Monopeptides Give Pores for Thought

Tiny peptide structures team up to form channels that allow rapid transit of ions in and out of cells

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Molecules that can insert themselves into cell membranes, then self-assemble into selectively permeable pores, could offer a new therapeutic option for diseases such as cystic fibrosis, research from A*STAR suggests. These channel-forming molecules are small and simple to produce, yet form some of the most active anion-transporting channels known.

Nature contains many pore-forming peptides, consisting of short chains of amino acids that group together to create selectively porous channels that tunnel through cell membranes. “Precisely regulated transport of anions such as chloride, nitrate, sulfate, iodide and bicarbonate is crucial for many physiological processes,” says Huaqiang Zeng from the A*STAR Institute of Bioengineering and Nanotechnology, who led the current work.

“Malfunctioning protein channels cause diseases such as Liddle’s syndrome (impaired sodium ion transport), EAST syndrome (impaired potassium ion transport) and cystic fibrosis (impaired chloride ion transport),” he says. There is a strong need for artificial ion transport machinery able to transport ions across cell membranes with high selectivity and activity.

Natural pore-forming peptides are typically 12 to 50 amino acids long. Zeng and his team created the simplest pore-forming peptides known, called monopeptides — just a single amino acid long. By making and testing a library of these monopeptides with different side chains attached, the team identified pore-forming monopeptides that showed exceptionally high activity for transporting anions across membranes.

The key to the pore-forming monopeptides’ performance was the hydrogen bonds that form between neighboring monopeptide molecules, Zeng explains. These bonds form between the bottom face of one amino acid and the top face of the next, so that the monopeptides stack together into columns that can span the membrane.

“The monopeptides contain a built-in hydrogen-bonding motif, which packs the molecules into one-dimensional H-bonded stacks with the same type of side chains on the same side,” Zeng says. The side chains stick out of the side of the stack, and bond neighboring stacks together. Clusters of three or more stacks group together to form a cylindrical structure with a hole through the middle less than one nanometer in diameter. That’s just large enough for negatively charged ions such as chloride or iodine ions to whizz through the pore, thereby crossing from one side of the membrane to the other.

As well as optimizing the performance of these pore-forming peptides by testing a wider range of structures — including di- and tri-peptides — the team plans to test their potential for punching holes in cancer cells, Zeng says.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Bioengineering and Nanotechnology.

Source : A*STAR Research