The performance of perforin-like proteins

This feature appeared in the May/June 2009 issue of Australian Life Scientist. To subscribe to the magazine, go here.

The great Belgian immunologist Jules Bordet first described the workings of the complement system and how it functions in the innate immune system back it the early 1900s. Bordet was awarded the Nobel Prize in 1919 for this and other discoveries, but since then the actual mechanism behind the lytic factor of complement has not been understood.

In 2007, Professor James Whisstock and his team, including his partner Dr Michelle Dunstone, solved the structure of a perforin protein called Plu-MACPF, and by doing so uncovered the mechanism by which these proteins work. They found that perforins are actually descended from bacterial pore-forming cholesterol cytolysins (CDCs), a specialised family of proteins used by bacteria such as anthrax, Clostridium perfringens and Clostridium difficile to disrupt cell membranes.

They found that the MACPF domain folds in the same way as CDCs and is probably used by vertebrates for defense against infection. The finding was momentous, Whisstock says.

“This project has trotted along for about 10 years,” Whisstock says. “We started working on finding out how perforin-like proteins actually work. When you look at proteins and protein sequences it is actually quite easy to find their relatives using bioinformatic approaches so there are very few surprises, but occasionally, just occasionally, we will solve a structure and the structure remembers its roots long after sequence motifs have disappeared.

“This is what happened to us – we solved the structure, looked at it and went ‘oh my God’ – it’s a bacterial cytolysin. We found that this is actually a common fold found in almost all life, with the exception of viruses and nematodes. Everything has a MACPF protein and they are actually bacterial descendents, the same as bacterial CDCs.”

Whisstock says the discovery raises a couple of very interesting questions, which his lab is now pursuing. “First of all, it is clear that bacteria and mammals use a common mechanism to defend themselves and attack one another.

“Secondly, we can explain how these things work. Complement was discovered over a hundred years ago by Jules Bordet, but the actual mechanism for that lytic factor has been a mystery ever since. Our work addresses that mystery.

“And finally, when you look at the roles of these things there are a lot of immunity proteins involved, but there is also a lot of proteins involved in processes where you don’t expect a MACPF protein to play a part, in developmental biology, in embryonic development, in neural development and so forth.

Then there are these MACPF proteins that perform an unknown role. For example, in Nature the other day a genome-wide association study pulled up one of our MACPF proteins in autism. So suddenly you’ve got this almost entirely new field and it is very rare in one’s career to have that opportunity.”

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