Cellular Code on Protein Folding Unlocked

There are hundreds of diseases affecting humans that result from errors in cellular production of its proteins. A team led by researchers at the University of Massachusetts, Amherst, report they recently unlocked the carbohydrate-based code that governs how certain classes of proteins form themselves into the complex shapes necessary to keep us healthy.

The research is published in Molecular Cell in an article titled, “ER chaperones use a protein folding and quality control glyco-code.”

Scientists once thought that the single code governing life was DNA, and that everything was governed by how DNA’s four building blocks—A, C, G, and T—combined and recombined. But in recent decades, it has become clear that there are other codes at work.

The discovery of the carbohydrate-based chaperone system in the endoplasmic reticulum (ER), was due to the pioneering work that Daniel Hebert, PhD, professor of biochemistry and molecular biology at UMass Amherst and one of the paper’s senior authors, initiated as a postdoctoral fellow in the 1990s. “The tools we have now, including glycoproteomics and mass spectrometry at UMass Amherst’s Institute for Applied Life Sciences, are allowing us to answer questions that have remained open for over 25 years,” said Hebert. “The lead author of this new paper, Kevin Guay, is doing things I could only dream of when I first started.”

Among the most pressing of these unanswered questions is: how do chaperones know when 7,000 different origami-like proteins are correctly folded?

We know now that the answer involves an “ER gatekeeper” enzyme known as UGGT, and a host of carbohydrate tags, called N-glycans, which are linked to specific sites in the protein’s amino acid sequence.

Guay, who is completing his PhD in the molecular cellular biology program at UMass Amherst, focused on two specific mammalian proteins, known as alpha-1 antitrypsin and antithrombin. Using CRISPR-edited cells, he and his co-authors modified the ER chaperone network to determine how the presence and location of N-glycans affected protein folding. They watched as the disease variants were recognized by the ER gatekeeper UGGT and, in order to peer more closely, developed a number of innovative glycoproteomics techniques using mass spectrometry to understand what happens to the glycans that stud the surface of the proteins.

What they discovered is that the enzyme UGGT “tags” misfolded proteins with sugars placed in specific positions.

“This is the first time that we’ve been able to see where UGGT puts sugars on proteins made in human cells for quality control,” said Guay. “We now have a platform for extending our understanding of how sugar tags can send proteins for further quality control steps and our work suggests that UGGT is a promising avenue for targeted drug therapy research.”

The discovery of the role that UGGT plays may lead to new studies that further our understanding and may help to treat the hundreds of diseases that result from misfolded proteins.