PROTEASE (PR) IN VIRUS LIFE CYCLE AND AIDS PATHOGENESIS
The human immunodeficiency virus (HIV) genome consists of a single-stranded positive sense ribonucleic acid (RNA) molecule that is organized into three major coding elements: Gag, Pol, and Env.
Enzymes necessary for viral replication are included within the Pol gene and include protease, reverse transcriptase, and integrase. During the viral assembly and maturation, the P55 Gag precursor and P160 Gag-Pol precursors are protealitically cleaved by preformed protease, which is contained within the HIV virion and introduced into the cell at the time of initial infection. The preformed protease consequently generates new protease by virtue of new cleavages between the protease and reverse transcriptase regions of the Pol gene. HIV protease is critical to the HIV life cycle, as demonstrated by observations that mutation or inhibition of HIV protease results in formation of structurally disorganized noninfectious viral particles.HIV protease (PR) is also a cytotoxic protein that leads to the death of human and bacterial cells after transfection. These findings may suggest that HIV PR contributes to the death of HIV- infected T cells by directing induced cell death. This concept is further supported by the recent finding that inducible expression of HIV protease causes growth arrest, loss of plasma membrane, and lysis, both in Saccharomyces cerevisiae and in human cell lines. Although morphologically the mode of death in these cells is consistent with necrosis (as determined by electron microscopy), other features of apoptosis, such as caspase activation, mitochrondrial transmembrane potential change, and DNA fragmentation, were not directly assessed. Thus, although it is clear that HIV PR is cytotoxic, the mechanisms by which HIV PR induces cell death are unknown.
There are two subcellular locations of PR within infected cells that correspond to the two locations where viral polyproteins are processed: first, as membrane-associated polyproteins that are cleaved for virus assembly and maturation, and second, as free polyproteins processed within cytosols. Active PR within the cytosolic fraction of infected cells cleaves not only the viral Gag-Pol precursor but also nonviral proteins, including both structural and cell-death regulatory proteins. This chapter will review the science concerning the cytotoxic effects of HIV PR putative mechanisms responsible for cell death induced by PR and will review evidence that PR-induced killing might contribute to the T cell depletion seen in HIV infection.