Split-luciferase assays were undertaken in 293Ts seeded in 24-well plates. We cotransfected 30 ng each of PB2, PA, and PB1, with the N terminus of Gaussia luciferase (Gluc1) tagged to its C terminus after a GGSGG linker cotransfected using Lipofectamine 3000 along with ANP32A, tagged with the C terminus of Gaussia luciferase (Gluc2) on its C terminus (after a GGSGG linker). Twenty-four hours later, cells were lysed in 100 μl of Renilla lysis buffer (Promega), and Gaussia activity was measured using a Renilla luciferase kit (Promega) on a FLUOstar Omega plate reader. Normalized luminescence ratios (NLRs) were calculated by dividing the values of the tagged PB1 and ANP32 wells by the sum of the control wells, which contained (i) untagged PB1 and free Gluc1, and (ii) untagged ANP32A and free Gluc2 as described elsewhere (15 (link), 37 (link)).
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