word count: 197Text word count: 9541 author/funder. All rights reserved. No reuse allowed without permission.
AbstractInterferon-induced proteins with tetratricopeptide repeats (IFITs) are highly expressed during the cell-intrinsic immune response to viral infection. IFIT1 inhibits translation by binding directly to the 5' end of foreign RNAs, particularly those with non-self cap structures, precluding the recruitment of the cap-binding eukaryotic translation initiation factor 4F and subsequent 40S recruitment. Interaction of different IFIT family members is well described, but little is known of the molecular basis of IFIT association or its impact on function. Here, we reconstituted different complexes of IFIT1, IFIT2 and IFIT3 in vitro, which enabled us to reveal critical aspects of IFIT complex assembly. IFIT1 interacts rapidly and strongly with IFIT3 forming a stable heterotetramer. IFIT2 and IFIT3 homodimers dissociate to form a more stable heterodimer that associates with IFIT1, forming an IFIT1:IFIT2:IFIT3 trimer. Site-directed mutagenesis revealed a C-terminal 'YxxxL' motif in IFIT1 that mediates its association with IFIT3. Using various reporter mRNAs, we demonstrate for the first time that IFIT3 stabilises IFIT1 binding to cap0-mRNA and enhances its translation inhibition activity. Disrupting the binding interface between IFIT1 and IFIT3 abrogated this enhancement. This work reveals molecular aspects of IFIT assembly and provides an important 'missing link' between IFIT interaction and function.author/funder. All rights reserved. No reuse allowed without permission. doi: bioRxiv preprint PCR amplified to contain 5' NdeI and 3' XhoI restriction sites for cloning into pET28b (Novagen) producing a full-length protein with thrombin cleavable N-terminal 6-His tag. For reporter RNA transcription, the firefly luciferase reporter gene (Fluc) was PCR amplified using primers containing the 5' UTR and 3' UTR sequences of human -globin (NM_000518.4), including a 5' T7 promotor and EcoRI and PstI sites to facilitate cloning into pUC57. pUC57-ZIKV-Fluc was previously described (30).
Protein expression and purificationRecombinant IFITs were expressed in Rosetta 2 (DE3) pLysS (Novagen). Cells were grown to an OD600 of approximately 1 in 2x TY media at 37 C. Expression was induced by adding 1 mM isopropyl β-D-1thiogalactopyranoside. The induced culture was incubated at 22 C for 16 hours. Cells were harvested and lysed in a buffer containing 20 mM Tris-Cl pH 7.5, 400 mM KCl, 5 % glycerol, 1 mM DTT and 0.5 mM phenylmethylsulfonyl fluoride and 0.5 mg/ml lysozyme (from hen egg). IFITs were isolated by affinity chromatography on Ni-NTA Agarose beads (Qiagen). IFIT1 and IFIT1 mutants were additionally purified by FPLC on MonoQ (Q buffer: 20 mM Tris-Cl pH 7.5, 5 % glycerol, 1 mM DTT and 100-500 mM KCl), followed by MonoS 5/50 GL (S buffer: 30 mM HEPES pH 7.5, 5 % glycerol, 1 mM DTT and 100-500 mM KCl). IFIT2 and IFIT3 were treated with thrombin (from bovine plasma). author/funder. All rights reserved. No reuse allowed with...