Single particle cryo-EM image processing was carried out using the EMAN/EMAN2 and SPIDER/WEB image processing program packages [54, 55] . Particle selection (EMAN) and contrast transfer function correction (EMAN2) were conducted on an Apple Inc. Mac Pro computer (12-core, Intel Xeon Nehalem processors 2.93 GHz, 32 GB Ram, Mac OS X 10.6.7). All subsequent calculations were performed on a Dell PowerEdge R900 4-way 64-bit Xeon X7460 processors, Six Core 2.67 GHz CPUs with 256 GB Ram running Linux (CentOS 5.2). Images were corrected for contrast transfer function (ctf) using the ''e2ctf.py'' function in the EMAN2 software package 5 , which estimates defocus and corrects for ctf by phase-flipping. Images of the spike (n = 8084 side perspective; n = 234 end-on perspective) and nucleocapsid (n = 34,605) were selected for image analysis. The resolution of the cryo-EM reconstruction was estimated by Fourier shell correlation using the FSC 0.5 criteria. In all subsequent sections image analysis procedures were conducted using the SPIDER software package unless otherwise stated.