‘Hit-and-run’ interaction between proteins appears to be key trigger for cell death

A 'hit-and-run' interaction between two proteins could be an important trigger for cell death, according to new research from Walter and Eliza Hall Institute researchers.

The researchers investigated the chain of events that activates the protein BAX, which is a crucial driver of apoptosis, the major form of cell death. Addressing a long-standing question in the field, they discovered that BAX is activated for cell death by transient interactions with so-called 'BH3-only' proteins at two distant sites on BAX.

The research, led by Dr Michael Dengler and Professor Jerry Adams, was published today in the journal Cell Reports.

At a glance

• Activation of the protein BAX is a key step in apoptosis, the programmed death of cells.
• Our researchers revealed a previously controversial 'hit-and-run' interaction between BAX and 'BH3-only' proteins that initiates the conversion of BAX into a lethal protein for cells.
• The discovery, which clarifies a long-standing question in apoptosis research, could underpin the development of new approaches to trigger or prevent the death of cells in certain diseases.

Triggering cell death

Apoptosis is the major way our bodies remove damaged or unwanted cells. Many different stimuli trigger apoptosis by turning on cell signaling pathways that activate BAX and its close relative BAK. Activated BAX and BAK create holes in the cell's energy factories, the mitochondria. Once mitochondria are damaged, cells are compelled to die.

Professor Adams said a long-standing question in apoptosis research had been how BAX is triggered to move to mitochondria once cell death is triggered. "The events activating BAX once it has embedded on the surface membrane of mitochondria have been well-characterized - we know that death-inducing BH3-only proteins bind to BAX, changing its shape to damage the mitochondrial membrane," he said.

"There had been hints that BH3-only proteins are also the signal for BAX to move from its location in a healthy cell's cytosol - the liquid interior of the cell - to the mitochondria, but the experimental data supporting this were controversial and weak."

Two-step activation

To understand how BAX interacts with BH3-only proteins, Dr Dengler and colleagues strategically altered different regions of BAX, subtly changing the protein's structure. By comparing the behavior of these mutant forms of BAX with that of normal, unmutated BAX, they could determine the function of different regions of BAX, he said.

"We discovered that two different parts of BAX could bind to BH3-only proteins," he said. "Intriguingly, these sites functioned at different stages of BAX activation.

"One site prompted BAX to move to the mitochondrial membrane. Binding of BH3-only proteins to this site on BAX changed BAX's structure, releasing a 'tail' that anchors BAX to mitochondria. When BH3- only proteins bound the other site on BAX, BAX became able to damage the mitochondria."

These two distinct steps in BAX activation had not previously been clearly distinguished.

"The first, early activation step had never been well characterized, because it appears to involve a transient 'hit-and-run' interaction between BAX and a BH3-only protein. We think this first step might be a way that BAX activation can be fine-tuned," Dr Dengler said.

The research involved collaborations with structural biology and proteomics researchers at the Institute, aided by the Australian Synchrotron and the CSIRO Collaborative Crystallisation Centre. "Structural biology and proteomics were critical technologies for understanding how BAX is activated," Dr Dengler said.

As well as explaining a key detail in how cell death is executed, the research may in the future lead to new classes of drugs that modify BAX function.

"BAX is a key mediator of cell death, and many major diseases involve either too little or too much cell death. Our discovery may eventually underpin the search for drugs that promote apoptosis by activating BAX, which may have potential for treating cancer. Conversely, drugs that block BAX activation could help to prevent the harmful cell death that occurs in neurodegenerative disorders or stroke," Professor Adams said.