FURTHER EVIDENCE OF ICOSAHEDRAL SYMMETRY IN HUMAN AND SIMIAN IMMUNODEFICIENCY VIRUS

Specimens of HIV and SIV have been examined by electron microscopy, us ing the techniques of conventional thin sectioning, freeze-substitutio n, cryosectioning, and cryomicroscopy of frozen hydrated specimens. In addition freeze-drying and critical point drying were used for both s hadowed replicas and scanning electron microscopy. Thin sections revea led hexagonal, pentagonal, or near-spherical profiles. Angular particl es were seen in shadowed replicas and also by scanning electron micros copy. The images observed were consistent with an icosahedral shape of the virus. It is proposed that mature HIV (SIV) is an icosadeltahedro n with flat triangular facets. Size measurements of the specimens show ed a wide range of values for conventional embedding, but a narrow ran ge for specimens prepared by low-temperature techniques.