PSI researcher Yasushi Kondo is investigating how the BRAF signalling protein influences the growth of cancer cells and how this can be prevented.
Photo Credit: © Paul Scherrer Institute PSI/Markus Fischer
Scientific Frontline: Extended "At a Glance" Summary: Asymmetric BRAF Dimer Conformation
The Core Concept: An asymmetric structural conformation of the BRAF protein that forms during cellular signaling cascades and is responsible for driving uncontrolled cell division in certain cancers when the protein is mutated.
Key Distinction/Mechanism: In healthy cells, BRAF proteins require specific upstream signals to form active dimers and initiate cell growth. Mutated BRAF bypasses this requirement by creating an asymmetric dimer where an NtA sequence motif acts as a bridge, linking two uniquely shaped BRAF proteins. This complex then perpetually binds to the MEK1 protein, forcing a constant signaling loop for cellular proliferation.
Major Frameworks/Components:
- BRAF Protein: A central signaling kinase that functions as a critical regulatory switch for cellular growth and division.
- NtA Sequence Motif: A short sequence within the BRAF protein that extends outward to structurally bridge and link with a partner protein.
- Asymmetric Dimerization: The pairing of two differently shaped BRAF proteins, joined by the NtA sequence, representing the active configuration of the complex.
- MEK1 Interaction: A downstream protein that physically docks with the asymmetric BRAF dimer to propagate the signal across the cellular network.
Branch of Science: Molecular Biology, Structural Biology, and Oncology.
Future Application: The precise molecular mapping of this complex will direct the development of next-generation, targeted cancer drugs designed to bind specifically to the active components of the BRAF-MEK1 pathway, providing new options when tumors develop resistance to existing therapeutics.
Why It Matters: BRAF mutations drive approximately half of all malignant melanomas and seven percent of other cancers, creating aggressive and difficult-to-treat tumors. Characterizing the exact structure of this oncogenic driver provides a crucial molecular blueprint for halting therapy-resistant tumor growth.
Malignant melanoma is one of the most dangerous forms of cancer. Around half of melanomas and about 7 percent of all other cancers carry a mutation in a protein called BRAF. This mutation disrupts the regulation of cell division, causing cells to multiply in an uncontrolled manner. Researchers at PSI and the University of Zurich have now examined the BRAF protein in greater detail, gaining valuable insights into its complex regulation.
"Cancerous tumors affected by BRAF mutations are considered particularly aggressive and difficult to treat," says Yasushi Kondo, a researcher at the PSI Center for Life Sciences and lead author of the new publication. There are currently very few active substances available for the targeted treatment of these tumors—and furthermore, the cells typically develop resistance to these drugs after a few months.
A Traffic Light That Is Always Green
Proteins change shape and bind to other proteins and molecules to trigger signaling cascades. The BRAF protein is part of a key signaling pathway in all human cells that functions like a series of traffic lights and regulates cell growth and division. Upon receiving the appropriate upstream signal, two BRAF proteins bind together to form what is known as an active dimer, giving a green light to the next switch. Without the earlier signals, however, the traffic comes to a halt.
Mutated BRAF breaks this control. Instead of waiting for instructions, it turns the traffic light green and remains in this state—irrespective of the rest of the traffic network. This sends the cells a constant signal to proliferate, driving uncontrolled tumor growth.
Scientists already knew that a specific short section of the BRAF protein, known as the NtA sequence motif, plays a crucial role in this cancer-promoting process. Now, Kondo's team of researchers has succeeded in establishing the role of the NtA motif in greater detail.
They accomplished this by investigating the molecular structure of BRAF dimers at the Swiss Light Source (SLS) at PSI and at the Diamond Light Source in Didcot, England, and by carrying out experiments in human cells at the University of Zurich.
The researchers discovered a previously unknown form of BRAF dimer that forms during the signaling cascade. This consists of two BRAF proteins of different shapes, in which the NtA sequence of one BRAF extends toward its partner in such a way that it forms a kind of bridge connecting the two.
An Intricate Structure
The researchers did not examine this asymmetric BRAF dimer in isolation, but rather in its intricate linkage with another protein called MEK1. "This interaction with MEK1 is a crucial step in the signaling cascade leading to cell division," explains Kondo. This is because the MEK1 protein subsequently moves on, giving the next green light in the signaling chain.
"In this study, we succeeded for the first time in producing a high-resolution visualization of the exact structure of this complex in its active, working form, which consists of the two BRAF proteins, linked asymmetrically via NtA, and the MEK1 protein," says Kondo.
This is helpful for drug development: targeted therapeutic agents work by binding to the active components of a signaling pathway, interrupting signal transmission. This sets one of the "traffic lights" to red and prevents further cell division in the tumor. "The more precisely we know the structure of these components, the more precisely we can design drugs that fit perfectly."
More Options for Cancer Treatment
The researchers hope that a wider range of more effective therapeutic agents targeting BRAF will become available in the future. "This would give us more options when resistance sets in," says Kondo.
In fact, the researchers at the PSI Center for Life Sciences are already searching for molecules that bind to the newly characterized BRAF-MEK1 complex and could thus interrupt the unwanted signaling. "Our findings open up new possibilities for developing drugs that target BRAF in new, different ways," explains Kondo. "Having a broader range of compounds could provide more options for treatment and help address the diverse ways in which cancers evade existing therapies."
Funding: The current study was supported by the Swiss Cancer Research Foundation.
Published in journal: Molecular Cell
Title: Mechanism of MEK1 phosphorylation by the N-terminal acidic motif-mediated asymmetric BRAF dimer
Authors: Yasushi Kondo, Judith Notbohm, Ignacio Navas Camacho, Gabriela Nagy-Davidescu, Thomas Mason, onas Mühle, Jörg Standfuss, and Tina Perica
Source/Credit: Paul Scherrer Institute | Dirk Eidemüller, and Laura Hennemann
Edited by: Scientific Frontline
Reference Number: mbio072126_01
.jpg)