protein that carries iron in the blood. Cell-surface transferrin receptors deliver transferrin with its bound iron to early endosomes by receptor-mediated endocytosis. The low pH in the endosome induces transferrin to release its bound iron, but the iron-free transferrin itself (called apotransferrin) remains bound to its receptor. The receptor–apotransferrin complex enters the tubular extensions of the early endosome and from there is recycled back to the plasma membrane. When the apotransferrin returns to the neutral pH of the extracellular fluid, it dissociates from the receptor and is thereby freed to pick up more iron and begin the cycle again. Thus, transferrin shuttles back and forth between the extracellular fluid and early endosomes, avoiding lysosomes and delivering iron to the cell interior, as needed for cells to grow and proliferate.
plasma Membrane signaling receptors are down-regulated by degradation in lysosomes
A second pathway that endocytosed receptors can follow from endosomes is taken by many signaling receptors, including opioid receptors and the receptor that binds epidermal growth factor (EGF). EGF is a small, extracellular signal protein that stimulates epidermal and various other cells to divide. Unlike LDL receptors, EGF receptors accumulate in clathrin-coated pits only after binding their ligand, and most do not recycle but are degraded in lysosomes, along with the ingested EGF. EGF binding therefore first activates intracellular signaling pathways and then leads to a decrease in the concentration of EGF receptors on the cell surface, a process called receptor downregulation, that reduces the cell’s subsequent sensitivity to EGF (see Figure 15–20).
Receptor downregulation is highly regulated. The activated receptors are first covalently modified on the cytosolic face with the small protein ubiquitin. Unlike polyubiquitylation, which adds a chain of ubiquitins that typically targets a protein for degradation in proteasomes (discussed in Chapter 6), ubiquitin tagging for sorting into the clathrin-dependent endocytic pathway adds just one or a few single ubiquitin molecules to the protein—a process called monoubiquitylation or multiubiquitylation, respectively. Ubiquitin-binding proteins recognize the attached ubiquitin and help direct the modified receptors into clathrin-coated pits. After delivery to the early endosome, other ubiquitin-binding proteins that are part of ESCRT complexes (ESCRT = Endosome Sorting Complex Required for Transport) recognize and sort the ubiquitylated receptors into intralumenal vesicles, which are retained in the maturing late endosome (see Figure 13–47). In this way, addition of ubiquitin blocks receptor recycling to the plasma membrane and directs the receptors into the degradation pathway, as we discuss next.
The endosomal compartments can be made visible in the electron microscope by adding a readily detectable tracer molecule, such as the enzyme peroxidase, to the extracellular medium and allowing varying lengths of time for the cell to endocytose the tracer. The distribution of the molecule after its uptake reveals the sequence of events. Within a minute or so after adding the tracer, it starts to appear in early endosomes, just beneath the plasma membrane (Figure 13–53). By 5–15 minutes later, it has moved to late endosomes, close to the Golgi apparatus and near the nucleus.