The carbohydrate chains, or glycans, attached to most cell-surface proteins aren’t there for decoration. They control protein activity and act as binding sites for other molecules such as the causative toxin of whooping cough. Now, researchers have combined two chemical tagging strategies to monitor glycans on individual proteins of interest. Their work could help scientists better define how sugars regulate biology.
Glycans have been imaged on proteins in general but rarely on specific proteins. Typically, researchers co-opt a cell’s metabolism to attach a fluorescent dye to a given sugar, which is then incorporated into different proteins. Now, Xing Chen and colleagues at Peking University, in China, have adapted that method to image only the sugars on one specific protein (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja410086d).
Chen’s technique uses FRET (fluorescence resonance energy transfer), which produces a signal only when two fluorescent dyes, a donor and acceptor, are close together. Chen and coworkers attach an acceptor to cell-surface glycans with traditional metabolic techniques. They then use a site-specific, enzyme-catalyzed probe-ligation method developed by another lab to place the donor only on a desired target protein. Even though many cell-surface carbohydrates get labeled with acceptors, only those on the protein of interest give a FRET signal because they are the only ones close enough to the donor dye.
The researchers have demonstrated the technique in living cells on three glycoproteins involved in cell attachment and communication. Chen says his team plans to obtain more control over where the donor dye attaches to a protein. They believe better control will also allow them to extend the method to additional glycans.
Another team has published a protein-specific glycan-imaging technique, notes Caltech carbohydrate chemist Linda C. Hsieh-Wilson. But Chen’s approach, she says, “has superior sensitivity because the FRET donor-acceptor pairs are closer and reside on the same side of the cell membrane.”
Glycoscience expert Chi-Huey Wong of Taiwan’s Academia Sinica also praises the work. “This is an important step forward to look at specific glycoproteins in live cells,” he says.