Palm Clasp: DARPins, or designed ankyrin repeat proteins, consist of an N-capping repeat (green ribbon), many internal repeats whose number can be freely chosen (three shown here) (dark blue ribbon), and a C-capping repeat (cyan ribbon). The molecular model shows a classic DARPin library design. ANNU REV PHARMACOL TOXICOL, 55:489–511, 2015Antibodies are the immune system’s foot soldiers, the first line of defense against foreign invaders. With their unique arms that bind only to specific ligands, antibodies screen thousands of proteins to find the one that clasps perfectly.
This bit of biology also makes antibodies a powerful tool for detecting and capturing proteins in the lab. But they have some significant drawbacks. For one thing, it takes between six months and a year to develop lab-ready antibodies, and the process, which was developed in the 1970s and ’80s, often involves using animal hosts, such as rabbits, to generate the molecules. Also, antibodies’ unwieldy structure of light and heavy chains and their large size—most are about 150 kDa—makes it hard to fuse them with target proteins, or to use them inside live cells. What’s more, antibodies often cannot be produced as genetically encoded reagents within cells of interest because they have disulfide bonds that fail to form in the reducing environment of the cytoplasm.
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