Battling Cattle Bloat

The 3D structure of Bovine Salivary Protein 30b (BSP30b) and its interaction with specific rumen bacteria.
Vic Arcus web
Professor Vic Arcus

For years scientists have been trying to find out why cattle get bloat. It’s one of the most common causes of death in adult animals. University of Waikato and AgResearch scientists have joined forces to research why some cattle are more susceptible than others.

Cattle produce more than 56 litres of saliva a day, and AgResearch’s Dr Tom Wheeler reckoned all that liquid going into a cow’s rumen might be responsible for bloat. So he tested the saliva from two groups of cattle, one where the animals experienced little bloat, and the other that had regular sufferers. He found the cattle that weren’t prone to bloat had elevated levels of a particular protein, Bovine Salivary Protein (BSP30).

So the take-home story was that BSP protected cows from bloat, but the next question was to find out how BSP was doing that. This is where University of Waikato professor of biological sciences Vic Arcus enters the story. “I suggested we use structural biology to get clues about BSPs.”

Easier said than done. “You have to purify your protein, crystalise it and then use x-ray diffraction to solve the structure. So we developed ways to do the first two steps, and we collected the diffraction data, but we just couldn’t solve the structure of the protein. We were stuck.”

Professor Arcus recruited a PhD student Heng Zhang to try to clear the last hurdle in the protein puzzle. And then they decided to call in the heavy machinery, a synchrotron. It’s a huge particle accelerator, worth $250 million, housed in Melbourne “and uses about the same amount of power as the whole of Hamilton”, says Professor Arcus. “The synchrotron enabled us to collect data that allowed us to solve the structure.”

The scientists were able to see the position of every atom in the protein, about 3000 of them. “What we saw immediately was a big tunnel, or tube in the protein that looked like it was going to bind to lipids, which are fatty molecules, and we could see that the protein formed the tube, and it was the right size to take a lipid,” says Professor Arcus. The picture was slowly coming together. Plants are full of lipids, it’s where their energy comes from, but lipids are not very soluble. “And one of the things about bloat is that it’s caused by foaming which means these insoluble molecules, the lipids, were potentially responsible for forming this foam in the rumen.”

To test that idea, they did a very basic experiment; they put the protein in a tube with olive oil and water.  “Olive oil is completely insoluble, and the protein acted like a surfactant, like a detergent if you like, and it basically dissolved the oil, turning it into tiny, micro-droplets that made it so much more palatable. So we were thinking that cattle that produce huge amounts of these proteins in saliva dissipate the lipids and inhibit foaming.”

Having tested this in the lab, they needed to test it on the real thing. They wanted to know if the protein would bind to bacteria because bacteria have a cell wall made up of lipids. Dr Graeme Attwood at AgResearch provided some rumen contents and Professor Arcus and Heng Zhang did a purification of the bacteria. “We found that our protein, the BSP, only attached itself to a few bacteria. That was completely surprising,” says Professor Arcus. “We thought it would combine with all bacteria. So the protein is obviously doing something else as well. In a soup of tens of thousands of bacteria, the protein is picking out just one or two species in that huge soup. Which is incredibly intriguing and we don’t know why.” (Roll on another PhD!)

“But because BSP is a surfactant, I think we’ve potentially found the relationship between high levels of this protein (BSP) and low susceptibility to bloat,” Professor Arcus says.