“animals make and use prions in their nervous systems as part of an essential function: stabilizing the synapses that constitute long-term memories. These natural prions aren’t infectious but on a molecular level they chain up exactly the same way as their disease-causing brethren. (Some researchers call them “prionlike” to avoid confusion.) This week, work from neuroscientist Kausik Si of the Stowers Institute for Medical Research, one of Kandel’s former students, shows that the prion’s action is tightly controlled by the cell, and can be turned on when a new long-term memory needs to be formed…

Their self-propagating chain reactions are the solution to a dilemma the cell faces: how to maintain a permanent memory when the set of cellular processes that formed the memory are long completed Or, as Si puts it, “How can you create a permanent state with molecules that are going to disappear within two months?…

Before the memory is formed a fly’s neuron is full of a version of the prion called Orb2B. Although this version can switch shapes to form prions’ characteristic clumps, it can’t get started without the related protein Orb2A. In this week’s paper Si and colleagues untangled the multipartnered dance that controls Orb2A’s role. First, a protein called TOB binds to Orb2A, allowing it to persist intact in the cell. (Normally, it would be broken down within a few hours.) Once stabilized it needs to have a phosphate tag attached, and this is done by another protein called Lim kinase.

Crucially, Lim kinase is only activated when the cell receives an electrical impulse — and only targeted at that synapse, not any other synaptic connections the cell might also be making. That means that the prion chain reaction is turned on in the specific time and place it’s needed.”