The Endomembrane System and Protein Sorting
Becker’s World of the Cell Tenth Edition: Chapter 12 - The Endomembrane System and Protein Sorting
The Endomembrane System (1 of 2)
Definition: An appreciation of eukaryotic cells depends on understanding the role of intercellular membranes and the compartmentalization of function within organelles.
Trafficking: Movement of lipids and proteins between organelles (called trafficking) must be tightly regulated.
Components of the Endomembrane System
Endoplasmic Reticulum (ER): The endoplasmic reticulum and the Golgi complex are sites for protein synthesis, processing, and sorting.
Endosomes: Endosomes carry and sort material brought into the cell.
Lysosomes: Lysosomes digest ingested material and unneeded cellular components.
Structure: These organelles contribute to forming the endomembrane system.
The Endoplasmic Reticulum (1 of 2)
Structure: The endoplasmic reticulum (ER) is a continuous network of flattened sacs, tubules, and vesicles through the cytoplasm of a eukaryotic cell.
Components: The membrane-bound sacs are called ER cisternae (singular: ER cisterna), and the space inside them is referred to as the ER lumen.
Functions of the ER
Protein Synthesis:
Enzymes associated with the ER are involved in the biosynthesis of proteins for:
Incorporation into the plasma membrane or into the organelles of the endomembrane system.
Export from the cell.
Lipid Synthesis: The ER is also involved in lipid synthesis.
Types of Endoplasmic Reticulum
Rough ER (RER):
Characterization: Characterized by ribosomes on the cytosolic side of the membrane.
Transitional Elements (TEs): A subdomain of rough ER that plays a role in the formation of transition vesicles, shuttling lipids and proteins from the ER to the Golgi complex.
Smooth ER (SER):
Characteristics: Lacks ribosomes and has roles in processing and storing nonproteins.
Distinguishing Rough and Smooth ER
Rough ER Structures:
Rough ER membranes form large flattened sheets.
Smooth ER Structures:
Smooth ER membranes form tubular structures.
Transitional elements of the rough ER resemble the smooth ER, contrary to the overall structure of the two.
Continuity: The lumenal spaces of rough and smooth ER are continuous.
Variation in ER
Presence: Both types of ER are present in most eukaryotic cells, but there's variation in relative amounts.
Prominence of Rough ER: Cells involved in the synthesis of secretory proteins have pronounced rough ER networks.
Extensive Smooth ER: Cells producing steroid hormones tend to have extensive networks of smooth ER.
Rough ER and Protein Biosynthesis (1 of 2)
Ribosome Function: Ribosomes on the cytosolic side of the rough ER membrane synthesize both membrane-bound and soluble proteins for the endomembrane system.
Synthesis Mechanism: Newly synthesized proteins are inserted into the endomembrane system through a pore complex as they are synthesized, a process called cotranslationally insertion.
Rough ER and Protein Biosynthesis (2 of 2)
Initial Steps: The rough ER is the site for:
The initial stages of adding carbohydrates to glycoproteins.
The folding of polypeptides.
Recognition and removal of misfolded proteins.
Assembly of multimeric proteins.
Rough ER and Quality Control
Misfolding Response: Proteins that are incorrectly folded, modified, or assembled are exported for degradation in cytosolic proteasomes.
Smooth ER Functions
Roles:
Involved in drug detoxification, carbohydrate metabolism, calcium storage, and steroid biosynthesis.
Primarily involved in processing or storing non-protein molecules within cells.
Drug Detoxification
Hydroxylation Process:
Adding hydroxyl groups to hydrophobic drugs increases their solubility, making them easier to excrete from the body.
Cytochrome P-450: Hydroxylation is catalyzed by cytochrome P-450 proteins, also known as monooxygenases.
Hydroxylation Mechanism
Electron Transport:
Electrons from NADPH or NADH are transferred to a heme group in cytochrome P-450.
Chemical Reaction:
Drug Tolerance
Phenobarbital Effect: Injection of phenobarbital into rats causes rapid increases in barbiturate detoxifying enzymes and proliferation of smooth ER, leading to drug tolerance.
Smooth ER in Drug Detoxification
Aryl Hydrocarbon Hydroxylase:
One cytochrome P-450 is part of a complex that metabolizes polycyclic hydrocarbons, producing more toxic byproducts, some linked to spontaneous cancers.
Effect of Cigarette Smoke: A potent inducer of aryl hydrocarbon hydroxylase.
Pharmacogenetics
Study Field: Investigates how inherited differences in genes like P-450 can cause variable responses to drugs and medications.
Carbohydrate Metabolism (1 of 2)
Liver Cell Involvement: Smooth ER in liver cells breaks down stored glycogen, containing glucose-6-phosphatase.
Biochemical Reaction:
Glucose-6-phosphate is hydrolyzed to yield free glucose.
Carbohydrate Metabolism (2 of 2)
Glycogen Storage: Liver stores glucose as glycogen in granules associated with smooth ER.
Release Mechanism: Glycogen is broken down by phosphorolysis, producing glucose-6-phosphate, which must be converted to free glucose for entry into the bloodstream.
Role of the Smooth ER in the Catabolism of Liver Glycogen (1 of 2)
Catabolism Insight: Figure 12.2 illustrates role of smooth ER in liver glycogen catabolism.
Role of the Smooth ER in the Catabolism of Liver Glycogen (2 of 2)
Biochemical Process: Smooth ER regulates breakdown of glycogen for energy release in liver cells.
Calcium Storage
Specialization: Sarcoplasmic reticulum in muscle cells specializes in calcium storage.
Calcium Dynamics: ER contains high concentrations of calcium-binding proteins; calcium ions are pumped into the ER via ATP-dependent calcium ATPases and released for muscle contraction.
Steroid Biosynthesis (1 of 2)
Functionality: Smooth ER synthesizes cholesterol and steroid hormones.
Cell Types: Large amounts found in cells synthesizing steroid hormones; also associated with chloroplasts in some plants.
Steroid Biosynthesis (2 of 2)
Cholesterol Relation: Cholesterol, cortisol, and steroid hormones share a four-ring structure with variations in carbon side chains and hydroxyl groups.
Enzymatic Step: Hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase) is the committed step in cholesterol biosynthesis and is targeted by statin drugs.
Central Role of the ER in Membrane Biosynthesis
Composition: The ER is the primary source of membrane lipids, with exceptions including mitochondria, peroxisomes, and chloroplasts.
Membrane Biosynthesis (1 of 2)
Mechanism: Fatty acids for membrane phospholipids are synthesized in the cytoplasm, incorporated into the ER membrane, and transferred to the lumenal side by phospholipid translocators (flippases).
Membrane Asymmetry: The type of phospholipid transferred depends on the translocator present, leading to membrane asymmetry.
Membrane Biosynthesis (2 of 2)
Transfer to Organelles: Distinct compositions of cytosolic and lumenal monolayers are established in the ER and transferred to other cellular membranes.
Phospholipid Exchange Proteins: Facilitate conveyance of specific phospholipids to mitochondria, chloroplasts, or peroxisomes.
Composition of the ER and Plasma Membranes of Rat Liver Cells
Weight Percentages:
Carbohydrate: 10% in both ER and plasma membranes.
Protein: 62% (ER), 54% (plasma).
Total Lipid: 27% (ER), 36% (plasma).
Lipid Types:
Major lipids such as phosphatidylcholine (40% ER, 24% plasma), phosphatidylethanolamine (17% ER, 7% plasma), and more, as shown in Table 12.1.
The Golgi Apparatus
Definition: Functionally and physically linked to the ER.
Role: Glycoproteins and membrane lipids from the ER undergo further processing and are sorted and packaged for transport.
Central Role: Plays a major role in membrane and protein trafficking in eukaryotic cells.
Structure of the Golgi Apparatus
Composition: Series of flattened membrane-bound cisternae.
Golgi Stack: A series of cisternae, usually three to eight, is called a Golgi stack.
Variation: Some cells have one large stack, while secretory cells may have hundreds or thousands of stacks.
Transport Vesicles
Surrounding Structures: Both ER and the Golgi complex are surrounded by numerous vesicles that transport lipids and proteins between them.
Intracisternal Space: Golgi complex lumen is part of the endomembrane system.
Two Faces of the Golgi Stack
Cis Face: Oriented toward the ER; compartment called the cis-Golgi network (CGN).
Trans Face: Opposite side of the stack, referred to as the trans-Golgi network (TGN).
Golgi Network
Vesicle Budding: Proteins and lipids leave the Golgi in transport vesicles that bud from the tips of the TGN.
Processing: Between CGN and TGN are medial cisternae, where most protein processing occurs.
Distinct Biochemical Environment: Each compartment contains specific receptor proteins unique to its section.
Models of Golgi Function
Stationary Cisternae Model:
Each cisterna is stable; transport of materials between cisternae is mediated by shuttle vesicles.
Cisternal Maturation Model:
Golgi cisternae are transient compartments, change from CGN to TGN with enzymes returned in vesicles.
Integration of Models
Experimental Evidence: Suggests both models are valid as they facilitate the movement of cargo targeted for various destinations.
Research Tools: Time-lapse fluorescence microscopy and tracking substances aid in understanding these processes.
Anterograde and Retrograde Transport (1 of 2)
Anterograde Transport: Movement toward the plasma membrane, involves secretory granules fusing with the membrane via exocytosis.
Cell membrane must balance the flow of lipids.
Anterograde and Retrograde Transport (2 of 2)
Retrograde Transport: Flow of vesicles from Golgi back to the ER, maintaining lipid balance and providing materials for new vesicle formation.
ER and Golgi in Protein Processing
Processes: Includes protein folding, quality control, and glycosylation (addition of carbohydrate side chains to form glycoproteins).
Glycosylation Types
N-linked Glycosylation: Addition of oligosaccharides to the nitrogen atom of specific asparagine residues.
O-linked Glycosylation: Addition of carbohydrates to the hydroxyl group of serine, threonine, or rarely tyrosine residues.