Figure 12–31 Fluorescent micrographs of the endoplasmic reticulum. (a) an animal cell in tissue culture that was genetically engineered to express an er membrane protein fused to a fluorescent protein. The er extends as a network of tubules and sheets throughout the entire cytosol, so that all regions of the cytosol are close to some portion of the er membrane. The outer nuclear membrane, which is continuous with the er, is also stained. (B) part of an er network in a living plant cell that was genetically engineered to express a fluorescent protein in the er. (a, courtesy of patrick Chitwood and Gia Voeltz; B, courtesy of petra Boevink and Chris hawes.)
The er is structurally and functionally Diverse
While the various functions of the ER are essential to every cell, their relative importance varies greatly between individual cell types. To meet different functional demands, distinct regions of the ER become highly specialized. We observe such functional specialization as dramatic changes in ER structure, and different cell types can therefore possess characteristically different types of ER membrane. One of the most remarkable ER specializations is the rough ER.
Mammalian cells begin to import most proteins into the ER before complete synthesis of the polypeptide chain—that is, import is a co-translational process (Figure 12–32A). In contrast, the import of proteins into mitochondria, chloroplasts, nuclei, and peroxisomes is a post-translational process (Figure 12–32B). In co-translational transport, the ribosome that is synthesizing the protein is attached directly to the ER membrane, enabling one end of the protein to be translocated into the ER while the rest of the polypeptide chain is being synthesized. These membrane-bound ribosomes coat the surface of the ER, creating regions termed rough endoplasmic reticulum, or rough ER; regions of ER that lack bound ribosomes are called smooth endoplasmic reticulum, or smooth ER (Figure 12–33).
Most cells have scanty regions of smooth ER, and the ER is often partly smooth and partly rough. Areas of smooth ER from which transport vesicles carrying newly synthesized proteins and lipids bud off for transport to the Golgi apparatus are called transitional ER. In certain specialized cells, the smooth ER is abundant and has additional functions. It is prominent, for example, in cells that specialize in lipid metabolism, such as cells that synthesize steroid hormones from cholesterol; the expanded smooth ER accommodates the enzymes that make cholesterol and modify it to form the hormones (see Figure 12–33B).
The main cell type in the liver, the hepatocyte, also has a substantial amount of smooth ER. It is the principal site of production of lipoprotein particles, which carry lipids via the bloodstream to other parts of the body. The enzymes that synthesize the lipid components of the particles are located in the membrane of the smooth ER, which also contains enzymes that catalyze a series of reactions to detoxify both lipid-soluble drugs and various harmful compounds produced by metabolism. The most extensively studied of these detoxification reactions are carried out by the cytochrome P450 family of enzymes, which catalyze a series of reactions in which water-insoluble drugs or metabolites that would otherwise accumulate to toxic levels in cell membranes are rendered sufficiently water-soluble to leave the cell and be excreted in the urine. Because the rough ER alone cannot house enough of these and other necessary enzymes, a substantial portion of the membrane in a hepatocyte normally consists of smooth ER (see Table 12–2).
Another crucially important function of the ER in most eukaryotic cells is to sequester Ca2+ from the cytosol. The release of Ca2+ into the cytosol from the ER, and its subsequent reuptake, occurs in many rapid responses to extracellular
Figure 12–32 Co-translational and post-translational protein translocation.