Complementarity in the supramolecular design of arenaviruses and retroviruses revealed by electron cryomicroscopy and image analysis

Arenaviruses are rodent-borne agents of diseases, including potentially lethal human hemorrhagic fevers. These enveloped viruses encapsidate a bisegmented ambisense single-stranded RNA genome that can be packaged in variable copy number. Electron cryomicroscopy and image analysis of New World Pichinde and Tacaribe arenaviruses and Old World lymphocytic choriomeningitis virus revealed pleomorphic enveloped particles ranging in diameter from similar to400 to similar to2,000 Angstrom. The surface spikes were spaced similar to100 Angstrom apart and extended similar to90 Angstrom from the maximum phosphollipid headgroup density of the outer bilayer leaflet. Distinctive stalk and head regions extended radially similar to30 and similar to60 Angstrom from the outer bilayer leaflet, respectively. Two interior layers of density apposed to the inner leaflet of the viral lipid billayer were assigned as protein Z and nucleoprotein (NP) molecules on the basis of their appearance, spacing, and projected volume. Analysis of en face views of virions lacking the GP-C spikes showed reflections consistent with paracrystalline packing of the NP molecules in a lattice with edges of similar to57 and similar to74 Angstrom. The structural proteins of retroviruses and arenaviruses assemble with similar radial density distributions, using common cellular components.