New Technique Improves Folding of Membrane Proteins

Membrane proteins, part of the material that surrounds a cell, perform many essential processes, including transporting molecules in and out of cells and facilitating communication between cells.

So when synthetic biologists re-engineer cells or cellular mimetic membranes to take on new purposes—such as sensing biological molecules or acting as therapeutic delivery systems—using membrane proteins is essential.

Neha Kamat, assistant professor of biomedical engineering at Northwestern Engineering, and her team recently set out to synthesize membrane proteins outside of a cell, and in the process developed a new technique that improved the rate and yield of these proteins. The technique also gave rise to a new understanding of mechanical processes that could be occurring inside a cell.

“This work could help us understand and use membrane proteins for a variety of applications,” Kamat said. “Designing materials that incorporate membrane proteins could lead to a completely new class of materials that can sense, transport, or even target important biological signals and molecules within the body.”

The results were published February 11 in the Proceedings of the National Academy of Sciences. Kamat’s graduate students, Miranda Jacobs and Margrethe Boyd, were co-authors on the paper.

At the heart of the technique is protein folding, where a new protein folds into a structure that ultimately determines its function.

When researchers synthesize proteins in a cell-free manner, they put together the necessary components—including ribosomes, nucleotides, a gene, and more—to create proteins outside of a cell. Researchers have found that when a protein begins to emerge from the ribosome, if a membrane is nearby, it will insert itself into that membrane and can often spontaneously fold into the correct structure.

Scientists have been assembling membrane proteins into membranes this way for years, but until now, no one looked at the properties of the membrane to understand how it affected the protein folding and production. The membranes Kamat used in this research were created with a diblock copolymer (a designed material that only exists in the laboratory), which made the membrane material more elastic. That, in turn, had an effect on how quickly the proteins were made, and how much protein was made.

Researchers saw a four-fold increase in folding efficiency for one type of protein (MscL-GFP), and a 20-fold increase in folding efficiency for another membrane protein (chR2GFP) over previous techniques using membranes made from traditional phospholipids.

“That was an exciting result,” said Kamat, who is also a member of Northwestern’s Center for Synthetic Biology. “We’re pushing the membrane to behave in ways that it wouldn’t have in nature by working with molecules that don’t exist in nature. Then we can ask questions about how these features translate to cells and better understand the design rules that dictate protein folding.”

By showing that the mechanics of the membrane matter in membrane protein production, the researchers also showed that the mechanical properties of cells could play an important role in how quickly membrane proteins are created—a new idea within the field.

“It’s exciting to learn that the very first step in a protein’s life cycle — folding and insertion into a membrane—are potentially dictated, at least in part, by how elastic the membrane is,” she said. “What we’re wondering now is, does a cell change the mechanical properties of its membrane, and does that change protein dynamics inside of a cell?”

Understanding how the mechanics of membranes work within cells could lead to a better understanding of diseases that stem from protein misfolding, like cystic fibrosis or Alzheimer’s disease.

The technique could also be used to design materials that use membrane proteins to perform functions, like delivering drugs within the body. Next, the researchers hope to see if the effects they observed are reproducible with other types of proteins, as well as begin to design biosensors that use membrane proteins.

“We want to both understand more about protein folding and use that information to design new materials,” Kamat said. “These results could have exciting implications.”