translocated via the SecYEG translocon in bacterial cells, although direct living cell evidences remain lacking. Here, mainly viain vivoprotein photo-crosslinking analysis, we revealed that the never reported membrane-integrated SecANprotein apparently functions as the translocon for β-barrel OMPs. Additionally, SecANcontains a GXXXG motif known for mediating protein interactions in biomembranes, and processing of β-barrel OMP precursors was severely affected in cells producing an assembly-defective SecANvariant resulted from the GXXXG motif mutations. Furthermore, SecANwas demonstrated to directly interact with the Bam complex, thus likely be a part of the supercomplex that we revealed earlier to be responsible for β-barrel OMP biogenesis.