The human glutamine transporter ASCT2 is increased in several forms of cancer. It is the docking platform for a wide range of pathogenic retroviruses. The ASCT2 protein imports the amino acid glutamine in the human cells and maintains the amino acid balance in many tissues. The amount of ASCT2 is increased in several types of cancer usually because of an increased demand for glutamine. Moreover, several types of retrovirus infect human cells by first docking on this protein.
ASCT2 belongs to a larger family of similar transporters. To understand the working of this family of amino acid transporters and to help in designing the drugs that block glutamine transport by ASCT2 or its role as a viral docking station the scientists have resolved the 3D structure of the protein. They resorted to the technique of single particle cryo-electron microscopy, as they did not succeed in growing crystals from the protein, which are required for X-ray diffraction studies. 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 revealing the striking new insights. It was a challenging target for scientist as it is rather small for cryo-EM. But it also has a good symmetric trimeric structure, which helps.
The cryo-EM images reveal a familiar type of ‘lift-structure’, where a part of the protein travels up and down through the cell membrane. The substrate enters the lift in the upper position and then moves down to release the substrate inside the cell. The structure of ASCT2 depicts the lift in the lower position. It had been thought that the substrate enters and leaves the lift through different openings, but the results suggest it might well use the same opening.
This information could help design molecules that stop glutamine transport by ASCT2. Some tests in mice with small molecules that block transport have been published. Blocking glutamine transport would be a way to kill cancer cells. This new structure allows for a more rational design of transport inhibitors.
Another observation was the surprise for the scientists as they observed the spikes that protrude on the outside of each of the three monomers. These are the places where retroviruses dock. This proved to be consistent with mutagenic studies performed by others. By knowing the shape of the spikes could help design molecules which block the viruses from docking. The protein structure was resolved in some time duration, which is remarkably fast for cryo-EM.
Future studies will be done to capture ASCT2 in different configurations, like inside a lipid bilayer rather than the detergent micelles used in the present study and with the lift in different positions. Thus, it was concluded that studying different states will help them understand how this protein functions.