bio sphere
Fishing for MAGIC targets nspired by magnets and iron filings, Tae Kook Kim and colleagues at the Korea Advanced Institute of Science and Technology and CGK Co. (Korea) have found a way to identify intracellular targets of small molecules (Science 2005, 309, 121–125). With the new technology, called magnetism-based interaction capture (MAGIC), the researchers introduce ligands labeled with magnetite nanoparticles into living cells and use a magnet to attract any proteins the ligands hook. Because the interacting proteins are labeled with fluorescent markers, movement toward the magnet can be easily visualized. Carolyn Bertozzi at the University of California, Berkeley, says the technique is the first, to her knowledge, that can visualize molecular interactions inside a cell and perturb the locations of these interactions. “[By providing] a new means of literally fishing for the cellular target of a bioactive molecule, this [method] may be the first step toward studying cellular processes by ‘intracellular surgery’ using nanoprobes,” she says. Many of the current methods for identifying intracellular targets give high background signals and false-positive results, have poor sensitivity, or require the use of proteins that lack important posttranslational modifications. Moreover, in vitro binding conditions or the need to use nonmammalian cells can give misleading results. “But MAGIC can measure physiologically and pharmaceutically relevant molecular interactions inside cells in real time,” Kim says. To make their superparamagnetic nanoparticles (MNPs), the researchers combined biotin-conjugated ligands with streptavidin-conjugated magnetite nanoparticles. They added a viral peptide to encourage HeLa cells to take in © 2005 AMERICAN CHEMICAL SOCIETY
the particles by endocytosis, although the MAGIC particles could alternatively be microinjected directly into cells. When they briefly exposed treated cells to a magnet, the MAGIC particles moved through the cytoplasm toward the magnet. As
TAE KOOK KIM, KOREA ADVANCED INSTITUTE OF SCIENCE AND TECHNOLOGY
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MNPs fish out targets in living cells when a magnetic force is applied.
expected, nonmagnetic quantum dots, which were used as a negative control, did not move. In a MAGIC screen, the investigators identified targets for FK506, an immunosuppressant. First, they fused complementary DNA from many human tissues to an enhanced green fluorescent protein gene and expressed them in HeLa cells. When the cells were exposed to MAGIC particles coated with FK 506 and placed in a magnetic field, migrating green fluorescence was seen in 19 clones. By analyzing the messenger RNA from the positive clones, the researchers identified 7 known protein targets and 4 unknown targets for FK 506. In addition to identifying drug targets, the technology proved capable of detecting signal-induced protein phos-
phorylation and protein–protein interactions. With MAGIC, “various types of dynamic molecular interactions can be detected inside live cells without timeconsuming and invasive procedures such as cell lysis or fixation,” Kim says. He adds that researchers could use an automated microscope to monitor phenotypic changes while simultaneously using MAGIC to monitor the intracellular interactions that cause those changes. Simon Robson at the Beth Israel Deaconess Medical Center says that MAGIC seems like a quantum jump in sophistication compared with other systems. He adds, “The one major limitation is that one still has to introduce the MNPs into cells, and even with [the viral peptide used] there will be cellular damage that might be problematic, particularly when readouts are related to apoptosis or modulation of cellular injury.” Although Stanley Shaw at Massachusetts General Hospital and the Broad Institute finds the experiments very encouraging, he says another possible caveat exists: The target proteins are greatly overexpressed in the current implementation, and this could produce some nonphysiological interactions. Angela Koehler at the Broad Institute says, “As with most affinity-capture methods, the orientation of display for the small molecule on the surface of the MNPs may determine whether or not protein partners are identified.” Meanwhile, Kim and colleagues have developed the second and third generations of MAGIC. “In the near future,” Kim says, “we will start to systematically screen the binding of target proteins to bioactive small molecules or the binding of small molecules to specific target proteins inside living human cells.” a —Linda Sage
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