Research Interests

Many organisms produce potent toxic proteins that exert their action, and sometimes this is a lethal activity, towards other cell types. By doing this, such proteins help increase the chance of survival or proliferation of the producing organism.  Furthermore, many toxins have an exquisitely specific action. For example, the proteins we principally work with - ricin and other ribosome inactivating proteins (RIPs) produced by plants and bacteria, have evolved to selectively hit a key molecule within the cells of susceptible organisms, enabling a fatal disruption of protein synthesis. As we and others have discovered, these particular toxins exploit existing intracellular transport pathways to travel from the cell surface to their substrates in the cytosol. Once delivered they catalyse irreversible damage, and thereby promote the death of the cell.  

Our research group is interested in elucidating molecular details of the cellular uptake of ricin and other protein synthesis inhibiting toxins, and of using these proteins as tools to study certain intracellular transport processes in mammalian, yeast and plant cells. This work includes investigation of not only the vesicle trafficking aspects of toxin uptake into eukaryotic cells, but also of the membrane transport step that must ultimately deliver the catalytic subunits into the cytosol. In this final transport step the toxins appear to exploit a natural process where defunct ER proteins are detected and dislocated to the cytosol for proteasomal degradation. It isn't yet entirely clear how the toxins we study accomplish this step and then avoid the normal fate of such ejected proteins, namely degradation, although preliminary studies suggest a role of substrate ribosomes in preventing this fate.

One fascinating side to the problem of cells and toxins concerns the strategies used by RIP- producing plants in making and storing these deadly poisons. How do they survive the presence of the poison? In situations where toxins are active on endogenous substrates in the cytosol, they must be efficiently transported from their site of synthesis on cytosolic ribosomes to their site of storage. We are currently investigating the biosynthesis of ricin and its targeting to the safe haven of plant cell vacuoles.

The enzymatic activity of RIPs is another area under investigation.  The RIPs we study are capable of inactivating ribosomes by catalysing a specific depurination of rRNA, although the activity depends on the source of ribosomes.  Interestingly, ricin A chain and pokeweed antiviral protein, which are virtually indistinguishable in 3D structure, have very different substrate specificities. Discovery of the ribosome-recognition determinant(s) is therefore of interest. Structural studies are also underway to determine the three-dimensional structure of a number of catalytic mutants of ricin and abrin A chain.

We are also keen to exploit the potency and/or trafficking properties of these proteins in therapeutic applications. For instance, over the years it has been shown possible to redirect toxins such as ricin towards unwanted classes of cells by making conjugates with other cell binding molecules. This provides opportunities to target and destroy cancer cells.  We are currently attempting to use disarmed versions of ricin and Shiga-like toxin as intracellular carriers of viral epitopes or tumour antigens to elicit a cytotoxic T lymphocyte response in a strategy that may provide novel T cell vaccines.