Membrane proteins are essential for a wide range of biological processes and represent a significant portion of drug targets, making their study incredibly important. However, analyzing these proteins has historically been difficult. Researchers at the Hefei Institutes of Physical Science, led by Wang Junfeng, have published a new study in Analytical Chemistry detailing a promising solution that significantly improves the reliability of surface plasmon resonance (SPR) analysis of membrane proteins.
Why Membrane Proteins are Difficult to Study
About one-third of all human proteins are membrane proteins, and they are involved in vital functions like cellular signaling and transport. Nearly 60% of the proteins that drugs target are membrane proteins. This highlights their key role in health and disease, and therefore underscores the need for accurate study.
Understanding how these proteins interact with other molecules—a process called binding—is crucial for developing effective treatments. A technique called surface plasmon resonance (SPR) provides a valuable tool for doing so.
SPR is considered a “gold standard” in the field because it allows scientists to monitor these interactions in real-time without having to chemically label the proteins. However, a major challenge has been finding reliable ways to immobilize—or attach—membrane proteins to the SPR sensor chip while preserving their natural structure and function. If the protein’s shape or behavior changes during attachment, the results of the analysis are unreliable.
A Novel Approach: Nanodisks and SpyTag-SpyCatcher
To overcome this challenge, the research team developed a novel immobilization method. They combined two established technologies—the SpyCatcher-SpyTag covalent conjugation system and membrane scaffold protein (MSP)-based nanodisks—to create a simple, efficient, and stable process.
Here’s a breakdown of the process:
- Creating Nanodisks: The team engineered a fusion protein combining MSP with the SpyTag molecule. This engineered protein was then used to incorporate the target membrane protein into nanodisks. Nanodisks are tiny, artificial lipid structures that mimic the environment where membrane proteins normally reside within cell membranes.
- Specific Attachment: These nanodisks carry the SpyTag label. The researchers then pre-immobilized SpyCatcher proteins—which have a strong affinity for SpyTag—on a standard CM5 sensor chip using a conventional chemical coupling process.
- Stable Immobilization: This design allows the nanodisks, carrying the membrane protein, to be specifically and efficiently captured by the SpyCatcher proteins. The result is robust and stable immobilization of the membrane protein within a near-native lipid environment—essentially, mimicking the protein’s natural surroundings.
Demonstrating the Method’s Effectiveness
To demonstrate the method’s capabilities, the research team performed SPR analysis of three different types of membrane protein interactions:
- Protein–lipid interactions
- Transmembrane protein–antibody interactions
- Transmembrane protein–small molecule interactions
The results consistently produced high-quality SPR data, enabling precise quantification of binding kinetics—how fast the interactions occur—and affinities—how strongly the proteins bind to each other.
Significance and Future Potential
This innovative approach effectively addresses longstanding limitations of SPR technology when studying membrane proteins. By providing a reliable method for immobilization, this research holds significant potential for accelerating both membrane protein research and drug discovery efforts. This technique should lead to a greater understanding of the complex role of membrane proteins and lead to the development of better and more targeted therapies.
