Ubiquitination is the post-translational modification of substrate proteins and involved in the regulation of almost all cellular signalling pathways. It is catalysed by a three protein enzymatic cascade, comprised of E1 activating, E2 conjugating and E3 ligating enzymes. TRIM family proteins are RING-type E3 ubiquitin ligases that are characterised by the presence of an N-terminal tripartite motif that harbours the catalytic RING domain and a C-terminal variable substrate-binding domain. TRIM28 is a part of class VI TRIM proteins that has unique properties: Instead of substrate binding domains, it harbours C-terminal PHD-Bromo domains which mediate its role in transcriptional repression. Additionally, while most TRIM proteins characterised so far require RING dimerisation for ubiquitination activity, all three members of class VI of TRIM ligases, including TRIM28, have recently been shown to contain monomeric RINGs and indeed show no apparent E3 ligase activity. Nevertheless, ubiquitination activity has been described for TRIM28 in a cellular context, especially in cancer cells, in conjunction with MAGE proteins. MAGE proteins are aberrantly expressed in many cancer types and have been proposed to regulate and activate TRIM28 E3 ligase activity to mediate proteasomal degradation of substrates, such as p53 and AMPK. However, the regulation and molecular details of MAGE dependent activation of TRIM28 E3 ligase activity remain unknown. Furthermore, it is unclear, how TRIM28 suppresses E3 ligases activity while being part of transcriptional complexes, as there is currently no evidence in the literature that suggests that both functions of TRIM28 are functionally connected.
The goal of the action was to use a multi-disciplinary approach to study MAGE-dependent regulation of TRIM28 ligase activity. Unbiased proteomic studies together with in vitro biochemical, biophysical, and structural experiments tried to identify cellular signals such as posttranslational modifications and interaction partners that can modulate MAGE-TRIM28 complexes and regulate the interplay between its transcriptional activity and E3 ligase activity. To uncover how MAGE proteins modulate substrate recognition, E2 recruitment and ubiquitin ligase activity of TRIM28, and to establish a molecular model of the MAGE-TRIM28 complex, structural and mechanistic studies were performed.
