The enzymes that synthesize the lipid components of lipoproteins 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. It is the principal site of production of lipoprotein particles, which carry lipids via the bloodstream to other parts of the body. The main cell type in the liver, the hepatocyte, is another cell with an abundant smooth ER. (B) A three-dimensional reconstruction of a region of smooth ER and rough ER in a liver (more.) This electron micrograph is of a testosterone-secreting Leydig cell in the human testis. (A) Abundant smooth ER in a steroid-hormone-secreting cell. Individual ribosomal subunits are thought to move randomly between these two segregated populations of mRNA molecules ( Figure 12-37). Therefore, only those mRNA molecules that encode proteins with an ER signal sequence bind to rough ER membranes those mRNA molecules that encode all other proteins remain free in the cytosol. In contrast, if an mRNA molecule encodes a protein that lacks an ER signal sequence, the polyribosome that forms remains free in the cytosol, and its protein product is discharged there. The mRNA itself, however, remains attached to the ER membrane by a changing population of ribosomes, each transiently held at the membrane by the translocator. The individual ribosomes associated with such an mRNA molecule can return to the cytosol when they finish translation near the 3′ end of the mRNA molecule. Since many ribosomes can bind to a single mRNA molecule, a polyribosome is usually formed, which becomes attached to the ER membrane, directed there by the signal sequences on multiple growing polypeptide chains ( Figure 12-36B). When a ribosome happens to be making a protein with an ER signal sequence, the signal directs the ribosome to the ER membrane. They differ only in the proteins they are making at any given time. Membrane-bound and free ribosomes are structurally and functionally identical. Free ribosomes, unattached to any membrane, synthesize all other proteins encoded by the nuclear genome. Membrane-bound ribosomes, attached to the cytosolic side of the ER membrane, are engaged in the synthesis of proteins that are being concurrently translocated into the ER. There are therefore two spatially separate populations of ribosomes in the cytosol. The cytosol is filled with closely packed sheets of ER membrane studded with ribosomes. (A) An electron micrograph of the rough ER in a pancreatic exocrine cell that makes and secretes large amounts of digestive enzymes every day. These membrane-bound ribosomes coat the surface of the ER, creating regions termed rough endoplasmic reticulum, or rough ER ( Figure 12-36A). The ribosome that is synthesizing the protein is directly attached to the ER membrane. Thus, in contrast to the posttranslational import of proteins into mitochondria and chloroplasts, chaperone proteins are not required to keep the protein unfolded. Since one end of the protein is usually translocated into the ER as the rest of the polypeptide chain is being made, the protein is never released into the cytosol and therefore is never in danger of folding up before reaching the translocator in the ER membrane. This distinguishes the process from the import of proteins into mitochondria, chloroplasts, nuclei, and peroxisomes, which are posttranslational processes. In mammalian cells, the import of proteins into the ER begins before the polypeptide chain is completely synthesized-that is, import is a co-translational process. All of these proteins, regardless of their subsequent fate, are directed to the ER membrane by the same kind of signal sequence and are translocated across it by similar mechanisms. The water-soluble proteins are destined either for the lumen of an organelle or for secretion. Some of the transmembrane proteins function in the ER, but many are destined to reside in the plasma membrane or the membrane of another organelle. These proteins are of two types: transmembrane proteins, which are only partly translocated across the ER membrane and become embedded in it, and water-soluble proteins, which are fully translocated across the ER membrane and are released into the ER lumen. The ER captures selected proteins from the cytosol as they are being synthesized. Membrane-bound Ribosomes Define the Rough ER
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