Proteins are the working machinery of every cell. They build structures, speed up chemical reactions, transport molecules, send signals, and help cells respond to changes in their environment. To perform these jobs, a protein must fold into a particular three-dimensional shape.
When a protein fails to reach or maintain its proper shape, it is called misfolded. A misfolded protein may lose its normal function, interfere with other cellular processes, or form abnormal clumps. Cells have sophisticated systems for preventing and correcting these problems, but those defenses are not perfect. When misfolded proteins accumulate faster than cells can manage them, they can contribute to disease and, in severe cases, cell death.
Why protein shape matters
Proteins are long chains of amino acids. The sequence of amino acids is determined by the gene that encodes the protein, but the chain itself is only the starting point. It normally folds into a specific shape based largely on interactions among its amino acids and with its surrounding environment.
That shape determines what the protein can do. An enzyme, for example, needs the right arrangement of atoms to bind its target molecules and catalyze a reaction. A receptor on a cell’s surface needs a particular structure to recognize its signaling molecule. A structural protein needs the right shape and interactions to contribute to the architecture of a cell or tissue.
A misfolded protein can therefore be compared with a tool that has been bent out of shape: its chemical components are still present, but they may no longer be positioned correctly to perform the intended job.
Misfolding does not necessarily mean that every part of a protein is permanently distorted. Proteins can fluctuate between different shapes, and some changes are reversible. The important issue is whether the protein can reach and maintain the functional structure required for its role.
How proteins become misfolded
Protein folding is influenced by the protein’s amino acid sequence, the cellular environment, and other molecules that assist the process. Errors can arise at several points.
Sometimes the amino acid sequence itself is altered by a genetic mutation. A single change can make a protein less stable or cause it to adopt an abnormal structure. In other cases, a protein has a normal sequence but encounters conditions that make proper folding difficult.
The cellular environment can change with aging, infection, metabolic stress, changes in temperature, or other forms of cellular damage. Such conditions can increase the likelihood that proteins will unfold or fold incorrectly.
Newly produced proteins can also occasionally fail to fold correctly simply because folding is a complex molecular process. Cells produce enormous numbers of proteins, so some folding errors are inevitable.
Importantly, misfolding is not always caused by a single dramatic event. A cell is constantly producing, folding, repairing, and removing proteins. Protein quality depends on the balance among these processes.
What cells do about misfolded proteins
Cells have several layers of protein quality control, a collection of systems that identify damaged or incorrectly folded proteins and attempt to deal with them.
One important group of helpers is called molecular chaperones. Despite the name, chaperones do not generally determine the final shape of a protein themselves. Instead, they can bind to vulnerable protein chains, prevent inappropriate interactions, and give proteins better opportunities to fold correctly.
If a protein remains misfolded, the cell can often mark it for destruction. A common pathway involves attaching small molecules called ubiquitin to the unwanted protein. This marking can direct the protein to the proteasome, a molecular machine that breaks proteins down into smaller components that can be recycled.
Cells also use other disposal systems, including autophagy, in which cellular material is delivered to lysosomes for degradation. Different quality-control pathways handle different types of damaged or aggregated proteins.
These systems are essential because a misfolded protein is not merely a defective version of the original molecule. Its exposed chemical surfaces can interact abnormally with other proteins and cellular structures.
When misfolded proteins start sticking together
One of the most important consequences of misfolding is aggregation, in which abnormal proteins associate with one another and form larger assemblies.
Proteins normally hide many water-avoiding, or hydrophobic, regions inside their folded structures. Misfolding can expose some of these regions. Exposed surfaces can cause proteins to stick together instead of remaining as separate molecules.
Aggregates can take many forms. Some are relatively disorganized clusters, while others can develop into highly ordered structures known as amyloid fibrils. Amyloids contain proteins arranged in a characteristic structure and are associated with several human diseases.
The effects of aggregation depend on the protein, the type of aggregate, where it forms, and how effectively the cell can remove it. Large deposits can physically disrupt cells, but the largest visible aggregates are not necessarily the most harmful species. Smaller abnormal protein assemblies can also interfere with cellular functions.
How misfolding can damage cells
Misfolded proteins can cause harm in several ways at once.
First, the cell may lose the normal function of the affected protein. If the protein performs an essential task, its absence can disrupt an entire cellular pathway.
Second, abnormal proteins can interfere with other proteins. They may bind to molecules they were never supposed to interact with, disrupt cellular structures, or interfere with transport and signaling.
Third, the cell must spend energy and resources trying to identify, refold, or destroy the defective proteins. If misfolding becomes extensive, the quality-control machinery itself can become overwhelmed.
Misfolded proteins can also place stress on the endoplasmic reticulum, a cellular compartment where many proteins are made and processed. Accumulation of improperly folded proteins there activates a protective response known as the unfolded protein response. This response can temporarily reduce protein production and increase the cell’s capacity to handle misfolded proteins.
If the problem cannot be resolved, however, prolonged cellular stress can activate pathways leading to cell death.
Misfolding and human disease
Abnormal protein folding is involved in several important diseases, although the precise mechanisms differ substantially from one condition to another.
In Alzheimer’s disease, abnormal forms of amyloid-beta and tau are associated with pathological changes in the brain. In Parkinson’s disease, aggregates containing the protein alpha-synuclein are a characteristic feature of affected neurons. Huntington’s disease results from a genetic change that produces an altered huntingtin protein prone to abnormal behavior and aggregation.
Some diseases arise because a mutation makes a particular protein unstable or prone to misfolding. Cystic fibrosis, for example, can result from mutations in the CFTR gene that cause certain versions of the CFTR protein to fold improperly and be eliminated by cellular quality-control systems before they reach the cell surface.
There is also a distinct group of disorders called prion diseases. Prions involve abnormal forms of a normal protein that can promote abnormal conformational changes in other copies of that protein. This unusual mechanism allows the misfolded state to propagate through molecular interactions.
These conditions should not be viewed as simply “diseases caused by protein clumps.” In many cases, loss of normal protein function, toxic interactions, cellular stress, impaired degradation, and inflammation can all contribute to disease.
Why aging makes protein quality control harder
Cells become more vulnerable to protein misfolding as they age, although aging does not mean that proteins inevitably become misfolded.
Protein quality-control systems can become less effective over time. Changes in cellular metabolism, damage to proteins and other molecules, and reduced efficiency of degradation and repair can make it harder to maintain a healthy population of proteins.
This creates a difficult feedback loop. More damaged proteins increase the workload on quality-control systems, while weakened quality control makes it harder to remove damaged proteins efficiently.
The result can be a gradual increase in abnormal protein accumulation and cellular stress, particularly in long-lived cells such as neurons.
Can a misfolded protein ever be fixed?
Sometimes.
Many proteins that initially fold incorrectly can refold into a functional structure, particularly when the underlying amino acid sequence is normal and the damaging conditions are temporary. Molecular chaperones can assist this process.
Other proteins are too unstable or too severely damaged to be rescued. In those cases, destruction and replacement are safer than trying to restore the protein.
This distinction is important because the cell’s goal is not to save every misfolded protein. Its priority is to maintain a functional and safe protein population. When repair is unlikely to succeed, disposal is often the better option.
What happens when the cleanup systems fail
A healthy cell maintains a dynamic balance between protein production, folding, repair, and degradation. Misfolded proteins become particularly dangerous when that balance breaks down.
If production of an unstable protein increases, if a mutation makes it unusually difficult to degrade, or if cellular quality-control systems become overwhelmed, abnormal proteins can accumulate. Aggregates may then interfere with essential processes, cellular stress responses may remain activated, and damaged cells may eventually die.
In tissues such as the brain, the consequences can be especially serious because many neurons are long-lived and cannot simply be replaced. Progressive accumulation of abnormal proteins is therefore one important feature of several neurodegenerative disorders.
Protein misfolding is not, by itself, an unusual event. It is a normal challenge that cells continuously manage. The problem arises when misfolding becomes too frequent, the resulting proteins become unusually difficult to remove, or the cell’s protective systems can no longer keep pace.
At its core, protein misfolding is a failure of molecular shape with consequences that can extend far beyond a single protein. Because protein structure determines protein function, an error in folding can become a problem of cellular chemistry, signaling, energy use, and ultimately cell survival.
