St. Jude Scientists Uncover Neurotransmitter Transporter Structures

The many faces of monoamine transporters

There are two types of VMAT: VMAT1 and VMAT2. VMAT1 is more specialized and found only in neuroendocrine cells, while VMAT2 is found throughout the neuronal system and has significant clinical relevance.

“We knew that VMAT2 is very important physiologically,” Lee said. “This transporter is a target for pharmacologically relevant drugs used in the treatment of hyperkinetic disorders such as chorea and Tourette’s syndrome.”

Despite their importance, VMAT2’s structure, which would allow researchers to explore how it fully works, has remained elusive. Lee and his team used cryo-electron microscopy (cryo-EM) to obtain structures of VMAT2 bound to the monoamine serotonin and the drugs tetrabenazine and reserpine, which are used to treat chorea and hypertension, respectively. This was no easy task.

“VMAT2 is a small membrane protein,” explains co-first author Yaxin Dai, PhD. St. Jude Department of Structural Biology. “This makes it a very challenging target for determining the cryo-EM structure.”

Despite the difficulty and with the help of some clever tricks, the team captured multiple structures of VMAT2, allowing them to discover how the protein functions and explore how exactly these drugs work. “VMAT transporters adopt multiple conformations (shapes) as they transport their substrate. This is called alternating access transport, where the protein faces ‘outward’ or ‘inward’,” explains co-first author Shabareesh Pidathala, Ph.D ., out. St. Jude Department of Structural Biology. “To fully gain mechanistic insight at the atomic level, we needed to capture multiple conformations of this transporter.”

Answer to a question from forty years ago

The researchers discovered that this dynamic mechanism ensures that drugs have multiple options for binding. They confirmed that reserpine and tetrabenazine bind two different conformations of VMAT2. “30 or 40 years of pharmacological research had suggested that these two drugs bind to the transporter in different ways,” said Pidathala, “but no one knew the atomic details of how this works. Our structures nicely show that these two drugs stabilize two different conformations . of the carrier to block its activity.”

The serotonin-binding structure of VMAT2 allowed the researchers to identify specific amino acids that interact with the neurotransmitter and direct its transport. “We believe this is a common mechanism that this transporter uses to engage all monoamines,” says Lee.

While this work represents a huge step forward in understanding monoamine transport, Lee and his team are delving deeper into its mechanism. For example, the uptake of monoamines into vesicles is fueled by protons moving in the other direction. “We have identified amino acids that are important for this proton-independent process,” said Lee, “but we still don’t know how exactly protons drive this transport. Determining this mechanism is our future direction, which will help us fully understand how these transporter works.”

Authors and funding

The other first author of the study is Shuyun Liao of Peking University’s School of Life Sciences. The co-corresponding author of the study is Zhe Zhang of the School of Life Sciences, Beijing University. Other authors include Xiao Li and Chi-Lun Chang St. Jude, and Changkun Long of the School of Life Sciences, Beijing University.

The study was supported by grants from National Institutes of Health (R01GM143282), the National Key Research and Development Program of China (2021YFA1302300), the National Natural Science Foundation of China (32171201), the SLS-Qidong innovation fund, the Li Ge-Zhao Ning Life Science Youth Research Foundation, the State Key Laboratory of Membrane Biology of China, and ALSAC, the fundraising and awareness organization of St. Jude.