Golgi apparatus: From the RER, proteins and lipids destined for other organelles or for extracellular export are shuttled into the Golgi apparatus. This organelle consists of stacked cisternae that progressively modify proteins in an orderly fashion from cis (near the ER) to trans (near the plasma membrane); macromolecules are shuttled between the various cisternae within membrane-bound vesicles. As molecules move from cis to trans, the N-linked oligosaccharides originally added to proteins in the ER are pruned and further modified in a stepwise fashion; O-linked oligosaccharides (sugar moieties linked to serine or threonine) are also appended. Some of this glycosylation is important in directing molecules to lysosomes (via the mannose-6-phosphate receptor); other glycosylation adducts may be important for cell–cell or cell–matrix interactions, or for clearing senescent cells (e.g., platelets and red cells). In addition to the stepwise glycosylation of lipids and proteins, the cis Golgi network is where proteins are recycled back to the ER, and the trans Golgi network is where proteins and lipids are dispatched to other organelles (including the plasma membrane), or to secretory vesicles destined for extracellular release. The Golgi complex is especially prominent in cells specialized for secretion, including goblet cells of the intestine, bronchial epithelium (secreting large amounts of polysaccharide-rich mucus), and plasma cells (secreting large quantities of antibodies).
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Smooth ER (SER): The SER in most cells is relatively sparse, forming the transition zone from RER to transport vesicles moving to the Golgi. However, in cells that synthesize steroid hormones (e.g., in the gonads or adrenals), or that catabolize lipid-soluble molecules (e.g., in the liver), the SER may be particularly conspicuous. Indeed, repeated exposure to compounds that are metabolized by the SER (e.g., phenobarbital, which is catabolized by the cytochrome P-450 system) leads to a reactive hyperplasia of the SER. The SER also is responsible for sequestering intracellular calcium; subsequent release from the SER into the cytosol can mediate a number of responses to extracellular signals. In addition, in muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for the cyclical release and sequestration of calcium ions that regulate muscle contraction and relaxation, respectively.
WasteDisposal:LysosomesandProteasomes
As already mentioned briefly, cellular waste disposal depends on the activities of lysosomes and proteasomes (
Fig. 1.10
Lysosomes are membrane-bound organelles containing roughly 40 different acid hydrolases (i.e., enzymes that function best in acidic pH ≤5), including proteases, nucleases, lipases, glycosidases, phosphatases, and sulfatases. Lysosomal enzymes are initially synthesized in the ER lumen and then tagged with a mannose-6-phosphate (M6P) residue within the Golgi apparatus. Such M6P-modified proteins are subsequently delivered to lysosomes through trans-Golgi vesicles that express M6P receptors. The other macromolecules destined for catabolism in the lysosomes arrive by one of three other pathways (
Fig. 1.10
Material internalized by fluid-phase pinocytosis or receptor-mediated endocytosis passes from plasma membrane to early endosome to late endosome, and ultimately into the lysosome, becoming progressively more acidic in the process. The early endosome is the first acidic compartment encountered, whereas proteolytic enzymes only begin significant digestion in the late endosome; late endosomes mature into lysosomes.