Recombinant NA and Fabs were diluted with buffer (5mM HEPES, 150 mM NaCl pH 7.3) to approximately 0.02 mg/mL and 0.04 mg/mL respectively. To prepare Fab bound samples, equal volumes of NA and Fabs 1F2 or 4F11 were mixed and incubated for 5–10 minutes. The samples were adsorbed to plasma cleaned (Solarus Model 950 cleaner, Gatan Inc., Pleasanton, CA) EM grids coated with continuous carbon film that were subsequently washed with buffer and stained with 0.75% uranyl formate. Images were collected using EPU software (FEI Company, Hillsboro, OR) on a Tecnai T12 electron microscope (FEI Company) fitted with a 4K CCD camera (Gatan Inc.) at an effective pixel size of 0.18 nm in the specimen plane. The software package RELION 1.4 (50 (link)) was used to obtain 3D reconstructions. The maps for unbound NA, and for the complexes with 1F2, and 4F11 were constructed using 47,592, 4,326, and 13,665 particles, respectively and visualized using UCSF Chimera software (51 (link)).