IN-VITRO CELL-FREE CONVERSION OF NONINFECTIOUS MOLONEY RETROVIRUS PARTICLES TO AN INFECTIOUS FORM BY THE ADDITION OF THE VESICULAR STOMATITIS-VIRUS SURROGATE ENVELOPE G-PROTEIN

In the absence of envelope gene expression, retrovirus packaging cell lines expressing Moloney murine leukemia virus (MLV) gag and pol genes produce large amounts of noninfectious virus-like particles that cont ain reverse transcriptase, processed Gag protein, and viral RNA (Sag-p ol RNA particles). We demonstrate that these particles can be made inf ectious in an in vitro, cell-free system by the addition of a surrogat e envelope protein, the G spike glycoprotein of vesicular stomatitis v irus (VSV-G). The appearance of infectivity is accompanied by physical association of the G protein with the immature, noninfectious virus p articles. Similarly, exposure in vitro of wild-type VSV-G to a fusion- defective pseudotyped virus containing a mutant VSV-G markedly increas es the infectivity of the virus to titers similar to those of conventi onal VSV-G pseudotyped viruses. Furthermore, similar treatment of an a mphotropic murine leukemia virus significantly allows infection of BHK cells not otherwise susceptible to infection with native amphotropic virus. The partially cell-free virus maturation system reported here s hould be useful for studies aimed at the preparation of tissue-targete d retrovirus vectors and will also aid in studies of nucleocapsid-enve lope interactions during budding and of virus assembly and virus-recep tor interactions during virus uptake into infected cells. It may also represent a potentially useful step toward the eventual development of a completely cell-free retrovirus assembly system.