A08: Large BCL-2 family protein complexes on the outer mitochondrial membrane and their role in the decision to undergo apoptosis

Project Leader: Georg Häcker

The decision to undergo mitochondrial apoptosis can be made by activating a sufficient amount of BH3-only proteins, such as BIM, which will activate BAX and/or BAK. BIM plays a role in this decision in many different situations of apoptosis. BIM often appears to be activated in the absence of substantial changes to protein abundance, but how this activation is achieved is unclear. We have made a number of observations that may be relevant here: in epithelial cells, BIM is not found in heterodimeric complexes with anti-apoptotic BCL-2 proteins but forms large protein complexes at the outer mitochondrial membrane, which are coordinated by dynein light chain 1 (DLC1). BIM binds to anti-apoptotic BCL-2 proteins only upon DLC1-binding. In epithelial cells, exclusively MCL-1 is recruited into the BIM-complexes, which can also contain the BH3-only protein BMF. MCL-1 can further directly bind DLC1. Intriguingly, the complexes are different in lymphoid progenitor cells, with non-exclusive MCL-1-BIM interaction. We believe that this complex-formation is important in the decision to trigger mitochondrial apoptosis and will here test this hypothesis. In the first part, we will investigate the molecular structure of the DLC1-organized, BCL-2 protein family-containing complexes in a cell-free system using purified components. In the second part, we will analyse these complexes in intact cells, using the different cell types and deletion and complementation strategies. In the third part, we will endeavour to advance our understanding of the role of BIM-MCL-1-BMF-DLC1-complexes in the induction of apoptosis and will test how these complexes sense, integrate and transmit upstream signals to undergo apoptosis. Models of mitochondrial apoptosis operate on the basis of heterodimeric binding within the members of the BCL-2 family, and no model can comprehensively explain mitochondrial apoptosis. We believe that understanding BIM-containing complexes will provide answers to this question of the regulation of decisions in mitochondrial apoptosis.