Proteins, often referred to as the building blocks of cells, require intricate processes to form correctly. One crucial step in protein formation is glycosylation, where sugar molecules, known as glycans, are attached to a developing protein. This process significantly influences how proteins fold and function, and errors can lead to diseases. A recent study by Robert Keenan’s team at the University of Chicago, in collaboration with Rajat Rohatgi’s lab at Stanford University, offers new insights into the regulation of this essential cellular mechanism.
“It’s a complicated story that has many interesting layers, but it’s yet another example where curiosity-driven research reveals the underlying mechanism of a very basic cellular process that is linked to human disease,” said Keenan, a Professor of Biochemistry and Molecular Biology at UChicago. The research was published this week in Nature.
Unveiling the Protein Formation Process
Keenan’s research has primarily focused on protein synthesis within cells, particularly the role of ribosomes. These cellular machines translate genetic information into proteins, docking to the endoplasmic reticulum (ER) to facilitate protein transport. Of the approximately 20,000 proteins encoded by the human genome, about 7,000 are synthesized on ribosomes attached to the ER, a crucial organelle for molecular transit within cells. Once docked, the nascent protein chain is threaded into the ER, where it begins to fold and undergo modifications like glycosylation.
Last year, Mengxiao Ma from the Rohatgi lab at Stanford published findings on GRP94, a protein aiding in the folding and maturation of proteins in the ER. The study revealed how GRP94 avoids “hyperglycosylation,” a condition where excessive sugar molecules lead to protein degradation. This degradation can disrupt other proteins, including those involved in tissue development and immune responses.
Discovering the Role of Chaperone Proteins
When GRP94 forms, it partners with CCDC134 to inhibit the oligosaccharyl transferase complex (OST), which facilitates glycosylation. Mutations in CCDC134 can cause GRP94 hyperglycosylation, linked to osteogenesis imperfecta, a bone disorder.
Meanwhile, Keenan’s team explored OST’s function, discovering that FKBP11 often associates with ribosome machinery during protein synthesis. Surprisingly, GRP94 and CCDC134, studied by Rohatgi’s group, were also involved. Using cryogenic electron microscopy (cryo-EM), postdocs Mel Yamsek and Roshan Jha captured images showing GRP94 in a nascent form, recruiting CCDC134 and FKBP11 as “chaperones” to shield it from glycosylation during formation.
“We trapped GRP94 in the process of being made,” Keenan noted. “There are very few examples of any protein being observed like this. So, this was serendipity, a bit of good fortune.”
Implications for Future Treatments
Due to its links to diabetes and cancer, GRP94 is a target for potential drug treatments. This study provides insights into how future therapies could selectively target GRP94 without affecting other cellular processes. Previous attempts have failed due to drugs binding to other GRP94-like proteins, causing unintended effects. Targeting CCDC134 or FKBP11 might offer a new strategy to disrupt GRP94 by removing its protection from hyperglycosylation.
“Thinking about it in terms of evolution, maybe the early function of FKBP11 and CCDC134 was to shield any nascent protein chain as it enters the ER, to prevent any sort of inappropriate interactions with other stuff in the cell that could cause problems,” Keenan explained.
“Later, GRP94 might have evolved the ability to bind much more tightly so it could inhibit its own glycosylation,” he continued. “It’s the first example we’ve ever seen for directly regulating the activity of OST, which is fascinating because this is such a fundamental process in cells.”
The study, titled “Structural basis of regulated N-glycosylation at the secretory translocon,” received support from the National Institutes of Health, the American Cancer Society, the AP Giannini Foundation, and the National Science Foundation.
This groundbreaking research not only enhances our understanding of protein formation but also opens doors for innovative therapeutic approaches targeting diseases linked to glycosylation errors.
