Unit 4: Endoplasmic Reticulum, Golgi Complex and Lysosomes - Subjective Questions

Definition: The endoplasmic reticulum (ER) is an interconnected membrane-bound network of flattened sacs, tubules, and vesicles extending throughout the cytoplasm and continuous with the outer nuclear membrane.\n\nStructure and distribution:\n- The ER encloses a lumen called the cisternal space.\n- Its membrane is a phospholipid bilayer containing proteins involved in synthesis, transport, and signaling.\n- The ER is usually abundant in cells that synthesize large quantities of proteins or lipids.\n- It forms functional contacts with mitochondria, the Golgi complex, endosomes, and the plasma membrane.\n- The ER provides a major intracellular pathway for the movement of newly synthesized proteins and lipids.

Rough ER:\n- Has ribosomes attached to its cytosolic surface.\n- Consists mainly of flattened cisternae.\n- Synthesizes proteins destined for secretion, lysosomes, or cellular membranes.\n- Begins protein folding, quality control, and initial glycosylation.\n\nSmooth ER:\n- Lacks attached ribosomes.\n- Has a more tubular structure.\n- Produces lipids, phospholipids, steroids, and some membrane components.\n- Participates in detoxification, carbohydrate metabolism, and calcium-ion storage.\n\nRelationship: The rough and smooth regions are continuous parts of the same membrane system and can convert from one form to another according to cellular requirements.

The smooth ER performs several specialized functions:\n\n- Lipid synthesis: It synthesizes phospholipids, cholesterol, triglycerides, and steroid hormones.\n- Membrane production: Newly formed lipids are distributed to other membranes through vesicles or membrane-contact sites.\n- Detoxification: In liver cells, enzymes such as cytochrome P450 modify drugs and toxic substances to make them more soluble.\n- Calcium storage: In muscle cells, the specialized smooth ER called the sarcoplasmic reticulum stores and releases calcium ions required for contraction.\n- Carbohydrate metabolism: It contains enzymes involved in glycogen breakdown and glucose release, particularly in liver cells.\nThus, the smooth ER is structurally adapted for lipid metabolism, detoxification, ion storage, and metabolic regulation.

The signal hypothesis explains how proteins destined for the ER are selectively recognized and transported during translation:\n\n1. Translation begins on a free ribosome in the cytosol.\n2. An ER signal sequence, usually a hydrophobic segment near the amino terminus, emerges from the ribosome.\n3. Signal-recognition particle (SRP) binds the signal sequence and temporarily pauses translation.\n4. SRP guides the ribosome to an SRP receptor on the ER membrane.\n5. The ribosome is transferred to a protein-conducting channel called the translocon.\n6. Translation resumes, and the growing polypeptide enters the ER lumen or becomes inserted into the ER membrane.\n7. In many soluble proteins, the signal sequence is removed by signal peptidase.\n\nThis mechanism couples protein synthesis with translocation and prevents inappropriate accumulation of secretory proteins in the cytosol.

Membrane proteins are inserted into the ER through signal sequences and stop-transfer or start-transfer sequences.\n\n- A hydrophobic signal-anchor sequence can initiate translocation and remain in the membrane as a transmembrane segment.\n- A stop-transfer sequence halts movement of part of the polypeptide through the translocon and causes the sequence to exit laterally into the lipid bilayer.\n- Multiple start-transfer and stop-transfer sequences can produce multipass membrane proteins.\n- The orientation of a protein is determined by the distribution of positively charged residues and by the order of targeting sequences.\n- Regions synthesized after translocation may remain in the ER lumen, whereas other regions remain in the cytosol.\n- The inserted protein can undergo folding, glycosylation, disulfide-bond formation, and quality-control inspection in the ER.\n\nTherefore, the translocon acts both as a protein-conducting channel and as a lateral gate for membrane insertion.

Proteins enter the ER through a process called co-translational translocation. A cytosolic ribosome begins translation, and an ER signal sequence directs the ribosome to the ER membrane. The ribosome binds to a translocon, allowing the growing polypeptide to pass into the ER lumen or insert into the membrane.\n\nProteins using this pathway include:\n- Soluble proteins secreted outside the cell.\n- Proteins destined for the ER, Golgi complex, endosomes, or lysosomes.\n- Integral membrane proteins of the plasma membrane and internal membranes.\n- Some proteins that remain resident within the ER.\n\nProteins that function in the cytosol, nucleus, mitochondria, or peroxisomes generally use different targeting mechanisms and do not enter the ER pathway.

The ER provides an environment in which newly synthesized proteins acquire their correct structure and undergo early processing.\n\n- Molecular chaperones: Proteins such as BiP bind exposed hydrophobic regions and prevent incorrect aggregation.\n- Disulfide bonds: Protein disulfide isomerase promotes the formation and rearrangement of disulfide bonds in the oxidizing ER lumen.\n- Glycosylation: N-linked oligosaccharides are transferred to selected asparagine residues of many proteins.\n- Folding sensors: Calnexin and calreticulin help glycoproteins fold correctly.\n- Quality control: Correctly folded proteins are released for transport, whereas misfolded proteins are retained.\n- ER-associated degradation: Persistent misfolded proteins are moved back to the cytosol, ubiquitinated, and degraded by proteasomes.\n\nThese processes ensure that only properly assembled and functional proteins leave the ER.

The unfolded protein response (UPR) is a signaling pathway activated when misfolded or unfolded proteins accumulate in the ER lumen. It protects the cell by restoring protein-folding capacity and reducing the incoming workload.\n\n- Translation of many nonessential proteins is temporarily reduced.\n- Production of ER chaperones and folding enzymes is increased.\n- ER-associated degradation is enhanced to remove defective proteins.\n- Membrane production and ER expansion may increase the capacity of the organelle.\n- If stress is severe or prolonged, the UPR can activate programmed cell death.\n\nThus, the UPR maintains ER homeostasis by balancing protein synthesis, folding, transport, and degradation.

Proteins and lipids leave the ER in transport vesicles that bud from specialized ER exit sites.\n\n- Cargo proteins are concentrated by cargo receptors or sorting signals.\n- The cytosolic coat protein complex COPII assembles on the ER membrane.\n- Small GTPase activation helps recruit the coat and bend the membrane.\n- A budding vesicle pinches off and carries selected proteins and lipids toward the ER-Golgi intermediate compartment and Golgi complex.\n- Vesicle uncoating allows the vesicle to interact with the correct target membrane.\n- Rab GTPases, tethering proteins, and SNARE proteins provide specificity for docking and fusion.\n- Some ER-resident proteins that escape are returned by COPI-coated vesicles.\n\nThis selective vesicular transport maintains the identity of both the ER and Golgi compartments.

The Golgi complex is an organized stack of flattened membrane-bound sacs called cisternae, surrounded by associated tubules and vesicles. It has distinct polarity:\n\n- Cis face: The receiving side, located near the ER, accepts transport vesicles from the ER.\n- Medial cisternae: Intermediate compartments where many processing reactions occur.\n- Trans face: The exiting side, where proteins and lipids are sorted into different transport carriers.\n- Trans-Golgi network: A tubular region that distributes cargo to the plasma membrane, endosomes, lysosomes, or secretory vesicles.\n\nThe cisternae contain different enzymes in an ordered arrangement. Cargo may progress through the stack by cisternal maturation, while resident enzymes are recycled backward by transport vesicles.

Cisternal maturation model:\n- A cisterna forms at the cis face by fusion of incoming ER-derived vesicles.\n- It gradually matures from cis to medial to trans identity.\n- Cargo proteins remain within the maturing cisterna.\n- Golgi-resident enzymes move backward in COPI-coated vesicles to maintain their correct locations.\n\nVesicular transport model:\n- Golgi cisternae are considered relatively stable compartments.\n- Cargo moves forward from one cisterna to the next in transport vesicles.\n- Resident enzymes remain mainly within their corresponding cisternae.\n\nModern view: Evidence suggests that both mechanisms may operate, with cisternal maturation being especially important for large cargo that cannot easily fit into small transport vesicles.

Protein glycosylation in the Golgi is the enzymatic modification of carbohydrate chains attached to proteins.\n\n- Many proteins receive an initial N-linked oligosaccharide in the ER.\n- As the protein passes through Golgi cisternae, specific glycosidases remove selected sugars.\n- Glycosyltransferases add new sugars in a defined sequence.\n- The final carbohydrate structure depends on the enzymes present in each Golgi compartment and the time spent there.\n- O-linked glycosylation is initiated mainly in the Golgi by attachment of sugars to serine or threonine residues.\n- Glycosylation can influence protein folding, stability, activity, recognition, and intracellular targeting.\n\nThe Golgi therefore acts as a processing assembly line that produces diverse and cell-specific glycoprotein structures.

N-linked glycosylation:\n- Carbohydrate is attached to the nitrogen atom of an asparagine side chain, usually in the sequence Asn-X-Ser or Asn-X-Thr.\n- An oligosaccharide is transferred as a preassembled unit in the ER.\n- The carbohydrate chain is subsequently trimmed and modified in the Golgi.\n\nO-linked glycosylation:\n- Carbohydrate is attached to the oxygen atom of a serine or threonine side chain.\n- It is initiated mainly in the Golgi.\n- Sugars are generally added one at a time by specific glycosyltransferases.\n\nBoth forms can alter protein stability, folding, protection from degradation, and recognition by other molecules.

The trans-Golgi network sorts proteins according to their molecular signals and destination.\n\n- Lysosomal enzymes: They receive mannose-6-phosphate, which is recognized by mannose-6-phosphate receptors. These receptors direct the enzymes into clathrin-coated vesicles bound for endosomes.\n- Constitutive secretion: Some proteins and lipids are continuously delivered to the plasma membrane.\n- Regulated secretion: Other cargo is concentrated in secretory granules and released only after a stimulus, such as a rise in cytosolic calcium.\n- Endosomal delivery: Selected membrane proteins and soluble proteins are transported to endosomes.\n- Sorting machinery: Adaptins, clathrin, Rab proteins, tethering factors, and SNAREs ensure correct packaging, targeting, docking, and fusion.\n\nSorting at the trans-Golgi network ensures that cargo reaches the correct cellular or extracellular destination.

Coat proteins shape transport vesicles and help select their cargo:\n\n- COPII: Forms vesicles that transport newly synthesized proteins and lipids from the ER to the Golgi complex.\n- COPI: Mediates mainly retrograde transport from the Golgi to the ER and recycling between Golgi cisternae. It helps return escaped ER-resident proteins.\n- Clathrin: Forms vesicles from the trans-Golgi network and plasma membrane. It participates in transport to endosomes and in receptor-mediated endocytosis.\n\nCoat assembly is controlled by small GTPases and adaptor proteins. After budding, the coat is usually removed so that the vesicle can be targeted and fused with the appropriate membrane.

Lysosomes are membrane-bound organelles containing hydrolytic enzymes that digest cellular and extracellular materials.\n\nStructure:\n- They are surrounded by a single membrane.\n- Their lumen is acidic, commonly maintained near pH by proton pumps.\n- The membrane contains transport proteins and protective glycoproteins.\n- Their size and content vary according to cell type and digestive activity.\n\nFunctions:\n- Degradation of proteins, nucleic acids, carbohydrates, and lipids.\n- Breakdown of material taken up by endocytosis or phagocytosis.\n- Digestion of damaged organelles during autophagy.\n- Recycling of amino acids, sugars, nucleotides, and fatty acids.\n- Participation in membrane repair, signaling, and regulated cell death.\n\nThe acidic lumen activates lysosomal enzymes while limiting their activity in the cytosol if they escape.

Lysosomal enzymes are synthesized in the rough ER and transported to the Golgi complex, where they receive a specific targeting signal.\n\n1. In the cis-Golgi, selected lysosomal hydrolases are modified by addition of a phosphate group to mannose residues.\n2. The resulting mannose-6-phosphate marker is recognized by mannose-6-phosphate receptors in the trans-Golgi network.\n3. The receptor-enzyme complexes enter clathrin-coated transport vesicles.\n4. These vesicles fuse with early or late endosomes.\n5. The acidic endosomal environment causes the enzyme to dissociate from its receptor.\n6. The enzyme is delivered to the lysosome, where it becomes fully active.\n7. The mannose-6-phosphate receptor returns to the Golgi for reuse.\n\nThis pathway prevents lysosomal hydrolases from being secreted outside the cell.

Autophagy is a cellular recycling process in which cytoplasmic components, including damaged organelles and protein aggregates, are delivered to lysosomes for degradation.\n\n- During macroautophagy, a double-membrane structure called an isolation membrane surrounds the material.\n- The membrane closes to form an autophagosome.\n- The autophagosome fuses with a lysosome to form an autolysosome.\n- Lysosomal hydrolases digest the enclosed material.\n- The resulting amino acids, fatty acids, sugars, and nucleotides are transported back into the cytosol for reuse.\n\nAutophagy removes damaged structures, supplies nutrients during starvation, maintains organelle quality, and contributes to cellular adaptation and survival.

Role in phagocytosis:\n- Specialized cells such as macrophages and neutrophils engulf large particles, microorganisms, or cellular debris.\n- The engulfed material is enclosed in a phagosome.\n- The phagosome matures and fuses with a lysosome to form a phagolysosome.\n- Lysosomal enzymes and an acidic environment digest the contents.\n- Useful products are recycled, while indigestible residues may remain or be expelled.\n\nDifference from autophagy:\n- Phagocytosis removes material originating outside the cell.\n- Autophagy primarily removes and recycles material originating inside the cell.\n- Both processes depend on fusion with lysosomes and lysosomal hydrolytic enzymes.

Lysosomes maintain acidity through vacuolar-type proton ATPases located in their limiting membrane. These pumps use ATP to transport hydrogen ions from the cytosol into the lysosomal lumen. Counter-ion movement helps balance electrical charge and supports continued proton entry.\n\nThe acidic environment is important because:\n- It provides the optimum pH for lysosomal acid hydrolases.\n- It promotes the dissociation of cargo from some sorting receptors during endosomal transport.\n- It helps denature and destabilize macromolecules for digestion.\n- It protects the cytosol because most lysosomal enzymes function poorly at neutral cytosolic pH.\n\nIf acidification is disrupted, lysosomal digestion, autophagy, endosome maturation, and cellular recycling are impaired.