The RTP Site Shared by the HIV-1 Tat Protein and the 11 S Regulator Subunit α is Crucial for their Effects on Proteasome Function Including Antigen Processing

The human immunodeficiency virus-1 Tat protein inhibits the peptidase activity of the 20 S proteasome and competes with the 11 S regulator/PA28 for binding to the 20 S proteasome. Structural comparison revealed a common site in the Tat protein and the 11 S regulator α-subunit (REGα) called the REG/Tat-proteasome-binding (RTP) site. Kinetic assays found amino acid residues Lys51, Arg52 and Asp67 forming the RTP site of Tat to be responsible for the effects on proteasomes in vitro. The RTP site identified in REGα consists of the residues Glu235, Lys236 and Lys239. Mutation of the REGα amino acid residues Glu235 and Lys236 to Ala resulted in an REGα mutant that lost the ability to activate the 20 S proteasome even though it still forms complexes with REGβ and binds to the 20 S proteasome. The REGα RTP site is needed to enhance the presentation of a cytomegalovirus pp89 protein-derived epitope by MHC class I molecules in mouse fibroblasts. Cell experiments demonstrate that the Tat amino acid residues 37–72 are necessary for the interaction of the viral protein with proteasomes in vivo. Full-length Tat and the Tat peptide 37–72 suppressed 11 S regulator-mediated presentation of the pp89 epitope. In contrast, the Tat peptide 37–72 with mutations of amino acid residues Lys51, Arg52 and Asp67 to Ala was not able to reduce antigen presentation.