FURTHER EVIDENCE FOR HEXAGONAL ORGANIZATION OF HIV GAG PROTEIN IN PREBUDDING ASSEMBLIES AND IMMATURE VIRUS-LIKE PARTICLES

The fullerene-like model for the organization of HIV gag encoded precu rsor pr55gag was based on the study of prebudding assemblies at the pl asma membrane of cells infected with a recombinant baculovirus express ing HPV-1 gag protein. The objective of the present study was to suppo rt the model by image processing of virus-like particles (VLP). In thi s work we used VLP purified by density gradient centrifugation, which caused partial or occasionally complete loss of the lipid bilayer in s ome VLP without the use of detergent. In addition more plasma membrane -associated pr55gag protein assemblies were processed. Image processin g of negatively stained specimens revealed the presence of threefold s ymmetry and a hexagonal network of rings with a resolution of 29 A in VLP and better than 25 A in membrane associated assemblies. The center -to-center spacing of the rings was 67 A in VLP and 70 A in membrane a ssemblies. Patches of gag protein oligomers at the plasma membrane wer e usually round and varying in size, but some of them were triangular. Indication of triangular-shaped gag protein assemblies was also seen in partly dissociated VLP. Since the hexagonal network is formed by th e uncleaved gag polyprotein, we conclude that the threefold symmetry a pplies to all domains including p24gag. The presence of threefold symm etry and the hexagonal network in VLP are consistent with the hypothes is that immature HIV particles possess icosahedral symmetry. (C) 1998 Academic Press.