The human immunodeficiency virus type 1 carboxyl-terminal third of capsid sequence in Gag-Pol is essential but not sufficient for efficient incorporation of Pr160gag-pol into virus particles

To elucidate the role of the C-terminal portion of Gag in the incorporation of human immunodeficiency virus type 1 (HIV-1) Gag-Pol into virus particles, a series of HIV-1 Gag-Pol mutants with deletions in the C-terminalgag sequence was constructed and viral incorporation of the Gag-Pol deletion mutants was analyzed using cotransfecting 293T cells with a Pr55^gag expression plasmid. The biological function of the incorporated HIV-1pol gene product was tested using an infectivity assay of the released virus particles which were pseudotyped with the murine leukemia virus Env. Analysis indicated that Gag-Pol deletion mutants, with a removal of the matrix (MA) and/or nucleocapsid (NC) or of the N-terminal two thirds of thegag coding sequence, could be incorporated efficiently into virus particles and produce significant amounts of infectious virions when assayed in a single-cycle infection assay. In contrast, mutations involving a deletion of the major homology region and the adjacent C-terminal capsid sequence significantly affected Gag-Pol incorporation. However, incorporation into virus particles of a Gag-Pol deletion mutant retaining both the major homology region and the adjacent C-terminal capsid intact was still severely impaired. This suggests that the capsid major homology region and the adjacent C-terminal capsid sequence in Gag-Pol are necessary but not sufficient for the incorporation of HIV-1 Pr160^gag-pol into virus particles.