How a Crucial Protein Protects Itself While It’s Being Built Inside Cells

A new study from researchers at the University of Chicago and Stanford University uncovers how a major cellular chaperone protein called GRP94 recruits two helper proteins to shield itself from receiving too many sugar molecules during its formation. This process, known as N-glycosylation, is a fundamental part of protein maturation inside the endoplasmic reticulum (ER), and the new findings reveal a previously unknown regulatory mechanism that prevents errors that could lead to disease.

Proteins go through many steps from the moment they begin forming on ribosomes to finally becoming functional molecules. One of these steps is the attachment of sugar molecules—called glycans—by a cellular machine known as the oligosaccharyl transferase complex, or OST. When too many glycans are added, known as hyperglycosylation, proteins can become unstable or get marked for destruction. GRP94 faces exactly this risk during its own formation, and this study shows in clear detail how it avoids that fate.

The research, published in Nature, captured GRP94 in a rare intermediate state using cryo-electron microscopy (cryo-EM), allowing scientists to view how the protein interacts with two partner proteins—CCDC134 and FKBP11—as it emerges from the ribosome into the ER. Together, these three components form a protective shield that blocks OST from modifying GRP94 too aggressively.

Below is a straightforward breakdown of all the specific details uncovered in the study, followed by additional background information to help readers understand the broader context of protein glycosylation and its importance in cell biology.

What the Study Found About GRP94’s Protection Mechanism

The researchers discovered that as GRP94 is being synthesized on ribosomes docked to the ER membrane, it temporarily adopts a special partially folded shape. This immature form of GRP94 does not yet resemble the structure it will take on as a fully formed protein. In this transitional state, it is particularly vulnerable to getting hyperglycosylated by the OST machinery.

To prevent this, GRP94 recruits two specific chaperone proteins, CCDC134 and FKBP11. These proteins attach directly to the emerging GRP94 chain and create a physical barrier around the parts of GRP94 that OST typically targets with glycan attachments.

The new cryo-EM images show all three proteins interacting at the secretory translocon, the tunnel-like ER structure where nascent proteins enter the organelle. By sitting in key positions around GRP94, CCDC134 and FKBP11 effectively block OST from performing excessive glycosylation during this early phase.

The researchers emphasize that this captured moment—GRP94 frozen in mid-synthesis—is extremely rare. Nearly all previous structural studies focus on fully completed proteins, not ones still forming. The team described this as a fortunate, serendipitous capture, revealing biology in action rather than a final product.

Why Hyperglycosylation Matters

If GRP94 becomes hyperglycosylated, the cell’s quality-control systems treat it as defective and direct it to degradation pathways. That has serious consequences. GRP94 is not just any protein—it is a major ER chaperone, essential for the folding and maturation of many other proteins, including cell-surface receptors involved in tissue development and immune responses. Disrupting GRP94 indirectly disrupts many other systems that depend on it.

Past work by Stanford researchers showed that when the protective protein CCDC134 is mutated or absent, GRP94 becomes hyperglycosylated and destroyed. This leads to osteogenesis imperfecta, a bone-fragility disorder, because crucial developmental receptors fail to form or reach the cell surface correctly. The new study reinforces that connection and expands the understanding of how GRP94 and CCDC134 interact structurally during protein synthesis.

The Role of FKBP11 and CCDC134 in More Detail

Before this study, CCDC134 was already known to play a protective role for a related ER chaperone, HSP90B1. Now researchers show that CCDC134’s role is broader—it binds tightly to the early structure of GRP94, helping prevent inappropriate glycosylation.

FKBP11, on the other hand, was known to appear frequently at ribosome-translocon sites where proteins are being formed, but its specific functional role was unclear. The new work shows that FKBP11 acts alongside CCDC134 to enhance GRP94’s protection, forming a two-protein barrier surrounding the nascent chain.

Together, these two chaperones appear to be evolutionarily conserved guardians that prevent proteins entering the ER from engaging too early or too inappropriately with glycosylation machinery.

How the Findings Change Our Understanding of Glycosylation

Until now, scientists widely viewed N-glycosylation as a relatively automatic process: if a protein’s sequence included glycosylation sites, OST would modify them as the protein moved through the ER.

But this study shows that glycosylation can be actively regulated—some proteins can prevent glycan addition until they are ready for it. GRP94 uses a structural intermediate and helper proteins to selectively block glycosylation at the exact moment when it would be harmful.

This is the first known example of a protein directly controlling the activity of OST on itself.

It suggests that other proteins may have similar regulatory strategies that have simply gone undetected because catching proteins during synthesis is extremely difficult.

Why This Discovery Matters for Medicine

GRP94 has been a target of interest in diseases such as diabetes and cancer, but drugs designed to inhibit GRP94 often end up binding to other similar proteins, creating unwanted side effects. The discovery of CCDC134 and FKBP11 as selective regulators opens new therapeutic possibilities.

Instead of targeting GRP94 directly, future treatments might instead modulate the helper proteins that protect or regulate it. This could lead to more precise interventions with fewer unintended consequences.

Understanding how OST can be selectively inhibited during specific stages of protein formation also creates new opportunities in diseases that involve misfolded or misglycosylated proteins.

What GRP94 Does Inside the Cell

To give readers more context, here is a clearer picture of GRP94’s known roles inside the ER:

• It helps fold and stabilize newly synthesized proteins.
• It assists with the maturation of immune receptors, growth-factor receptors, and secreted signaling molecules.
• It helps maintain protein quality control inside the ER.
• It plays a role in managing ER stress responses.

Because GRP94 supports so many essential cell-surface receptors, disruptions to its function can affect development, immunity, and cell communication.

Background on N-Glycosylation and the OST Complex

N-glycosylation is one of the most common modifications proteins undergo. It involves attaching sugar chains to an asparagine (N) residue within a specific sequence pattern. This modification influences how proteins fold, how stable they are, and where they end up in the cell.

Two forms of OST exist in the ER membrane:

• OST-A, which performs glycosylation while the protein is still being synthesized.
• OST-B, which typically glycosylates proteins after they are fully made.

The new study shows that GRP94 uses CCDC134 and FKBP11 to inhibit the OST-A form specifically while it is emerging into the ER. This prevents errors that could destabilize GRP94 before it has a chance to fold properly.

The Significance of Capturing a Nascent Protein

One of the most impressive technical achievements of the study is visualizing GRP94 during synthesis. Cryo-EM has advanced rapidly but is still rarely able to capture proteins in such fleeting intermediate states.

This work provides one of the few real examples of a protein being observed precisely as it is being made. This makes the findings uniquely valuable for structural biology, offering a rare glimpse into live protein biogenesis rather than a finished snapshot.

Research Reference

Structural basis of regulated N-glycosylation at the secretory translocon (Nature, 2025)
https://doi.org/10.1038/s41586-025-09756-8