The Ebola nucleocapsid structure was solved to 19 Å resolution (FSC 0.5 criteria) using linear regions of 34,605 images (taken at 3-4 mm defocus, Figure 2 ). Since virus particles were not straight enough for conventional helical image processing, a combination of tomography, sub-tomogram averaging, single particle averaging, and the iterative helical real-space construction method were used [26, 27] . The EBOV nucleocapsid is a right-handed doublelayered helix with an outer diameter of 41 nm and a hollow inner channel 16 nm in diameter as determined by image analysis and tomography (Figures 2, 3 , 4, S2, S4, movie S1). The pitch is 6.96 nm, with 10.81 repeats per helical turn ( Figure 2 ). The inner nucleocapsid, composed of large subunits, is linked by vertical and horizontal contacts between the large subunits ( Figure 2E ,F). The horizontal contacts occur between the large subunits at a diameter of 22.3 nm. The vertical contacts linking the coils have a higher density than the horizontal ones, so we interpret the horizontal contacts as involving viral RNA (white arrow, Figures 2E, 4E ; movie S1) and the vertical contacts as protein-protein interactions (yellow arrow, Figure 2E ). The NP subunits are also linked by an outer horizontal layer at a diameter of 37 nm. This layer consists of a ring of bridges between adjacent large subunits ( Figure 2 ). The bridges joining the NP subunits are composed of two lobes, one of which of which is slightly bigger than the other. Previous studies that produced recombinant nucleocapsid-like structures showed that expressed VP24 and VP35 both independently associate with NP, but that all three proteins together are necessary to produce ,50 nm diameter helical nucleocapsid-like structures. When VP35, VP30, VP24, and NP were transfected together, approximately 50 nm diameter helical nucleocapsid-like structure was also generated, whereas NP alone generated helical NP-RNA complexes ,20-25 nm in diameter, which were nuclease sensitive [14, 28, 29] . Taken together these results suggest that VP24 and VP35 are the structural components of the bridge located on the periphery of the nucleocapsid ( Figure 2G ). It is not possible to accurately delineate VP24 and VP35 within the bridge at this resolution, however the density of the bridge is consistent with a predicted total mass of 60 kD, thus each bridge is composed of one molecule of VP24 and one molecule of VP35. It is likely that the larger lobe is VP35 and that VP24 therefore resides within the smaller lobe ( Figure 2F ). Thus, each bridge is composed of a VP24-VP35 heterodimer that holds adjacent NP molecules together horizontally. This structure explains how VP24 and VP35 are able to independently interact with NP, and why all three are required for the formation of a double-layered nucleocapsid. In addition, it implies that VP24 and VP35 can interact with each other as well as each interacting with a different site on the NP molecule in order to make an oligomeric structure.