Tracing the transport pathway for cholesterol in a cancer-associated membrane protein

In a new study, researchers have provided the first molecular description of how cholesterol travels into a protein called Smoothened (SMO), in a process that is linked to cancer and birth defects. Graduate student Prateek Bansal is first author on the study, which was led by chemical and biomolecular engineering professor Diwakar Shukla.

The SMO receptor is a protein in the human body that mediates Hedgehog signaling, a system that transmits information during embryonic development to ensure proper cell division. Insufficient activation of SMO in a developing fetus leads to significant birth defects, while excessive activation can lead to brain cancer in children or skin cancer in adults. Studying the activation of this protein is pivotal to understanding the mechanisms of cancer in order to develop better cancer therapies.

Why It Matters

SMO plays a crucial role in cell division, yet the mechanism that activates it remains poorly understood. Research has suggested that the activation trigger could be linked to cholesterol molecules moving through the protein. Using a combination of laboratory experiments and advanced computer simulations, the team investigated this transport process to identify the specific regions of the protein’s cell membrane directly involved in initiating activation. Their results could lead to the development of targeted drugs, paving the way for better, more effective cancer therapies.

The Details

• The researchers mapped the entire cholesterol transport process by combining unprecedented two-millisecond atomistic molecular dynamics simulations with biochemical experiments.
• They identified that cholesterol could enter the Smoothened protein through two distinct entry points, either from the outer layer (leaflet) or the inner layer of the cell membrane. 
• By calculating the energy required for both routes, they determined that entering from the outer membrane layer requires less energy.
• The team discovered that the protein actively moves the cholesterol using a “squeezing” mechanism, where the internal tunnel opens around the cholesterol and closes right behind it.

The Bottom Line

“Cholesterol is the essential trigger for activating the Smoothened protein, and we now have a comprehensive, step-by-step molecular blueprint of exactly how it travels from the cell membrane to the protein's activation site. By understanding the exact pathways, energy requirements, and the protein's ‘squeezing’ mechanism, we have uncovered specific sites in the protein that can be used to design the next generation of drugs to block this transport and fight associated cancers.” – Prateek Bansal

Contributors

In addition to Bansal and Shukla, Maia Kinnebrew and Rajat Rohatgi from Stanford University are co-authors on the paper. Computing power was provided by the Blue Waters Petascale Computing Facility. The authors thank the citizen scientists worldwide who donated their personal computing hours through the Folding@Home project, which made large-scale simulations possible.

This work was supported by funding from the National Science Foundation, the National Institutes of Health, and the Cancer Center at Illinois.

Notes:

To contact Diwakar Shukla, email diwakar@illinois.edu

The study Multiple modes of cholesterol translocation in the human Smoothened receptor, was published in eLife. DOI: 10.7554/eLife.108030.3