Cellular quality control misses half of misfolded proteins, study finds

Inside every cell, proteins are constantly being assembled from chains of amino acids, folding into precise three-dimensional shapes that allow them to do their work. Sometimes this folding process fails. When it does, the misshapen protein becomes useless, and the cell's maintenance systems are supposed to recognize the problem, tag the defective protein, and either repair it or break it down for recycling. But a new study from Penn State reveals that this quality control mechanism is far from perfect—it misses roughly half of a particularly dangerous class of misfolded proteins, allowing them to accumulate inside cells where they may contribute to aging and disease.

The misfolding in question involves a knot-like structure formed when a segment of a protein's amino acid chain loops back on itself and threads through that loop, creating an entanglement. This can happen in two ways: the knot forms where it shouldn't, or it fails to form where it should be part of the protein's natural design. Ed O'Brien, a chemistry professor at Penn State who led the research, describes the cell as a tiny factory with quality control mechanisms meant to catch errors on the production line. The team wanted to understand whether this newly identified class of misfolding disrupts the cell's ability to maintain what scientists call protein homeostasis—the balance between making, repairing, and recycling proteins.

To investigate, the researchers took an unusual approach. Rather than conducting new experiments, they repurposed four existing datasets that had been collected for different purposes. They cross-referenced a database of proteins tagged for degradation in human fibroblast cells with a database of protein structures, looking for patterns in which proteins with entanglements were being marked for removal. The work was part of a larger effort at Penn State's National Synthesis Center for Emergence in the Molecular and Cellular Sciences, which focuses on extracting new insights from publicly available data that might otherwise sit unused.

The findings were striking. Proteins containing an entanglement as part of their native structure were 93 percent more likely to be tagged for degradation than proteins without such structures. Computer simulations showed that tagged proteins with entanglements were four times more likely to misfold than untagged proteins without them. The team also discovered that the cell's quality control system acts quickly—some proteins are tagged for removal while they are still being synthesized. Yet despite this aggressive response, the system still fails. About one-third of proteins with entanglements escape detection and remain in the cell despite being nonfunctional.

Yang Jiang, the first author of the paper and an associate research professor at Penn State, explains why some misfolded proteins slip through. A misfolded entanglement buried deep within a protein's structure may be invisible to the quality control machinery, allowing the defective protein to persist. Over time, these nonfunctional proteins accumulate, disrupting the delicate balance of protein production and recycling. The consequences could be significant. Protein misfolding is already known to play a role in neurodegenerative diseases like Alzheimer's and Huntington's, where toxic protein aggregates accumulate in the brain. Understanding this newly discovered class of misfolding could reveal new disease mechanisms and point toward novel treatments. For now, the study reveals a gap in the cell's defenses—one that may have been operating undetected for years, silently allowing damaged proteins to build up in our cells.