The Hedgehog signaling pathway transmits information to embryonic cells. Insufficient signaling during development leads to birth defects, while unrestrained signaling occurs in many cancers, such as basal cell carcinoma, brain cancer, breast cancer, and prostate cancer.

Cryo-EM microscopy technology revealed the molecular mechanism of Hedgehog signaling. Having a clearer view of the structure could help pharmaceutical companies develop drugs that target Hedgehog signaling.

In a study published in Science, researchers used advanced microscopes to determine at atomic resolution the structure of a molecular complex implicated in birth defects and several cancers.

The researchers, using cryo-electron microscopy (cryo-EM) technology, showed that two Patched-1 (PTCH1) molecules simultaneously engage a single Hedgehog (HH) molecule, but at two distinct sites. This unique 2-to-1 ratio PTCH1-HH complex is required for efficient Hedgehog signaling in cells.

Authors show that 3.5-Å resolution cryo-EM structure of native Sonic Hedgehog (SHH-N) in complex with PTCH1 at a physiological calcium concentration. Functional assays using PTCH1 or SHH-N mutants that disrupt the individual interfaces illustrate that simultaneous engagement of both interfaces is required for efficient signaling in cells.

Cryo-EM uses enormous microscopes equipped with robotics to determine the structure of molecular samples that are frozen at temperatures so low that ice crystals cannot form.

In another paper published in Nature last month, the group reported a cryo-EM structure of the 1-to-1 PTCH1-HH complex. Their biochemical assays indicated that HH binding to one PTCH1 molecule may not be sufficient for full activity. HH may need to recruit either a different protein or another PTCH1 molecule, the group lead said.

"In the current Science paper, we report a 2-to-1 PTCH1-HH complex in which one Hedgehog molecule binds to two of its receptors (PTCH1) at two different spots. We used our cell biology assay to verify that this 2-to-1 complex is indeed the signaling generator for Hedgehog signaling. Combined with the earlier study published in Nature, we hope our new work will provide additional insights for physicians and scientists in this field," the lead author explained.

https://www.utsouthwestern.edu/newsroom/articles/year-2018/cell-signaling.html
https://www.nature.com/articles/s41586-018-0308-7

http://science.sciencemag.org/content/early/2018/08/22/science.aas8843