Efficient cell infection by moloney murine leukemia virus-derived particles requires minimal amounts of envelope glycoprotein

Retrovirus entry into cells is mediated by specific interactions between th e retrovirally encoded Env envelope glycoprotein and a host cell surface re ceptor. Though a number of peptide motifs responsible for the structure as well as for the binding and fusion activities of Env have been identified, only a few quantitative data concerning the infection profess are available . Using an inducible expression system, we have expressed various amounts o f ecotropic and amphotropic Env at the surfaces of Moloney murine leukemia virus-derived vectors and assayed for the infectivity of viral particles. C ontrary to the current view that numerous noncooperative Env-viral receptor interactions are required for cell infection, we report here that very sma ll amounts of Env are sufficient for optimal infection. However, increasing Env density clearly accelerates the rate at which infectious attachment to cells occurs. Moreover, our data also show that a surprisingly small numbe r of Env molecules are sufficient to drive infection, albeit at a reduced e fficiency, and that, under conditions of low expression, Env molecules act cooperatively. These observations have important consequences for our under standing of natural retroviral infection as web as for the design of cell-t argeted infection techniques involving retroviral vectors.