CLEAVAGE OF THE MURINE LEUKEMIA-VIRUS TRANSMEMBRANE ENV PROTEIN BY HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 PROTEASE - TRANSDOMINANT INHIBITION BY MATRIX MUTATIONS

We have identified mutations in the human immunodeficiency virus type 1 (HIV-1) matrix protein (MA) which block infectivity of virions pseud otyped with murine leukemia virus (MuLV) envelope (Env) glycoproteins without affecting infectivity conferred by HIV-1 Env or vesicular stom atitis virus G glycoproteins. This inhibition is very potent and displ ays a strong transdominant effect; infectivity is reduced more than 10 0-fold when wild-type and mutant molecular clones are cotransfected at a 1:I ratio. This phenomenon is observed with both ecotropic and amph otropic MuLV Env. The MA mutations do not affect the incorporation of MuLV Env into virions. We demonstrate that in HIV-I virions pseudotype d with MuLV Env, the HIV-I protease (PR) efficiently catalyzes the cle avage of the p15(E) tansmembrane (TM) protein to p12(E). Immunoprecipi tation analysis of pseudotyped virions reveals that the mutant MA bloc ks this HIV-1 PR-mediated cleavage of MuLV TM. Furthermore, the transd ominant inhibition exerted by the mutant MA on wild-type infectivity c orrelates with the relative level of p15(E) cleavage. Consistent,vith the hypothesis that abrogation of infectivity imposed by the mutant MA is due to inhibition of p15(E) cleavage, mutant virions are significa ntly more infectious when pseudotyped with a truncated p12(E) form of MuLV Env. These results indicate that HIV-1 Gag sequences can influenc e the viral PR-mediated processing of the MuLV TM Env protein p15(E). These findings have implications for the development of HIV-1-based re troviral vectors pseudotyped with MuLV Env, since p15(E) cleavage is e ssential far activating membrane fusion and virus infectivity.