Alice Ting was stymied by GFP's many deficiencies ? its large size, poor optical properties, and most importantly, its restriction to optical imaging. So the MIT assistant chemistry professor decided to roll her own labeling technology instead.

The objective, she says, is to migrate from the test tube to the live cell?to study proteins in their natural habitat. It took two years of "high drama," but Ting found a way to combine GFP's genetic flexibility with the nonintrusive elegance of small-molecule probes.

Now she can selectively, and covalently, couple almost any molecular label ? from Alexa fluors to MRI contrast agents ? to specific proteins exclusively on the cell surface.1 "Already we can do things GFP would never have let us do," she says, from monitoring synaptic receptor dimerization, to watching the trafficking of single receptor molecules.

In seeking a high-throughput way to chart the mammalian transforming growth factor-beta...


1. I. Chen et al., "Site-specific labeling of cell surface proteins with biophysical probes using biotin ligase," Nat Methods, 2:99?104, 2005. 2. M. Barrios-Rodiles et al., "High-throughput mapping of a dynamic signaling network in mammalian cells," Science, 307:1621?5, 2005. 3. J. Xie, P.G. Schultz, "Adding amino acids to the genetic repertoire," Curr Opin Chem Biol, 9:548?54, Nov. 2, 2005.

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