Glitches in the membrane

4 July 2016

Though the root causes of cancer may range from an infection to radiation exposure, it is the signalling behaviour of affected proteins and enzymes that directly lead to the errors in replication control that eventually lead to tumours. Understanding these pathways and the ways that proteins are activated, inactivated, or otherwise modulated, will give greater insight into the mechanics of cancer, hopefully provide more opportunities for treatment.

At the University of California, Berkeley, Monatrice Lam is studying the activity of Ras proteins and their effect on cancer-related cell processes like proliferation and apoptosis, or cell death. Studies have shown that almost 30% of cancers have some sort of Ras mutation, highlighting the importance of understanding this protein in the fight against cancer.

Ras proteins are ubiquitous in the body. They are found in all cell types and organs. Ras proteins belong to the group of proteins responsible for signal transduction, that is, they help send messages throughout the cell.  If a mutation causes Ras to ‘switch on’ permanently, there will be no end to the signal for growth and division, leading to the growth of a tumour.

We’re looking at how early stages in cancer signalling can be mis-triggered

To study the effects of Ras and better understand how best to combat potentially dangerous mutations, Lam and her team work with synthetic membranes on glass, allowing them to examine Ras protein’s activities while protein-membrane interactions are preserved. By tethering proteins to the synthetic membrane on the glass plate, membrane proteins like Ras can be tagged for observation and studied with greater ease.

Son of Sevenless

Ras is activated by Son of Sevenless (or SOS), a guanine exchange factor (or GEF) which drives Ras’s activation and subsequent conformational change. It appears that SOS is capable of activating thousands of Ras proteins, though the mechanism of this processive activation of Ras is still unclear.

SOS is a guanine nucleotide exchange factor, meaning it mediates the exchange of guanine diphosphate and guanine triphosphate (GDP and GTP respectively, two molecules that act as an energy currency within the body). While there are a few guanine nucleotide exchange factors present within cells, SOS is the most ubiquitous and its connection to Ras activity is critical to better understanding Ras related mutations and their effects on oncogenesis.

When SOS binds to Ras, it causes the guanine nucleotide that Ras was bound to to switch out, activating and re-activating Ras over and over again rapidly. While a lot of research revealing the structure of SOS has been done and its partnering with Ras clear, the positive feedback mechanism displayed by SOS and Ras is still unclear.

Fluorescent imaging

To visualise the activity of Ras and SOS, Lam and her team have developed a fluorescent sensor that can specifically bind to activated Ras, thereby illuminating the activation patterns and behaviour with respect to SOS. Building on studies that indicate Ras nanoclusters are present on cell membranes, these nanoclusters is likely to become activated all together by a single SOS molecule, resulting in a powerful signal. This signal even seems to be irreversible. Once SOS binds and the processive action is set in motion, it appears there is no going back and undoing the signal, eventually triggering the entire cell..

This activation is so thorough and rapid, it’s thought of as a digital signal, with one SOS leading to rapid and complete activation, with no way to stop it. Because of this powerful activation sequence, it’s the recruitment step of SOS that is most important. 

SOS has two Ras anchoring sites, the allosteric and the catalytic. When SOS binds to Ras at the allosteric site, the catalytic pocket of SOS is then brought closer to other Ras molecules allowing it to rapidly bind and activate one Ras protein after another. 

While the specifics are not completely understood, what is known is that once SOS binds, it has the potential to activate a huge number of Ras proteins on the cell membrane. In short, it’s that first binding of Ras to SOS that needs to be controlled, as that’s the tipping point for the eventual activation of possibly thousands of Ras proteins and the dangers that come with uncontrolled cell proliferation.

A graphic visualisation of the lipid bilayer membrane

Lam’s lab makes artificial membranes by creating supported lipid bilayers (or SLB). The lipid bilayer forms the cell membrane in human cells. Lipids are a large group of molecules encompassing fats, waxes, some vitamins, and more.

The lipid itself is made up of two parts, a hydrophilic head and a hydrophobic tail. In forming a membrane, the lipids come together tail to tail, with the heads facing the exterior and interior of the cell.

In the lab, Lam and her team create lipid bilayer vesicles which are used to make SLB on etched glass. The lipids can then easily be tagged with fluorescent molecules for easy identification and visualisation of the myriad processes that occur within, around, and across the cell membrane.

While options for treatment are still in a more speculative state at this point, it’s through a deeper understanding of our biological pathways that we can better arm ourselves in the fight against cancer.  

After graduating from Sha Tin College, Monatrice attended Washington University in St. Louis (Wash U) where she majored in Chemistry and Physics, and minored in Psychology. During the fours years of her undergraduate studies, she was a freshman Resident Advisor (RA) for two years and participated in the St. Louis Area Dance Marathon to raise money for children’s hospitals.

To view Monatrice’s personal Croucher profile, please click here.