Assembly of the matrix protein of simian immunodeficiency virus into virus-like particles.

To obtain a better understanding of the processes of assembly and morphogenesis of simian immunodeficiency virus (SIV), recombinant vaccinia viruses containing regions of the gag-pol open reading frame were constructed and their intracellular expression as well as the ability of the Gag polypeptides to be released into the culture medium as constituents of virus-like particles were studied. Biochemical and electron microscopy analyses of cells infected with a recombinant expressing only the SIV matrix (MA) domain of the Gag polyprotein (v-p17 gag) showed that this protein self-assembles into 100-nm virus-like particles which are released into the culture medium. Interestingly, coexpression of SIV MA and Env proteins resulted in incorporation of gp120 and gp41 proteins into the recombinant p17-made particles. In addition when a positively charged domain of SIV MA (residues 26-33), which is highly conserved among all HIV and SIV MA proteins, was mutated into an acidic region, particle release was abolished without affecting protein expression, processing, or stability. Further characterization of the phenotype of this mutant by electron microscopy indicated that this mutant was blocked at the stage of assembly. These results suggest that SIV MA protein, along with its function in myristic acid-mediated membrane targeting, has intrinsic information for self-assembly as well as incorporation of viral Env glycoproteins into particles.