Diagram, radial density plot, and the packing arrangements in each layer of Gag. (A) Diagram of the Gag polyprotein (from left to right: MA (blue), CA NTD (cyan), CA CTD (yellow), SP1 (white), NC (red), SP2 (white), and p6 (purple)). (B) (left): 11 nm slice through the reconstruction of an immature virion. The arrows point to the NC (red), CA CTD (yellow), CA NTD (cyan), and MA (blue) layers of the Gag lattice. (B) (right): average radial density plot calculated from solid angles where the Gag lattice was well ordered, with color-coded boxes labeling the peaks. (C) The density of each layer of the Gag lattice (from left to right: NC, CA CTD, CA NTD, MA) in the virion shown (B) was projected radially onto separate concentric spherical shells to reveal possible packing arrangements. The radial boundaries used for each layer are shown by the color-coded boxes on the radial density plot (A, right). Here red represents higher density. Only the CA layers appear periodic, with patches of hexagonal order. Scale bar (bottom) 50 nm.