Further evidence of icosahedral symmetry in human and simian immunodeficiency virus.

Specimens of HIV and SIV have been examined by electron microscopy, using the techniques of conventional thin sectioning, freeze-substitution, cryosectioning, and cryomicroscopy of frozen hydrated specimens. In addition freeze-drying and critical point drying were used for both shadowed replicas and scanning electron microscopy. Thin sections revealed hexagonal, pentagonal, or near-spherical profiles. Angular particles were seen in shadowed replicas and also by scanning electron microscopy. The images observed were consistent with an icosahedral shape of the virus. It is proposed that mature HIV (SIV) is an icosadeltahedron with flat triangular facets. Size measurements of the specimens showed a wide range of values for conventional embedding, but a narrow range for specimens prepared by low-temperature techniques.