Structure of an immature MLV particle. A segment from a rotationally averaged cryoelectron microscopic image of a single immature MLV particle is shown on the right. As indicated on the left, the particle is bounded by a lipid bilayer (“LB”), and the MA domain of Gag (pink) is associated with the inner leaflet of the bilayer. Interior to the MA domain is a zone of low density, presumably corresponding to the p12 domain. The most conspicuous feature of the image is the “railroad tracks”, representing the two domains within the CA domain (green), followed by the NC domain (blue) with bound RNA. The Pol and Env proteins are not visible in this image. As the particle is ~100 nm in diameter and the Gag molecules are ~20 nm in length, there is a region ~60 nm in diameter largely occupied by solvent in the center of the particle. (Reproduced from [9]. Copyright 1998, National Academy of Sciences, USA.)
