In search of a sugar's secrets

November 8, 2013
Figure 1: In the endoplasmic reticulum, N-glycosylation of proteins results in the formation of free oligosaccharides. Credit: RIKEN–Max Planck Joint Research Center

The discovery of the origin of enigmatic 'waste' sugar molecules within the cell also hints that they serve some as-yet-undefined function.

Within a cellular compartment called the (ER), certain proteins become decorated with sugar molecules by a process called N-glycosylation, before being delivered to the cell surface. This process also gives rise to a population of untethered known as free oligosaccharides (fOSs). Tadashi Suzuki, Yoichiro Harada and colleagues from the RIKEN–Max Planck Joint Research Center for Systems Chemical Biology have now determined how fOSs are produced and at the same time have uncovered some important new questions.

The oligosaccharyltransferase (OST) enzyme complex facilitates N-glycosylation by transferring preassembled sugar structures from dolichyl pyrophosphoryl-linked oligosaccharide (DLO) molecules onto the target protein. Scientists had long believed that fOSs result from the breakdown of DLOs until Suzuki discovered an enzyme in the cytoplasm called peptide:N-glycanase (PNGase) that can generate fOSs by acting on improperly folded glycosylated proteins. While initially skeptical, the scientific community eventually embraced this as the primary mechanism for fOS production. Suzuki, however, suspected that the biological picture was even more complicated, particularly since lacking PNGase still produce low levels of fOSs.

To clarify the situation, he and Harada set out in search of this yet-undiscovered source of fOSs. First, they verified that PNGase-deficient yeast cells produce fOSs within the ER, which are subsequently exported to and degraded in the cytosol. Prior studies suggested that OST—the same enzyme that drives N-glycosylation—can also break down DLOs to yield fOSs. After obtaining initial data to support this hypothesis from experiments with mutant yeast strains, Harada went through the laborious process of isolating the intact, eight-protein OST complex. Experiments with purified OST showed that the enzyme can indeed convert DLO molecules into fOSs, providing new support for the DLO-centered model of fOS production.

Importantly, the research revealed that a sharp increase in the concentration of available DLOs does not lead to an equivalent bump in fOS production, suggesting that this is not an accidental process. "Our data suggest that OST-mediated fOS release is a highly regulated reaction," says Suzuki. Given that mammalian cells appear to produce the majority of their fOSs through a similar non-PNGase mechanism, Suzuki hypothesizes that this pathway may serve some specific function in higher eukaryotes, which his team hopes to uncover in future studies. "It looks like a waste if fOSs are merely the junk produced by erroneous activity of OST," he says. "We believe there must be a reason for cells to engage in such an apparent waste of energy."

Explore further: Weighing up the causes of obesity

More information: Harada, Y., Buser, R., Ngwa, E. M., Hirayama, H., Aebi, M. & Suzuki, T. Eukaryotic oligosaccharyltransferase generates free oligosaccharides during N-glycosylation. The Journal of Biological Chemistry advance online publication, 23 September 2013 DOI: 10.1074/jbc.M113.486985

Related Stories

It takes a sugar to catch a sugar

December 2, 2011

After every meal, the hormone insulin is released into the bloodstream, issuing instructions to target cells to begin taking up excess sugar. In some situations, however, cells stop responding to these signals; and this insulin-resistant ...

Specific sugar molecule causes growth of cancer cells

September 16, 2013

The process of glycosylation, where sugar molecules are attached to proteins, has long been of interest to scientists, particularly because certain sugar molecules are present in very high numbers in cancer cells. It now ...

Cell nuclei harbor factories that transcribe genes

September 27, 2013

Our genetic heritage is contained—and protected—in the nucleus of the cells that compose us. Copies of the DNA exit the nucleus to be read and translated into proteins in the cell cytoplasm. The transit between the nucleus ...

Recommended for you

A composite thread that varies in rigidity

October 27, 2016

EPFL scientists have developed a new type of composite thread that varies in stiffness depending on its temperature. Applications range from multifunctional robots to knitted casts, and even tunable medical devices.

Turning CO2 to stone

October 25, 2016

Earth has limits to the amount of carbon dioxide in its atmosphere before the environment as we know it starts to change. Too much CO2 absorbed by the oceans makes the water more acidic. Too much in the atmosphere warms the ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.