New research identifies key protein complex in cellular quality control

Anyone who has ever tried to neatly gather a fitted sheet can tell you: folding is difficult. If you do the wrong job with your laundry, the result can be a crumpled, wrinkled mess of fabric, but if folding fails among the approximately 7,000 proteins of origami-like complexity that regulate essential cellular functions, the result can lead to a of many serious diseases ranging from emphysema and cystic fibrosis to Alzheimer’s disease. Fortunately, our bodies have a quality control system that identifies misfolded proteins and marks them for additional folding or destruction, but exactly how this quality control process functions is not fully understood. Researchers from the University of Massachusetts Amherst have now made a quantum leap in our understanding of how this quality control system works, by discovering the ‘hotspot’ where all the action happens. The research was recently published in the Proceedings of the National Academy of Sciences.

DNA may be the most important blueprint for life, but we are made up of proteins. Although many are structurally simple, there are approximately 7,000 proteins that must be made in a cell’s secretory pathway and that are either distributed throughout the cell or secreted into the extracellular space to perform their essential functions.

The story begins in the endoplasmic reticulum – the cellular protein factories responsible for properly building thousands of different proteins – and there are two main players involved: an enzyme known as UGGT and its partner protein Sep15. Senior authors Daniel Hebert, professor of biochemistry and molecular biology at UMass Amherst, and Lila Gierasch, professor of biochemistry and molecular biology and chemistry at UMass Amherst, along with co-author Kevin Guay, a graduate student in the molecular cellular biology program at UMass Amherst , had shown in previous research that UGGT acts as a “gatekeeper” by reading carbohydrate tags called N-glycans embedded in the protein to determine whether or not the protein is folded correctly.

“But there’s something else going on,” says lead author Rob Williams, a postdoctoral researcher with a joint appointment in both Hebert and Gierasch’s labs. “There is an exclusive club of proteins called ‘selenoproteins’, which contain the rare element selenium. Of the approximately 20,000 different proteins in our body, only 25 are selenoproteins. The UGGT partner Sep15 is a selenoprotein. Sep15 is always associated with UGGT But until now no one knew what it was doing there.”

Using an AI model called AlphaFold2, Williams and his co-authors predicted that the protein Sep15 has a complex spiral shape that resembles a catcher’s glove, and that this glove fits perfectly with a complementary site on the UGGT enzyme. The specific site where SEP15 and UGGT bind Also where UGGT reads the N-glycan code that indicates whether or not a protein is folded correctly.

“Basically,” says Hebert, “we’ve found the hot spot where all the action is happening; and September 15 is the key.”

To test their prediction generated by AlphaFold2, the research team designed an experiment using recombinant DNA reengineering of UGGT to disrupt its binding to Sep15; and indeed the modified UGGT failed to form a complex with Sep15.

So what exactly does Sep15 do? “There are two possibilities, both of which we are pursuing,” Hebert said. “Either Sep15 gives the misfolded protein a chance to correct its shape, or it marks that protein for destruction.”

“The complexity of the proteins we study allows higher life forms to function,” says Gierasch, “but the complexity of those proteins also means they are more susceptible to misfolding errors, and misfolding errors can have catastrophic consequences if the quality control process fails.” .”

Although much basic research remains to be done, the team’s research paves the way for new drug therapies that target the Sep15/UGGT interface. “This is an unexplored pharmaceutical area,” says Hebert, “and Williams’ research has put us in the right direction for an eventual treatment.”

This research was supported by the National Institutes of General Medical Sciences and made possible by the availability of instrumentation at the UMass Amherst Institute for Applied Life Sciences.