Chapter 10: Protein Sorting and Transport
Chapter Summary
THE ENDOPLASMIC RETICULUM
The Endoplasmic Reticulum and Protein Secretion: The endoplasmic reticulum is the first branch point in protein sorting. In mammalian cells, proteins destined for secretion, lysosomes, or the plasma membrane are translated on membrane-bound ribosomes and transferred into the rough ER as their translation proceeds.
Targeting Proteins to the Endoplasmic Reticulum: Proteins can be targeted to the ER either while their translation is still in progress or following completion of translation in the cytosol. In mammalian cells, most proteins are translocated into the ER while they are being translated on membrane-bound ribosomes. Ribosomes engaged in the synthesis of secreted proteins are targeted to the endoplasmic reticulum by signal sequences at the amino terminus of the polypeptide chain. Growing polypeptide chains are then translocated into the ER through protein channels and released into the ER lumen by cleavage of the signal sequence.
Insertion of Proteins into the ER Membrane: Integral membrane proteins of the plasma membrane or the membranes of the ER, Golgi apparatus, and lysosomes are initially inserted into the membrane of the ER. Rather than being translocated into the ER lumen, these proteins are anchored by membrane-spanning α helices that stop the transfer of the growing polypeptide chain across the membrane.
Protein Folding and Processing in the ER: Polypeptide chains are folded into their correct three-dimensional conformations within the ER. The ER is also the site of N-linked glycosylation and addition of GPI anchors.
Quality Control in the ER: Many secretory proteins are not folded correctly the first time. Chaperones detect incorrectly folded proteins and recycle them through the folding pathway. Those proteins that cannot be correctly folded are diverted from the secretory pathway and marked for degradation.
The Smooth ER and Lipid Synthesis: The ER is the major site of lipid synthesis in eukaryotic cells, and the smooth ER is abundant in cells that are active in lipid metabolism and detoxification of lipid-soluble drugs.
Export of Proteins and Lipids from the ER: Proteins and lipids are transported in vesicles from the ER to the Golgi apparatus. Resident ER proteins are marked by sequences that signal their return from the Golgi to the ER by a recycling pathway. Other targeting sequences mediate the selective packaging of exported proteins into vesicles that transport them to the Golgi.
THE GOLGI APPARATUS
Organization of the Golgi: The Golgi apparatus functions in protein processing and sorting as well as in the synthesis of lipids and polysaccharides. Proteins are transported from the endoplasmic reticulum to the cis Golgi network. From there they are transported to the Golgi stack, which represents the site of most metabolic activities of the Golgi apparatus. Modified proteins are transported from the Golgi stack to the trans Golgi network, where they are sorted and packaged in vesicles for transport to endosomes, lysosomes, the plasma membrane, or the exterior of the cell.
Protein Glycosylation within the Golgi: The N-linked oligosaccharides added to proteins in the ER are modified within the Golgi. Those proteins destined for lysosomes are specifically phosphorylated on mannose residues, and mannose-6-phosphate serves as a targeting signal that directs their transport to lysosomes from the trans Golgi network. O-linked glycosylation also takes place within the Golgi.
Lipid and Polysaccharide Metabolism in the Golgi: The Golgi apparatus is the site of synthesis of glycolipids, sphingomyelin, and the complex polysaccharides of plant cell walls.
Protein Sorting and Export from the Golgi Apparatus: Proteins are sorted in the trans Golgi network for packaging into transport vesicles targeted for secretion, the plasma membrane, lysosomes, or yeast and plant vacuoles. In polarized cells, proteins are specifically targeted to the apical and basolateral domains of the plasma membrane.
THE MECHANISM OF VESICULAR TRANSPORT
Experimental Approaches to Understanding Vesicular Transport: The mechanism of vesicular transport has been elucidated through studies of yeast mutants, reconstituted cell-free systems, and synaptic vesicles.
Cargo Selection, Coat Proteins, and Vesicle Budding: The cytoplasmic surfaces of vesicles are coated with proteins that drive vesicle budding and select the specific molecules to be transported.
Vesicle Fusion: Vesicle binding to the correct target membrane is mediated by interactions between pairs of transmembrane proteins, which leads to membrane fusion. Some types of fusion with the plasma membrane (exocytosis) occur at specific multiprotein complexes called exocysts.
LYSOSOMES
Lysosomal Acid Hydrolases: Lysosomes contain an array of acid hydrolases that degrade proteins, nucleic acids, polysaccharides, and lipids. These enzymes function specifically at the acidic pH maintained within lysosomes.
Endocytosis and Lysosome Formation: Extracellular molecules taken up by endocytosis are transported to endosomes, which mature to lysosomes as lysosomal acid hydrolases are delivered from the Golgi.
Phagocytosis and Autophagy: Lysosomes are responsible for the degradation of large particles taken up by phagocytosis and for the gradual digestion of the cell's own components by autophagy.
