Structure and Function of Transporter Implicated in Cancer and Immunity Uncovered

New research from St. Jude Children's Research Hospital and the University of Texas Southwestern Medical Center sheds light on the structure and function of sphingosine-1-phosphate (S1P), a transporter involved in cancer and immunity. The team captured six structures of the transporter, including its configuration while bound to an inhibitor, providing unprecedented insights into its workings.

Transporters play a crucial role in facilitating the movement of substances across cell membranes, enabling them to perform their functions. S1P is a vital signaling molecule that regulates various processes such as the immune system, blood vessel formation, auditory function, and the integrity of epithelial and endothelial membranes. It is also involved in promoting the progression and survival of cancer cells through chemoresistance and metastasis.

The S1P molecule is synthesized inside the cell but needs to traverse the cell membrane to carry out its signaling functions. Spinster homolog 2 (Spns2) acts as the transporter for S1P. This protein is situated on the membrane, facing the interior of the cell, and binds to S1P. It then reorients itself, facing the outside of the cell to release the S1P molecule.

Previous research has shown that manipulating Spns2 activity can have therapeutic effects against cancer, inflammation, and immune diseases. However, the transport mechanism of Spns2 and methods to inhibit it remained unclear.

"We hope our structural information will pave the way for the development of improved, more specific small molecules with higher potency against Spns2 in the future," explains Chia-Hsueh Lee, co-corresponding author of the study published in Cell.. "I think there is huge potential for inhibiting the Spns2 transporter therapeutically."

To understand how the transporter operates, the researchers employed cryo-electron microscopy and obtained six structures of Spns2, including two intermediate conformations that are functionally relevant. These intermediate shapes connect the inward and outward-facing states of the transporter, providing crucial insights into the S1P transport cycle.

"Capturing a particular transporter's major conformations is rare, so these results are quite satisfying," notes Lee. "By comparing these different structures, we now have a very detailed understanding of how this transporter captures the S1P signaling molecule."

The researchers also investigated an inhibitor called 16d, a specific small molecule with minimal off-target effects. They discovered that 16d halts transport activity by locking Spns2 in the inward-facing state, offering valuable structural data for the development of advanced Spns2 inhibitors.

"This inhibitor effectively blocks the protein in an inward conformation, preventing it from transitioning to the outward-facing state and inhibiting the transport of the signaling molecule from inside to outside the cells," adds Lee. "Furthermore, the inhibitor physically obstructs the binding of the signaling molecule, as both the inhibitor and the signaling molecule bind to the same cavity."

Cell surface molecules are an attractive target for drug development. G-protein coupled receptors (GPCRs) are a type of cell surface protein that is the target of one-third of all FDA-approved therapeutics. As cell surface molecules, transporters may have similar potential for drug development. Therefore, understanding their structure and function has the potential to make significant inroads for improving disease treatment.