Researchers are retracting a highly-cited 2004 Science paper describing a new way of adding sugars to proteins -- a longstanding challenge in molecular biology -- citing their inability to repeat the results and the absence of the original lab notebooks with the experiment details, they announced in Science last Thursday (November 26).

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"It is unfortunate that they cannot repeat it," said biochemist Lai-Xi Wang of the University of Maryland School of Medicine, who did not participate in the retracted research. "This method opened a new avenue for the preparation of glycoproteins [with] huge potential in this field."

Glycosylation, the addition of sugars to proteins, is a common posttranslational modification that is found on some 70% of human proteins. It can affect a variety of protein functions, including folding and biomolecular recognition, but the exact relationship between the structural and functional changes remains elusive.

One of the reasons studying glycosylation is so difficult is the seemingly random manner in which glycoproteins are created. The unpredictable process results in a mixture of different glycoproteins with the same peptide backbone, and reliably producing pure samples of a single form -- important for studying the structure-function relationship -- has proven difficult. "There's no efficient way to make a defined glycoprotein with a sugar attached," Wang said.

The 2004 Science study, published by Peter Schultz of The Scripps Research Institute in La Jolla, California and his colleagues, was heralded as a breakthrough because it proposed a method to artificially tack on sugars to a specific location of a protein in a predictable manner. Their strategy: Create a transfer RNA (tRNA) molecule that selectively binds a pre-modified amino acid -- one with the sugar already on it -- that would be incorporated during protein synthesis in E. coli, resulting in a glycosylated protein prior to any posttranslational additions.

Schultz's lab had previously used this technique to incorporate other modified amino acids into proteins, and this study, published in Science, was the "proof of concept" that the same strategy could be applied to glycosylation, Wang said. "It was a big deal."

But according to the retraction announcement released by Schultz and his colleagues last week, they have not been able to reproduce these results. "There are clearly complexities associated with suppression and cellular bioavailablity of these and other glycosylated amino acids that we did/do not understand," Schultz wrote in an email to The Scientist, "and, regrettably, we no longer have the notebooks to help resolve these issues (through no fault of any coauthors)."

Another one of Schultz's papers, published in the Journal of the American Chemical Society (JACS) less than nine months after the 2004 Science report, was also recently retracted. The JACS paper showed the applicability of the technique for the incorporation of a different glycosylated amino acid. ("The two withdrawal letters were submitted at the same time, but the processes of the two journals differ so the time to publication varied," Schultz said.) The Science paper has been cited 92 times, and the JACS paper has been cited 37 times, according to ISI.

Schultz said that he and his team "will continue to work to understand the difficulties associated with the incorporation of glycosylated amino acids." In the meantime, they will continue to send reagents that incorporate a large number of other modified amino acids to hundreds of labs worldwide, Schultz said.

"I'm still optimistic about the concept," said Wang. "But unfortunately so far it has not worked on the glycosylated amino acids."


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Sweet Attachments
[May 2008]

Off-the-shelf glycoprotein detection methods
[May 2008]

Sugars And Splice: Glycobiology: The Next Frontier
[19th July 1999]