Scattered across the surface of our cells are microscopic gateways known as clathrin-coated pits. These structures function like selective loading docks, sampling the environment to bring specific materials inside. This internalization is necessary for cellular activities, from absorbing nutrients to receiving molecular signals that govern cell behavior. The process ensures that only designated molecules, or cargo, are granted passage into the cell.
The Molecular Machinery of Formation
The primary structural component of these cellular pits is a protein named clathrin. This protein has a unique three-legged shape, referred to as a triskelion. Each triskelion is composed of three large proteins called heavy chains, which form the legs, and three smaller light chains that regulate the structure’s assembly and disassembly. The name clathrin is derived from the Latin for “lattice,” which describes the structure these molecules create.
These individual triskelions have an intrinsic ability to link together. The end of one leg can interact with the center of another, allowing them to self-assemble into a polyhedral cage. This network of interconnected proteins forms a scaffold with a pattern of hexagons and pentagons, similar to the surface of a soccer ball. The specific combination of five- and six-sided rings allows the clathrin coat to curve and form vesicles of different sizes, adapting to the cargo it needs to transport.
Clathrin does not work alone; it relies on a group of molecules called adaptor proteins to perform its function. The most prominent of these at the cell surface is the AP2 adaptor complex. AP2 acts as the physical link, bridging the outer clathrin cage to the cargo that needs to be brought into the cell. This complex is a heterotetramer, meaning it is built from four different protein subunits, each with a specialized role.
Adaptor proteins organize the process, ensuring the cell only internalizes specific molecules. They achieve this by recognizing and binding to particular sorting signals on the cytoplasmic tails of transmembrane cargo proteins. One AP2 subunit binds directly to the membrane surface, anchoring the complex, while other parts recognize the cargo and recruit clathrin triskelions. This dual-binding capacity allows AP2 to initiate the formation of a coated pit when and where it is needed.
The Process of Cellular Entry
The formation of a clathrin-coated vesicle is a dynamic sequence of events known as clathrin-mediated endocytosis (CME). The process begins at a specific location on the plasma membrane, guided by lipid molecules like phosphatidylinositol 4,5-bisphosphate (PIP2). These lipids act as a beacon to concentrate the necessary machinery. Once anchored, the adaptor proteins gather their designated cargo, clustering specific transmembrane receptors into a small area.
With the adaptors and cargo in place, clathrin triskelions are recruited from the cytoplasm to the site. They begin to assemble into their lattice structure on the inner surface of the membrane. As more clathrin molecules are added, the growing scaffold exerts a force on the membrane below it. This action pulls the membrane inward, causing it to deform and create the depression or “pit” associated with this process.
The pit continues to grow and invaginate as the clathrin coat expands, progressively enveloping the cargo-laden membrane patch. The structure eventually forms a deeply curved, flask-shaped bud connected to the cell surface by a narrow neck of membrane. This stage represents a point where the cell is committed to forming a vesicle. The entire process, from initial recruitment to this stage, often takes less than a minute.
To complete the entry, the newly formed vesicle must be severed from the parent membrane. This is accomplished by another protein, a large GTPase called dynamin. Dynamin assembles into a helical collar around the neck of the budding vesicle. Using energy from GTP hydrolysis, the dynamin collar constricts and pinches the membrane neck. This scission event releases a fully formed, clathrin-coated vesicle into the cell’s interior.
Sorting and Transporting Cellular Cargo
Once a clathrin-coated vesicle is released into the cytoplasm, its protein shell is quickly removed. This uncoating is an energy-dependent process required for the vesicle to fuse with its target destination. The disassembly of the lattice is carried out by a chaperone protein called Hsc70 and a cofactor named auxilin. Auxilin helps Hsc70 dismantle the clathrin cage, releasing the triskelions back into the cytoplasm for recycling.
After shedding its coat, the naked transport vesicle travels deeper into the cell to deliver its contents to an organelle called the early endosome. The early endosome functions as a major intracellular sorting station. Here, the vesicle fuses with the endosomal membrane, releasing its cargo into the lumen and integrating its receptors into the endosome’s membrane.
Within the endosome, a sorting process takes place that decides the ultimate fate of the internalized cargo. Some molecules, like nutrient receptors, are often sorted into tubular extensions that branch off the endosome. From there, they are shuttled back to the plasma membrane for reuse, which allows the cell to maintain the number of receptors on its surface.
Other types of cargo are destined for degradation. For instance, low-density lipoproteins (LDLs), which carry cholesterol, are transported via this pathway. Once inside the endosome, the LDL particle is released from its receptor and directed toward the lysosome. Signaling molecules, such as growth factors, are also often sent to the lysosome to be broken down, which turns off the signal.
Clinical Relevance and Disease
The efficiency of clathrin-mediated endocytosis makes it a target for pathogens seeking to invade a host. Many viruses have evolved to hijack this pathway to gain entry. Viruses such as influenza and Hepatitis C initiate infection by binding to specific cell surface receptors that are normally internalized through clathrin-coated pits. Once bound, the virus is treated as cargo and drawn into the cell within a vesicle.
After being internalized, the virus uses the changing environment of the endocytic pathway. The interior of the early endosome is mildly acidic, a condition that many viruses use as a trigger. For influenza and Hepatitis C, the low pH induces changes in viral proteins, activating their ability to fuse the viral envelope with the endosomal membrane. This fusion creates a pore through which the viral genetic material is released into the cytoplasm to begin replication.
Disruptions in the clathrin pathway from genetic defects can also lead to human diseases. A primary example is familial hypercholesterolemia (FH), a disorder characterized by high levels of blood cholesterol. In many cases, FH is caused by mutations in the gene for the low-density lipoprotein receptor (LDLR). These mutations can prevent the receptor from being synthesized correctly, binding to LDL particles, or clustering in clathrin-coated pits for uptake.
When the LDL receptor cannot be efficiently internalized by liver cells, LDL cholesterol remains in the bloodstream at elevated concentrations. This excess cholesterol is deposited in arteries, leading to accelerated atherosclerosis and an increased risk of premature heart disease. In some rare forms of the disease, the LDL receptor is normal, but a mutation occurs in an adaptor protein, LDLRAP1. This protein is required for the clathrin-mediated internalization of the receptor.