Deciphering the mechanism of the dual functional E3 ligase TRIM28 in cancer cells

TRIM family proteins are RING-type E3 ubiquitin ligases with important roles in immune responses, autophagy and cellular signalling.

They 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/KAP1 is a unique family member that is better known for its role in transcriptional repression, which is mediated via its C-terminal PHD-Bromo domains.

Nevertheless, ubiquitination activity has been described for TRIM28, especially in cancer cells, in conjunction with Melanoma Antigen (MAGE) proteins.

MAGE proteins are aberrantly expressed in many cancer types and have been proposed to regulate and activate TRIM28 E3 activity to mediate proteasomal degradation of prominent targets, including p53 and AMPK. Cellular and molecular details of TRIM28 MAGE-dependent ligase activity remain unknown.

Furthermore, it is unclear, how the interplay between the two functions of TRIM28 is regulated and how the protein E3 ligase activity is supressed when recruited as part of transcriptional silencing complexes.

I want to use a multi-disciplinary approach and combine unbiased proteomic studies with in vitro biochemical, biophysical and structural experiments to study MAGE-dependent regulation of TRIM28 E3 ligase activity.

I aim to identify cellular signals such as posttranslational modifications and interaction partners that can both modulate MAGE-TRIM28 complexes and regulate the interplay between the transcriptional and the E3 ligase activity.

By combining these findings with structural and mechanistic studies, I will uncover how MAGE proteins modulate substrate recognition, E2 recruitment and ligase activity of TRIM28, and create a model of MAGE-TRIM28 function.