MUTATIONS IN THE MATRIX PROTEIN OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 INHIBIT SURFACE EXPRESSION AND VIRION INCORPORATION OF VIRAL ENVELOPE GLYCOPROTEINS IN CD4(-LYMPHOCYTES() T)

Highly conserved amino acids in the second helix structure of the huma n immunodeficiency virus type 1 (HIV-1) MA protein were identified to be critical for the incorporation of viral Env proteins into HIV-1 vir ions from transfected COS-7 cells. The effects of these MA mutations o n viral replication in the HIV-1 natural target cells, CD4(+) T lympho cytes, were evaluated by using a newly developed system. In CD4(+) T l ymphocytes, mutations in the MA domain of HIV-1 Gag also inhibited the incorporation of viral Env proteins into mature HIV 1 virions. Furthe rmore, mutations in the MA domain of HIV-1 Gag reduced surface express ion of viral Env proteins in CD4(+) T lymphocytes. The synthesis of gp 160 and cleavage of gp160 to gp120 were not significantly affected by MA mutations. On the other hand, the stability of gp120 in MA mutant-i nfected cells was significantly reduced compared to that in the parent al, wild-type virus-infected cells. These results suggest that functio nal interaction between HIV-1 Gag and Env proteins is not only critica l for efficient incorporation of Env proteins into mature virions but also important for proper intracellular transport and stable surface e xpression of viral Env proteins in infected CD4(+) T lymphocytes. A si ngle amino acid substitution in MA abolished virus infectivity in divi ding CD4(+) T lymphocytes without significantly affecting virus assemb ly, virus release, or incorporation of Gag-Pol and Env proteins, sugge sting that in addition to its functional role in virus assembly, the M A protein of HIV-1 also plays an important role in other steps of viru s replication.