The release of retroviruses from cells requires ubiquitination of Gag and recruitment of cellular
proteins involved in endosome sorting, including the
ESCRT-III proteins and the Vps4
ATPase. In response to
infection, cells have evolved an
interferon-induced mechanism to block virus replication through expression of the
interferon-stimulated gene 15 (ISG15), a dimer homologue of
ubiquitin, which interferes with
ubiquitin pathways in cells. Previously, it has been reported that ISG15 expression inhibited the
E3 ubiquitin ligase, Nedd4, and prevented association of the
ESCRT-I protein Tsg101 with human immunodeficiency virus type 1 (HIV-1) Gag. The budding of
avian sarcoma leukosis virus and HIV-1 Gag virus-like particles containing L-domain mutations can be rescued by fusion to ESCRT
proteins, which cause entry into the budding pathway beyond these early steps. The release of these fusions from cells was susceptible to inhibition by ISG15, indicating that there was a block late in the budding process. We now demonstrate that the Vps4
protein does not associate with the
avian sarcoma leukosis virus or the HIV-1 budding complexes when ISG15 is expressed. This is caused by a loss in interaction between Vps4 with its coactivator
protein LIP5 needed to promote the formation of the ESCRT-III-Vps4 double-hexamer complex required for membrane scission and virus release. The inability of LIP5 to interact with Vps4 is the probable result of ISG15 conjugation to the
ESCRT-III protein, CHMP5, which regulates the availability of LIP5. Thus, there appear to be multiple levels of ISG15-induced inhibition acting at different stages of the virus release process.