Figure 12–30 A model that explains how peroxisomes proliferate and how new peroxisomes arise. peroxisomal precursor vesicles bud from the er. at least two peroxisomal membrane proteins, pex3 and pex15, follow this route. The machinery that drives the budding reaction and that selects only peroxisomal proteins for packaging into these vesicles depends on pex19 and other cytosolic proteins that are still unknown. peroxisomal precursor vesicles may then fuse with one another or with preexisting peroxisomes. The peroxisome membrane contains import receptors and protein translocators that are required for the import of peroxisomal proteins made on cytosolic ribosomes, including new copies of the import receptors and translocator components. presumably, the lipids required for growth are also imported, although some may derive directly from the er in the membrane of peroxisomal precursor vesicles.
preexisting peroxisomes, but others are first integrated into the ER membrane, where they are packaged into specialized peroxisomal precursor vesicles. New precursor vesicles may then fuse with one another and begin importing additional peroxisomal proteins, using their own protein import machinery to grow into mature peroxisomes, which can undergo cycles of growth and fission.
Peroxisomes are specialized for carrying out oxidation reactions using molecular oxygen. They generate hydrogen peroxide, which they employ for oxidative purposes—and contain catalase to destroy the excess. Like mitochondria and plastids, peroxisomes are self-replicating organelles. Because they do not contain DNA or ribosomes, however, all of their proteins are encoded in the cell nucleus. Some of these proteins are conveyed to peroxisomes via peroxisomal precursor vesicles that bud from the ER, but most are synthesized in the cytosol and directly imported. A specific sequence of three amino acids near the C-terminus of many of the latter proteins functions as a peroxisomal import signal. The mechanism of protein import differs from that of mitochondria and chloroplasts, in that even oligomeric proteins are imported from the cytosol without unfolding.
The enDoplasmiC reTiCUlUm
All eukaryotic cells have an endoplasmic reticulum (ER). Its membrane typically constitutes more than half of the total membrane of an average animal cell (see Table 12–2, p. 643). The ER is organized into a netlike labyrinth of branching tubules and flattened sacs that extends throughout the cytosol (Figure 12–31 and Movie 12.4). The tubules and sacs interconnect, and their membrane is continuous with the outer nuclear membrane; the compartment that they enclose therefore is also continuous with the space between the inner and outer nuclear membranes. Thus, the ER and nuclear membranes form a continuous sheet enclosing a single internal space, called the ER lumen or the ER cisternal space, which often occupies more than 10% of the total cell volume (see Table 12–1, p. 643).
As mentioned at the beginning of this chapter, the ER has a central role in both lipid and protein biosynthesis, and it also serves as an intracellular Ca2+ store that is used in many cell signaling responses (discussed in Chapter 15). The ER membrane is the site of production of all the transmembrane proteins and lipids for most of the cell’s organelles, including the ER itself, the Golgi apparatus, lysosomes, endosomes, secretory vesicles, and the plasma membrane. The ER membrane is also the site at which most of the lipids for mitochondrial and peroxisomal membranes are made. In addition, almost all of the proteins that will be secreted to the cell exterior—plus those destined for the lumen of the ER, Golgi apparatus, or lysosomes—are initially delivered to the ER lumen.