The interaction between HTLV-1 Tax protein and the proteasome
This thesis presents studies on the interaction between the human T cell lymphotropic virus type 1 (HTLV-1) Tax protein and the 20S proteasome and the role of the interaction in cellular processes and the cytotoxic T cell (CTL) response against HTLV-1. The rapid translocation of Tax into the nucleus is described. Tax accumulates in the nucleus and forms unique bodies involved in transcriptional activation. It was further found that Tax associated with assembled nuclear 20S proteasomes and stimulated the chymotryptic and tryptic activities of the 20S proteasome, independent of the induction of the LMP2 and LMP7 proteasome subunits. Confocal microscopy revealed a partial colocalisation of Tax with nuclear proteasomes. A panel of Tax mutants was generated and their subcellular localisation and association with the 20S proteasome analysed. This analysis revealed that both the N- and C-terminus of Tax play a role in proteasome binding of Tax and further showed that proteasome binding was not sufficient for nuclear localisation of Tax. Therefore, Tax probably translocates into the nucleus prior to and independent of proteasome association. Tax specific CTL clones were generated and characterised using tetrameric MHC class I/peptide complexes. These CTL clones were used to investigate the requirements for processing and presentation of Tax for recognition by CTL. It was found that Tax was a metabolically very stable protein and that the presentation of the immunodominant Tax 11-19 epitope was dependent on the transporter associated with antigen presentation (TAP), independent of the expression of LMP2 and LMP7 proteasome subunits and resistant to treatment with the proteasome inhibitor lactacystin. It is proposed that the interaction between Tax and the 20S proteasome plays a role in Tax mediated transcriptional activation, leading to cellular activation and proliferation, and may not determine the immunodominance of Tax in the CTL response against HTLV-1.