Antibodies are normally built inside cells, take up their final form as they are secreted from the cell, and function exclusively outside cells, binding to cell surfaces and extracellular molecules. But Terry Rabbitts at the MRC Laboratory of Molecular Biology in Cambridge, UK, wants to use antibodies' highly specific binding properties inside the cell itself. He has engineered cells to make fragments consisting of just a heavy-chain variable domain and hold them within the cell. These single-domain antibodies can interact with intracellular proteins at binding affinities of 10 nanomolar or better. At Iclectus, an MRC spin-off company based in London, Rabbitts' colleagues are developing these fragments as intracellular antibodies or 'intrabodies'.
They became interested in intrabodies while they were unravelling the role of transcription factors involved in producing chromosomal translocations in tumour cells. Using intrabodies to bind and block these transcription factors inside the cell opened up a new range of research possibilities as well as pointing the way to new anticancer therapies. Transcription factors operate by binding other proteins, so “if you have an intrabody that has an affinity close to or better than the affinity of the interacting partner, you should be able to block the interaction using the intrabody”, says Rabbitts.
Intrabodies offer the cell biologist the prospect of highly specific tools that can probe protein–protein interactions in situ more delicately than techniques that simply remove one of the proteins from the scene by antisense or gene knockout. Because an intrabody binds to a specific site on a protein, it can block a protein's interaction with one partner while allowing it to continue acting with another.
Another use is for the validation of potential drug targets, one of the first and vital steps in drug discovery. Most potential target proteins are modular, so intrabodies that bind to and block the function of individual modules could help to determine which part of the protein is key to causing the disease.
Placing localization signals on an intrabody allows it to be directed to various parts of the cell, such as the nucleus. “With intrabodies you can be more versatile in your experiments and ask more questions than, say, with RNA interference,” says Rabbitts.
About this article
Cite this article
Moore, P. Antibodies where you want them. Nature 426, 729 (2003). https://doi.org/10.1038/426729a