Molecule of the Month: AMPA Receptor

Nerve cells in our brain communicate with one another using small neurotransmitter molecules. These come in many shapes and sizes, such as acetylcholine, serotonin and endorphins, allowing the nervous system to shape the way neurons talk to one another. The amino acid glutamate is one of the most common neurotransmitters used to carry excitatory signals. It is released by neurons into synapses, where they stimulate neighboring neurons by binding to glutamate-specific receptors (an illustration of an excitatory synapse is included at PDB-101 in the gallery of Molecular Landscapes).

AMPA receptors (shown here from PDB entry 3kg2) are the most common excitatory glutamate receptors in the brain. They have modular structures, and each part has a specific task. The portion at the top recognizes and binds to glutamate (and similar neurotransmitters), and the portion at the bottom forms an ion channel through the membrane. When glutamate binds, it causes a change in shape that tugs on the channel, causing it to open and allow ions to flood through the membrane. There is also an additional tail at the bottom of the receptor, which is not seen in this structure because it is so flexible. It interacts with scaffolding proteins that organize the structure of the neuronal synapse.

Communication between neurons is highly complex, to capture the many subtle shades of thought and memory. At the simplest level, AMPA receptors each have four chains, each with a separate binding site for glutamate, so the response isn't simply on or off. Also, AMPA receptors close very quickly after receiving a message, becoming desensitized for a time against additional messages. Layers of complexity are added to these simple actions to store memories, by adding more receptors to a synapse or taking them away, or by modifying amino acids in individual receptors to tune their action.

To make things even more complex, our brains have additional options for tailoring glutamate signaling. Several accessory proteins bind to AMPA receptors, such as the TARP protein shown here from PDB entry 5weo. Each receptor can also be composed of different combinations of several similar forms of the protein. Additionally, there are two other classes of glutamate receptors with slightly different actions, known as NMDA receptors (shown here from PDB entry 4pe5) and kainate receptors (PDB entry 5kuf), both named after inhibitor molecules that were crucial for their discovery. Together, they help build up the rich environment of signaling that controls our inner world of thought.