Researchers from the University of Groningen have used cryo-electron microscopy to elucidate the structure of the protein, which may generate leads for drug development. The team relieved that the human glutamine transporter ASCT2 is upregulated in several forms of cancer and also the docking platform for a broad range of pathogenic retroviruses.
In human cells, the ASCT2 protein imports the amino acid glutamine which maintains the amino acid balance in many tissues. The increased amount of ASCT2 in many types of cancer, probably due to the increased demand for glutamine. Moreover, several types of retrovirus infect human cells by first docking on this protein.
ASCT2 is part of a larger family of similar transporters. University of Groningen scientists have resolved the 3D structure of the protein, to understand how this family of amino acid transporters works in turn to help in designing the drugs that block glutamine transport by ASCT2.The human gene for ASCT2 was expressed in yeast cells, and the human protein was purified for imaging.
"The structure was determined at a resolution of 3.85 Å, which revealed striking new insights. It was a challenging target, as it is rather small for cryo-EM. But it also has a nice symmetric trimeric structure, which helps". Credits: Dr. Cristina Paulino, Assistant Professor of Structural Biology and Head of the University's Cryo-EM Unit
The images from cryo-EM revealed a familiar type of 'lift-structure', in which part of the protein travels up and down through the cell membrane. In the upper position, substrate enters the lift, which then moves down to release the substrate inside the cell. The structure of ASCT2 revealed the lift in the lower position. "To our surprise, this part of the protein was further down then we had ever seen before in similar protein structures", says Dr. Dirk Slotboom, Professor of Biochemistry. "And it was rotated. It had been thought that the substrate enters and leaves the lift through different openings, but our results suggest it might well use the same opening".
"This information could help design molecules that stop glutamine transport by ASCT2. Blocking glutamine transport would be a way to kill cancer cells. This new structure should allow for a more rational design of transport inhibitors", says Dr.Albert Guskov, Assistant Professor in Crystallography
Another surprising observation was the spikes that protrude on the outside of each of the three monomers which they have never been seen before. By knowing the shape of the spikes that could help them to design molecules which will block the viruses from docking.
The protein structure was resolved in about four months, which is remarkably fast for Cryo-EM. Different scientists, each with their own specialty, worked in parallel, which speeded up the process. Furthermore, PhD student Alisa Garaeva, who played a central role in ensuring the project ran efficiently.
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