Structural organization of the influenza virus cRNP replicative intermediate. Structural organization of the influenza virus cRNP replicative intermediate. (A) One cRNP particle, extracted from an electron micrograph, is shown. NP-RNA loop end (L), central helical filament (C), and RdRp end (P) are labeled. (Scale bar, 20 nm.) (B) Class averages of cRNP particles. The two termini (T and T′) were not resolved in initial class averages. Further classification concentrated on subimages of the termini and the central filament (C). (C) Class averages of the central filament reveal variation due to variable curvature of the filament and different orientations around the filament long axis. Depending on the orientation, repeating triangular structural motifs were visible (triangle). (D) Class averages of the termini representing RdRp side views. Two globular densities, putatively corresponding to a large (white triangle) and small (black triangle) structural feature of RdRp are indicated. (E) Class averages of the NP-RNA loop side views reveal variation in loop diameter. The scale of B–E is the same as in A. Ashley York et al. PNAS 2013;110:45:E4238-E4245 ©2013 by National Academy of Sciences