The GP spike is necessary for cellular attachment and fusion of EBOV. A definitive cell surface ligand for the receptor binding domain has not been identified, and a number of different cell surface proteins are able to enhance infection [42, 43] . Cleavage of GP results in two domains: GP1 containing the receptor binding domain and GP2 that contains the fusion and transmembrane domains. The structure of a smaller engineered fragment of GP1-GP2 has recently been determined by x-ray crystallography (3CSY pdb [42] ). We determined the structure of the entire EBOV GP trimeric spike at a resolution of 14 Å (FSC 0.5 criteria), by combining sub-tomograms from the spikes of VP40-GP VLPs (234 images) with EBOV images for the side perspective data (8084 images) using projection matching as previously described [44] ( Figure 5 and movie S4). In our reconstruction, the spike is in situ in the viral membrane, thus the transmembrane region and base of the spike, adjacent to the membrane, are less well defined than the distal region of the spike, due to the smaller differences in contrast between lipid and protein versus water and protein, as well as Fresnel fringes at the edge of the viral membrane. The spike extends 10 nm from the surface of the envelope, with a clubshaped head 6.5 nm in height, and a 3.5 nm long stalk. Docking of the previously determined X-ray structure into our cryo-EM map shows a good fit, with a correlation coefficient of 0.75 using the docking and correlation program SITUS [45] . The difference map calculated between our cryo-EM map and the GP1-GP2 structure identified volumes corresponding to the domains deleted to generate the 3CSY GP1-GP2 structure ( Figure 5 ). We show that the mucin-like domains (connected at V310 and E502 -shown in Green in Figure 5 ) completely fill the previously described bowllike chalice which contains the putative receptor binding sites [42] . The proximity of glycosylation sites in the GP1-GP2 structure suggests that the distal density of the spike contains the glycans that were deleted in order to construct the GP1-GP2 structure. In addition, each mucin-like domain has an ''arm-like'' projection, which extends radially at the distal end of the spike, to a maximum diameter of 13 nm. The localization of the mucin-like domain is consistent with previous studies showing that endosomal proteolysis plays a role in enhancing infectivity as well as binding of the Ebola GP to the plasma membrane [46, 47] .