Scientists have devised a new way to make sugar-linked proteins, an important step towards understanding a key type of protein modification and refining drug therapies that utilize the so-called glycoproteins -- as well as the subject of two high-profile papers that were recently retracted.

Image: Wikimedia commons, S. Jähnichen

"It's a pretty important thing that they've done," said synthetic biologist Jason Chin of the Medical Research Council Laboratory of Molecular Biology in Cambridge, UK. Plus, after the previous controversy-ridden setback, the new findings might well be a welcome step forward for the field. "Any method where you install that first sugar [onto a protein] would be a big advance," Chin said.

The study, published online today (February 28) in Nature Chemical Biology, describes the addition of sugars to proteins, a process known as glycosylation.

Glycosylation is one of the most common posttranslational modifications in eukaryotes, known to affect protein functions such as cell adhesion and host-pathogen interactions. Studying this process, however, requires purified samples of a particular modification, and natural systems produce only highly mixed samples of sugar-linked proteins. Thus, a major goal of the field has been to engineer ways to produce individual glycoproteins in large quantities.

Taking advantage of the efficiency of the "workhorse" of protein synthesis, the Escherichia coli system, and the glycosylation machinery of another bacteria known as Campylobacter jejuni, biochemist Lai-Xi Wang of the University of Maryland School of Medicine and his colleagues developed a process by which proteins can be synthesized and glycosylated in E. coli, and then further modified in vitro to create "the exact linkage you find in natural eukaryotic glycoproteins," Wang said.

"In principle, you could make a lot of very homogenously glycosylated proteins [so that] you can really study the properties of one particular glycoform," Chin said.

Previous methods for creating such modified proteins were largely limited to synthetic methods, where the proteins are pieced together chemically, which are "technically pretty hard," Chin said.

Some drug therapies, such as therapeutic monoclonal antibodies used for cancer treatment, involve glycoproteins. "The glycosylation of therapeutic monoclonal antibodies is critical for [their] function," Wang said. "If we can find a way to make monoclonal antibodies that have [specific] glycans attached, then you would definitely have a better drug for cancer treatment."

In 2004, research from the lab of Peter Schultz at The Scripps Research Institute in La Jolla, California proposed a potential breakthrough by creating amino acids with the sugar already stuck on, and then incorporating those modified amino acids into full proteins in E. coli.

The 2004 paper "was a big deal," Wang told The Scientist last November. "This method opened a new avenue for the preparation of glycoproteins [with] huge potential in this field."

Unfortunately, however, both that paper, published in Science, and a similar one from the Journal of the American Chemical Society (JACS), were retracted late last year due to an inability to repeat the results and the loss of the lab notebooks with the original experimental conditions.

Although no charges of misconduct were filed in this case, rumors of deception flew around the scientific community after the surfacing of an anonymous email sent to Zhiwen Zhang, the first author on the Science paper and the third author on the JACS paper. The email accused Zhang of "fraud on at least 3 papers," and claimed to have proof, Nature reported.

Even with the new findings, there's still a lot to do, Wang admits -- such as improving the efficiency of the system, and tweaking it to more easily produce a mammalian glycoprotein.

"This is a just a start," Wang said. "This is just a proof of concept research [that] you can actually make eukaryotic glycoproteins using bacteria."


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Science paper pulled
[30th November 2009]

Sweet Attachments
[May 2008]

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

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