Separable mechanisms of attachment and cell uptake during retrovirus infection

In the absence of viral envelope gene expression, cells expressing human im munodeficiency virus type 1 HIV-1) gag and pol, accessory HIV functions, an d a vector genome RNA produce and secrete large amount of noninfectious vir us-like particles (VLPs) into the conditioned medium. After partial purific ation, such HIV-1 VLPs can be made infectious in cell-free conditions in vi tro by complex formation with lipofection reagents or with the G protein of vesicular stomatitis virus (VSV-G). The resulting in vitro-modified HIV-1 particles are able to infect nondividing cells. Infectivity of envelope-fre e HIV VLPs can also be induced by prior modification of target cells throug h exposure to partially purified VSV-G vesicles. Similarly, infection can b e carried out by attachment of envelope-free noninfectious VLPs to unmodifi ed cells followed by subsequent treatment of cells with VSV-G. We interpret these findings to indicate that interaction between a viral envelope and a cell surface receptor is not necessary for the initial virus binding to th e cells but is required for subsequent cell entry and infection.