Prions are proteins that cause brain-wasting diseases such as Creutzfeld-Jakob Disease, also known as CJD, and mad cow disease, also known as bovine spongiform encephalopathy or BSE.

Scientists have firmed up the evidence that misshapen protein are responsible for brain-wasting diseases by showing how these infectious prions are created.

Researchers from the United States and China have artificially created a disease-causing prion using proteins from mice.

Prions are proteins that occur naturally in the cells of mammals. Infectious prions are abnormal, misshapen versions of this protein that cause neurodegenerative diseases such as Creutzfeldt-Jakob disease and bovine spongiform encephalopathy, also known as mad cow disease.

The scientists used a mouse prion protein, called PrP, created through genetic engineering in bacterial cells in their experiments.

They found that the protein interacts with lipids, the fatty molecules in the structures of cell membranes, and becomes contorted and improperly folded, changing it into a disease-causing prion.

Jiyan Ma of Ohio State University said the experiment, published this week in Science, is the strongest evidence yet that prions are the cause of these brain-wasting diseases.

"The major thing we showed in this study is that the infectious agent in these diseases is truly a misfolded protein," Ma said in a statement.

Ma and his colleagues at Ohio State and East China Normal University injected the artificially created prions into the brains of mice.

The mice started showing symptoms of brain-wasting disease about four months later. After the mice died, the researchers dissected their brains and found microscopic holes throughout, the classic sign of spongiform encephalopathy.

"We folded recombinant mouse prion protein into its normal shape, then converted it into a different conformation and showed that when it infected an animal, it caused full-blown prion disease, with all of the characteristics," Ma said.

Ma said there is still much more research to be done on prions and brain-wasting disease.

"For example, we still don't know what actually makes prions infectious or how their propagation causes damage in the brain," he said.