Cryo-EM and specimen preparation was performed as previously described [11, 12] . Briefly, specimens on glowdischarged Quantifoil grids (Quantifoil MicroTools GmbH, Jena, Germany) were plunge cooled in liquid ethane using a Vitribot Mark IV (FEI Company). Specimens were imaged at 200 kV in a FEI Tecnai 20G 2 electron microscope equipped with a 4K CCD camera (FEI Company, Hillsboro, Oregon, USA). Images were recorded at a magnification of 80 000·, (corresponding to 108 240· at the CCD detector, or 1.353 Å /pixel) using a total dose of 10 electrons/Å 2 and a defocus of 3.5-6 l. A total of 13 455 particles were analysed. Correction was made for the contrast transfer function using EMAN [13] . T = 3 or T = 4 capsids were separated in SPIDER [14] (Fig. 1b,c) . Two populations of 1887, and 11 568 images of T = 3 and T = 4 capsids, respectively, were analysed.