A BIPARTITE MEMBRANE-BINDING SIGNAL IN THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 MATRIX PROTEIN IS REQUIRED FOR THE PROTEOLYTIC PROCESSING OFGAG PRECURSORS IN A CELL TYPE-DEPENDENT MANNER

It is unclear whether proteolytic processing of the human immunodefici ency virus type 1 (HIV-1) Gag protein is dependent on virus assembly a t the plasma membrane. Mutations that prevent myristylation of HIV-1 G ag proteins have been shown to block virus assembly and release from t he plasma membrane of COS cells but do not prevent processing of Gag p roteins. In contrast, in HeLa cells similar mutations abolished proces sing of Gag proteins as well as virus production, We have now addresse d this issue with CD4(+) T cells, which are natural target cells of HI V-1, In these cells, myristylation of Gag proteins was required for pr oteolytic processing of Gag proteins and production of extracellular v iral particles. This result was not due to a lack of expression of the viral protease in the form of a Gag-Pol precursor or a lack of intera ction between unmyristylated Gag and Gag-Pol precursors. The processin g defect of unmyristylated Gag was partially rescued ex vivo by coexpr ession with wild-type myristylated Gag proteins in HeLa cells. The cel l type-dependent processing of HIV-1 Gag precursors was also observed when another part of the plasma membrane binding signal, a polybasic r egion in the matrix protein, was mutated. The processing of unmyristyl ated Gag precursors was inhibited in COS cells by HIV-1 protease inhib itors. Altogether, our findings demonstrate that the processing of HIV -1 Gag precursors in CD4(+) T cells occurs normally at the plasma memb rane during viral morphogenesis, The intra cellular environment of COS cells presumably allows activation of the viral protease and proteoly tic processing of HIV-1 Gag proteins in the absence of plasma membrane binding.