Passive and active inclusion of host proteins in human immunodeficiency virus type 1 gag particles during budding at the plasma membrane.

Human immunodeficiency virus type 1 particles form by budding at the surface of most cell types. In this process, a piece of the plasma membrane is modified into an enveloped virus particle. The process is driven by the internal viral protein Pr55(gag). We have studied how host proteins in the membrane are dealt with by Pr55(gag) during budding. Are they included in or excluded from the particle? The question was approached by measuring the relative concentrations of host and viral proteins in the envelope of Pr55(gag) particles and in their donor membranes in the cell. We observed that the bulk of the host proteins, including actin and clathrin, were passively included into the virus-like Gag particles. This result suggests that budding by Pr55(gag) proceeds without significant alteration of the original host protein composition at the cell membrane. Nevertheless, some proteins were concentrated in the particles, and a few were excluded. The concentrated proteins included cyclophilin A and Tsg-101. These were recruited to the plasma membrane by Pr55(gag). The membrane-bound cyclophilin A was concentrated into particles as efficiently as Pr55(gag), whereas Tsg-101 was concentrated more efficiently. The latter finding is consistent with a role for Tsg-101 in Gag particle release.