It causes other proteins to multiply and clump together, damaging nerve cells in the brain.
The discovery offers hope of a cure for the devastating disease, which makes the brain shrink dramatically, leading to dementia.
The rogue substance, known as PAR-5, has a toxic “cross-seeding” effect on proteins called amyloid beta, German scientists found.
Clumps of these amyloid proteins are a distinctive feature of Alzheimer’s disease. Last night researchers in the UK welcomed the findings.
Dr Rosa Sancho, of Alzheimer’s Research UK, said: “We know that the build-up of sticky amyloid protein occurs in the brain in Alzheimer’s disease but why this is triggered remains unclear.
“This new study has investigated the build-up of amyloid in a test tube.
“We don’t know whether what the researchers saw in their test tubes reflects what takes place in people’s brains but these experiments have highlighted additional processes that could help explain why amyloid clumps together.”
The team of German scientists examined how certain proteins which can collect in the brain alter their behaviour with ageing.
They believe their discovery could pave the way for new approaches to tackling such “ageing seeds”.
Dr Frank Baumann, of the Hertie Institute For Clinical Brain Research in Tubingen, where the study was carried out, said it was now important for such proteins in the brain to be mapped.
If this could be done in both healthy people and those with dementia, it might hold the key to tackling the disease.
He explained: “This would clarify which ageing seeds need to be looked at and whether they would be more prone to associate with specific disease types in specific anatomical areas.”
The study, published in the online journal Frontiers In Ageing Neuroscience, examined brain processes in worms and in mice.
In the UK, around 850,000 people suffer from dementia and the figures are rising rapidly as the population ages.
It was revealed earlier this week that the number of people who die from the condition is set to almost quadruple by 2040, from 59,199 to 219,409.
Rob Burley, director of policy at Alzheimer’s Society, said: “Dementia is set to be the 21st century’s biggest killer. It is the only leading cause of death that we can’t cure, prevent or slow down.
“There is also scant palliative care for people with dementia. Just eight per cent die at home and less than one per cent die in hospices.”
He added: “Everyone has the right to a dignified death in a place of their choosing, yet we continue to see the sustained failure by the system to prepare and plan for end of life care for people with dementia.
“This predicted four-fold increase in dementia deaths is a wake-up call to provide more palliative care services in the community that are appropriate for those with dementia.”