Biosynthesis and Processing of the gag and pol Polyproteins of Human Immunodeficiency Virus Type 1 in Escherichia coli

The three major structural genes of human immunodeficiency virus type 1 (HIV-1) are arranged in the viral genome in the order of 5’ gag-pol-env 3’. The gag gene encodes four group specific antigens, MA(p17), CA(p24), NC(p7) and p6, while the pol gene codes for protease, reverse transcriptase/ribonuclease H (RT/RH, 66 kDa) and integrase (IN, 32 kDa) (Chassagne et al.., 1986; Henderson et al., 1988; Kramer et al., 1986; Lightfoot et al., 1986; Lillehoj et al., 1988). The primary translational product of the gag gene is a 55 kDa polyprotein (pr55gag) (Kalyanarman et al., 1984; Sarngadharan et al., 1985), and that of the pol gene is a gag-pol fusion protein 160 kDa in size (pr160gag-Pol) achieved by ribosomal frameshift near the 3’ end of the gag gene (Gendelman et al., 1987; Jacks et al., 1988). They are processed to the individual viral proteins by the protease encoded by the pol gene after the enzyme cleaves itself from the pr160gag-Pol precursor by autocatalysis (Farmerie et al., 1987; Graves et al., 1988). There are at least seven protease cleavage sites in the gag and pol polyprotein precursors (Darke et al., 1988), as depicted in Figure 1. The numbering of amino acid residues at the cleavage sites is based on the amino acid sequence predicted from the nucleotide sequence of clone HXB2 of HIV-1 isolate, HTLV HIB (Ratner et al., 1985).