Proline residues in human immunodeficiency virus type 1 p6(Gag) exert a cell type-dependent effect on viral replication and virion incorporation of Pol proteins

The C terminus of the HIV-1 Gag protein contains a proline-rich domain term ed p6(Gag). This domain has been shown to play a role in efficient virus re lease and incorporation of Vpr into virions. In a previous study (X. F. Yu, L. Dawson, C. J. Tian, C. Flexner, and hi. Dettenhofer, J. Virol. 72:3412- 3417, 1998), we observed that the removal of the p6 domain of Gag as web as drastic mutations in the PTAP motif resulted in reduced virion-associated Pol proteins from transfected COS cells. In the present study, amino acid s ubstitutions at residues 5 and 7 of p6(Gag) resulted in a cell type-depende nt replication of the mutant virus in CD4(+) T cells; the virus was replica tion competent in Jurkat cells but restricted in H9 cells and primary blood -derived monocytes. Established Jurkat and H9 cell lines expressing p6(Gag) mutant and parental virus were used to further understand this defect. Mut ant virions produced from H9 cells, which displayed no defect in extracellu lar virion production, showed an similar to 16-fold reduction in Pol protei n levels, whereas the levels of Pal proteins were only marginally reduced i n mutant virions produced from Jurkat cells. The reduction in the virion-as sociated Pol proteins could not be accounted for by differences in the leve ls of intracellular p160(Gag-Pol) Or in the interaction between p55(Gag) an d p160(Gag-Pol) precursors. Electron microscopic analysis of the p6(Gug) mu tant virions showed a predominately immature morphology in the absence of s ignificant defects in Gag proteolytic cleavage. Taken together, these data suggest that the proline-rich motif of p6(Gag) is involved in the late stag es of virus maturation, which include the packaging of cleaved Pol proteins in viral particles, a process which may involve cell-type-specific factors .