(A) Experimental strategy for formation of heterokaryons by expression of the fusogenic VSV-G protein. Heterokaryon formation between 293T cells expressing GFP and DF1 cells expressing RFP was monitored by the detection of dual-fluorescent cells (arrowheads). (B) Polymerase activity in heterokaryons expressing NP, PB1, PA, and no PB2, WT, or K627E PB2-TAP was determined by luciferase assay. Protein expression was monitored by western blot. n=3 +/− standard deviation.