Bulletin

Issue: March/April 2007

Research in Action

Understanding cell make-over

Understanding cell ‘make-over’ may yield dividends in fight against degenerative disease.

Dr Truscott, right, and Dr Dougan.

Over the past couple of decades, as the economic environment has changed, we’ve become used to the sight of old factories, warehouses and office blocks being turned into flats or town houses. Often, the outer structure of the building is left intact, but the interior is dismantled and remodelled. Some of the building materials are reused, while the rest are recycled elsewhere or used as landfill.

Such renovation and recycling happens in living systems too. As cells develop or encounter different environments, they are constantly changing to adapt to the new conditions. Instead of rearranging bricks and mortar, plasterboard and glass, cells break up old molecules - typically the proteins which regulate cellular activity - and make new and different replacements from the bits and pieces they recover. It’s a process which demands the biochemical equivalent of a jackhammer and recycling yard.

These exist in the form of proteases, the enzymes which dismantle proteins, and in disassembly machines - structures built from protein subunits which control the access to and flow through the degradation process.

‘Until recently it was thought that the regulation of cellular activities all happened at the level of activating genes and constructing new proteins,’ says Dr Kaye Truscott of the La Trobe University Biochemistry Department. ‘People tended to ignore the possibility that protein remodelling and degradation were important events as well.’

‘These molecular machines sculpt the cell’s response to its enviroment,’ says her scientific partner, Dr David Dougan. And they also act as a quality control system, able to dispose of proteins which have been damaged, incorrectly put together or are otherwise faulty.

The pair of researchers came back from Germany in 2004 as Queen Elizabeth II Fellows to establish a laboratory at the University supported by a five year Discovery Project Grant from the Australian Research Council to study protein quality control in bacteria. With the help of a second grant from the Australian Research Council, they are now embarking on a significant new line of study into the operation of protein disassembly or proteolytic machines - their role in maintaining the function of the cell’s energy factories known as mitochondria.

In addition to producing the energy to power all cellular activities, mitochondria play an important role in cell suicide, in detoxifying reactive compounds and in making important molecules. So the impact on cells of any disruption to the operation of the mitochondria can be dire. It’s not surprising that dysfunctional mitochondria have been implicated in many serious human diseases often associated with ageing, particularly conditions involving the degeneration of neurons, such as Parkinson’s disease, Alzheimer’s disease and motor neurone disease. Although the project is purely curiosity driven at this stage, it is not hard to see that it may provide considerable health dividends in the future.

The researchers are well suited to the project through their past experience. Dr Truscott spent more than five years at the University of Freiburg, Germany, working on the complex machineries that transport proteins into mitochondria.

Dr Dougan also worked at Freiburg, on bacteria and how they break down proteins. He then moved to the University of Heidelberg where he became interested in adaptor proteins, which bind to the disassembly machines and help deliver specific proteins to them.

Interestingly, mitochondria themselves are thought to have originated from an ancient free-living bacteria that became trapped inside a host cell. Mitochondria and bacteria share many features in common, and mitochondria even possess their own DNA formed into a typically circular bacterial chromosome. And their proteolytic machines are both formed from proteins belonging to the AAA+ superfamily - ATPases associated with a variety of cell activities.

The heart of a bacterial proteolytic machine is a barrel-shaped protein called a peptidase. Around its internal chamber are ranged two rings, each of which contains seven active sites which can break apart the links in the chain of amino acids which forms a protein. Sitting on top of the peptidase is another barrelshaped protein, the central chamber of which leads through a strategically-sized pore into the peptidase chamber.

The protein antechamber is known as an unfoldase. Proteins depend on their shape for their activity. When they are produced, like delicate works of origami, they fold up in a highly specific way. Before they can be chopped up and recycled, they must be unfolded. That’s the job of the unfoldase, which then feeds the unfolded chain into the peptidase.

But even before that, the protein must approach the unfoldase in the correct orientation. Some are recognised directly by the unfoldase and pulled into the machine. Others bind to a designated region of the unfoldase, and by doing so are pointed in the right direction. More typically an adaptor protein binds to the unfoldase and does the job of capturing the right protein and guiding it into the machine at the right time.

Why so complicated? It’s to ensure only those proteins that are defective or unwanted are chopped up and destroyed. Otherwise proteolytic machines could wreak havoc. And the presence of the adaptor proteins means that the machine can be retooled for different proteins when the pressures of development of environment change demand.

In their successful ARC grant application Drs Dougan and Truscott have outlined a program of research to investigate the operation of these machines in mitochondria. It includes checking to see how prevalent they are and identifying the proteins they process and exactly how they handle them. ‘As far as mitochondria go, we are only at the start of the journey to understand the importance of these machines,’ says Dr Dougan.

But the team already has a lead. The group recently identified a protein in mammalian mitochondria which interacts specifically with the mitochondrial unfoldase, mtClpX. This molecule also associates with two proteins known to form part of the mitochondrial chromosome. In fact, it is behaving suspiciously like an adaptor protein. Now it’s just a matter of years of painstaking research to trace just how it fits in with the rest of the machinery to operate the mitochondria’s wrecking yard and recycling works. - Tim Thwaites