Jerry Adams-Projects

Jerry Adams-Projects


Determining the structure of the oligomers of Bak and Bax

As we have reviewed recently (Adams and Cory, Cell Death and Differentiation, 2017), the pivotal step in commitment of cells to undergo apoptosis is the formation of oligomers of Bak or Bax that permeabilise the mitochondrial outer membrane, but the structure of these oligomers remains unknown. To gain insights into their structures, we have devised a way to isolate substantial amounts of pure oligomers from mammalian cells, and are attempting to define their structures by both x-ray crystallography and cryoEM.

Clarifying how oligomeric Bak and Bax perforate the mitochondrial outer membrane

Although permeabilisation of the mitochondrial outer membrane (MOM) by oligomers of Bak and Bax is the pivotal event in apoptosis, how the topology of these proteins with respect to the MOM changes on oligomerisation has been unknown. It had been thought that several helices of Bax and Bak penetrate the membrane to initiate pore formation.

However, our studies with Dr Ruth Kluck’s lab at the Institute suggest instead that these helices insert only shallowly into the MOM, in-plane with it, and most likely disrupt its integrity by crowding the outer leaflet and forming proteolipidic pores (Westphal et al, Proc Natl Acad Sci USA 2014).

Defining early steps in activation of Bax

In unstressed cells, Bax is mainly a cytosolic monomer with its membrane anchor (a9) tucked into a surface groove. To perform its apoptotic function, Bax must first dislodge a9, so that it can accumulate on the MOM, but the trigger for this is uncertain. It has been proposed that binding to a novel site remote from the cognate surface groove on Bax provokes release of a9, but this site remains poorly defined. 

By structural and mutagenic analysis, we are clarifying how this binding site contributes to Bax activation. Our recent work has identified several mutations in Bax that affect this non-canonical site. Their analysis suggests that engagement of this site controls the translocation of Bax to the mitochondria via the allosteric release of a9 and an altered balance of Bax conformers.