PARTICLE-SIZE DETERMINANTS IN THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1GAG PROTEIN

The retroviral Gag protein plays the central role in the assembly prof ess and can form membrane-enclosed, virus-like particles in the absenc e of any other viral products. These particles are similar to authenti c virions in density and size. Three small domains of the human immuno deficiency virus type 1 (HIV-1) Gag protein have been previously ident ified as being important for budding. Regions that lie outside these d omains can be deleted without any effect on particle release or densit y. However, the regions of Gag that control the size of HIV-1 particle s are less well understood. In the case of Rous sarcoma virus (RSV), t he size determinant maps to the CA (capsid) and adjacent spacer sequen ces within Gag, but systematic mapping of the HN Gag protein has not b een reported. To locate the size determinants of HIV-I, we analyzed a large collection of Gag mutants. To our surprise, all mutants with def ects in the MA (matrix), CA, and the N-terminal part of NC (nucleocaps id) sequences produced dense particles of normal size, suggesting that oncoviruses (RSV) and lentiviruses (HIV-1) have different size-contro lling elements. The most important region found to be critical for det ermining HIV-1 particle size is the p6 sequence. Particles lacking all or small parts of p6 were uniform in size distribution but very large as measured by rate zonal gradients. Further evidence for this novel function of p6 was obtained by placing this sequence at the C terminus of RSV CA mutants that produce heterogeneously sized particles. We fo und that the RSV-p6 chimeras produced normally sized particles. Thus, we present evidence that the entire p6 sequence plays a role in determ ining the size of a retroviral particle.