Comprehensive Study Guide for Intracellular Transport and Organelle Trafficking

Intracellular Transport and Membrane Compartmentalization

• Eukaryotic Cell Structure: The eukaryotic cell is subdivided into membrane-enclosed compartments that are functionally distinct. These compartments occupy nearly half the volume of the cell.
• Organelle Composition: Each organelle or compartment contains:
  • Its own characteristic set of enzymes.
  • Specialized molecules.
  • Complex distribution systems to transport specific products between compartments.
• Role of Proteins in Compartments:
  • Proteins provide the characteristic structural and functional properties of each compartment.
  • They catalyze specific reactions occurring within the organelle.
  • They selectively transport small molecules across the compartment membranes.
  • They serve as organelle-specific surface markers, directing the delivery of new proteins and lipids to the correct destination.
• Membrane Abundance and Regulation:
  • A large amount of intracellular membrane is required to form these compartments.
  • The shape and abundance of organelles are regulated based on the cell's metabolic needs.
  • Specific Examples of Membrane Surface Area:
    • Liver cells: The Endoplasmic Reticulum (ER) membrane surface area is $25$ times that of the plasma membrane.
    • Pancreatic cells: The ER membrane surface area is $12$ times that of the plasma membrane.
  • In most eukaryotic cells, the plasma membrane is considered a minor membrane in terms of total mass and area because internal organelles are packed so tightly.

Evolutionary Pathway of Eukaryotic Cells

• Step 1: Loss of Rigidity: An ancient anaerobic archaeon loses its rigid cell wall, facilitating horizontal gene transfers.
• Step 2: Phagocytosis: The cell begins phagocytosis and digestion of other prokaryotes (both archaeal and bacterial), which increases horizontal gene transfer and accelerates evolution.
• Step 3: Genome Protection: Membranes increasingly enclose the archaeon's chromosome to protect it, leading to the development of a nuclear envelope.
• Step 4: Endosymbiosis: An aerobic bacterium is taken up intact. It lives symbiotically as a promitochondrion.
• Step 5: Membrane Expansion: The development of multiple mitochondria provides the energy necessary for the evolution of additional membrane systems (ER, Golgi) and larger cell sizes.
• Result: The first eukaryotic cells are aerobic, containing a nucleus, cytosol, ER, and mitochondria.

Fundamentals of Protein Trafficking

• Protein Population: An animal cell contains approximately $10$ billion ($10^{10}$) protein molecules consisting of about $10,000$ different kinds.
• Site of Synthesis: Synthesis of almost all proteins begins in the cytosol (specifically on the rough ER), which is the space within the cytoplasm outside the membrane-enclosed organelles.
• Trafficking Modes: Proteins move between compartments through three fundamental ways:
  1. Gated Transport: Movement of proteins and RNA between the cytosol and nucleus through selective gates called nuclear pore complexes (NPCs) in the nuclear envelope.
  2. Protein Translocation: Transmembrane protein translocators directly move specific proteins from the cytosol into distinct organelles (e.g., ER lumen, mitochondria). These proteins usually cross the membrane as unfolded chains.
  3. Vesicular Transport: Transport vesicles or larger membrane fragments bud and pinch off from one organelle and discharge cargo into a second compartment (e.g., soluble proteins moving from ER to Golgi).
• Sorting Signals: These are specific sequences of amino acids ($15-60$ residues long) that direct proteins to their target location.

Transport Between the Nucleus and Cytosol

• Bidirectional Traffic:
  • Import: Histone proteins, DNA polymerases, RNA polymerases, transcriptional regulators, and RNA-processing proteins are imported from the cytosol into the nucleus.
  • Export: All RNAs (mRNA, rRNA, etc.) are synthesized in the nucleus and exported to the cytosol.
• Nuclear Envelope Structure:
  • Consists of two concentric membranes perforated by nuclear pores.
  • Inner Nuclear Membrane: Contains proteins serving as binding sites for chromosomes and the nuclear lamina.
  • Outer Nuclear Membrane: Continuous with the ER membrane and is studded with ribosomes.
  • Perinuclear Space: The $40\,nm$ wide space between the inner and outer membranes, continuous with the ER lumen.
• Nuclear Pore Complexes (NPCs):
  • Symmetry and Composition: Octagonal openings approximately $120\,nm$ in size.
  • Nucleoporins: Constructed from about $30$ different proteins, totaling $500-1000$ protein molecules per fully assembled NPC.
  • Filaments: Filamentous fibers ($3-5\,nm$) extend from both the cytoplasmic and nuclear sides. The nuclear side features a "nuclear basket."
  • Quantity: A typical mammalian cell contains $3000-4000$ NPCs. Numbers vary from a few hundred in glial cells to $20,000$ in Purkinje neurons. Oocytes have $70$ pores per $(\mu m)^2$, covering $30\%$ of the membrane.
  • Capacity: Each NPC can transport up to $1000$ macromolecules per second in both directions simultaneously.
• Diffusion Limits:
  • Molecules small enough ($\le 5000$ daltons) diffuse rapidly.
  • Proteins larger than $60,000$ daltons cannot enter by passive diffusion and require active transport.
  • Mature ribosomes ($30\,nm$ diameter) require active ferrying via receptor proteins.

The Nuclear Lamina and Related Diseases

• Structure: A thin filamentous meshwork ($10-20\,nm$ thick) on the inner surface of the nuclear envelope.
• Function: Provides mechanical support and serves as a chromatin attachment site.
• Composition: Composed of polypeptides called lamins ($10\,nm$ diameter). Types include Lamin A (LMNA), Lamin B (LMNB), and Lamin C (LMNC).
• Regulation: Phosphorylation of lamins causes the disassembly of the lamina prior to mitosis.
• Clinical Implications of LMNA Mutations:
  • Muscular Dystrophy (EDMD 2): Causes fragile nuclei in muscle cells.
  • Hutchinson-Gilford Progeria Syndrome (HGPS): Characterized by premature aging and death from heart attack or stroke in teenage years.

Mechanisms of Nuclear Import and Export

• Energy Requirements: Molecules larger than $10\,nm$ require energy (facilitated by ATPase and GTPase).
• Sorting Signals:
  • Nuclear Localization Signal (NLS): Short sequences rich in positively charged amino acids (Lysine and Arginine) that enable entry into the nucleus.
  • Nuclear Export Signal (NES): Directs the export of large molecules like ribosomal subunits and RNA-protein complexes.
• Receptors:
  • Importins: Nuclear import receptors.
  • Exportins: Nuclear export receptors.
• The Ran GTPase Cycle: Imposes directionality on transport.
  • Ran States:
    • Ran-GTP: High concentration in the nucleus due to Ran-GEF (guanine exchange factor) anchored to chromatin.
    • Ran-GDP: High concentration in the cytosol due to Ran-GAP (GTPase-activating protein).
  • Import Process: An import receptor binds cargo and enters the nucleus. Ran-GTP binds to the receptor, causing cargo release. The Ran-GTP-receptor complex exits the nucleus, where Ran-GAP triggers GTP hydrolysis, releasing the receptor.
  • Export Process: Ran-GTP promotes cargo binding to the export receptor in the nucleus. Once in the cytosol, Ran-GAP triggers hydrolysis, causing the release of cargo and Ran-GDP.
• Shuttling Proteins: Proteins with both NLS and NES shuttle back and forth; their localization depends on the relative rates of import versus export.

Protein Transport into Mitochondria

• Mitochondrial Properties: Semi-autonomous, spherical/elongated ($1-2\,μm$ long, $0.5\,μm$ wide). Present in all eukaryotic cells except mature RBCs.
• Genome: Contains its own DNA and ribosomes, but most mitochondrial proteins (95%) are encoded in the nucleus and imported.
• Mitochondrial Membranes:
  • Outer Membrane: Freely permeable to molecules up to $5000$ daltons due to porins.
  • Inner Membrane: Highly folded (cristae), contains electron-transport chain proteins, and is impermeable to most ions.
• Protein Translocators:
  1. TOM Complex: Translocator of the Outer Membrane; required for import of all nucleus-encoded proteins.
  2. SAM Complex: Sorting and Assembly Machinery; helps fold and insert $\beta$-barrel proteins (like porins) into the outer membrane.
  3. TIM23 Complex: Transports soluble proteins into the matrix and inserts some transmembrane proteins into the inner membrane.
  4. TIM22 Complex: Mediates insertion of a subclass of inner membrane proteins (e.g., ADP, ATP, and phosphate transporters).
  5. OXA Complex: Inserts inner membrane proteins synthesized inside the mitochondria or imported proteins initially sent to the matrix.
• Import Mechanism:
  • Proteins are kept unfolded in the cytosol by Hsp70 chaperones.
  • Signal sequence ($18$ AA residues forming an amphiphilic $\alpha$ helix) is recognized by TOM receptors.
  • Energy Sources: ATP hydrolysis (to release cytosolic Hsp70 and power the mitochondrial Hsp70 "motor") and the membrane potential (electrochemical gradient of $H^+$).

Metabolite Transport in Mitochondria

• Inner Membrane Transporters:
  1. ADP/ATP Exchange Carrier: Antiporter that exports ATP and imports ADP, driven by membrane potential.
  2. Phosphate Transporter: Antiporter that imports phosphate while exporting $OH^-$ ions, driven by the $H^+$ gradient.
  3. Pyruvate Carrier: Facilitated diffusion driven by concentration gradients.
  4. Dicarboxylate Carrier: Exchanges citrate/malate or other TCA intermediates.
  5. Fatty Acid Carrier: Specialized carnitine carrier system for activated fatty acids (acyl CoA).

Protein Transport into Chloroplasts

• Chloroplast Structure: Semi-autonomous, up to $10\,μm$ long. Contains a double membrane plus a third internal membrane system called thylakoids.
• Translocators:
  • TOC Complex: Translocon at the outer envelope membrane.
  • TIC Complex: Translocon at the inner envelope membrane.
• Thylakoid Transport Routes: Once in the stroma, proteins reach the thylakoid via four routes:
  1. Sec Pathway: Uses homologs of bacterial Sec proteins.
  2. SRP-like Pathway: Uses a chloroplast homolog of the signal-recognition particle.
  3. TAT (Twin Arginine Translocation) Pathway: Driven by the $H^+$ gradient across the thylakoid membrane; involves two critical arginines in the signal sequence.
  4. Spontaneous Insertion: Requires no specialized protein translocator.
• Chloroplast Metabolite Transport:
  • The Phosphate Exchange Carrier is the most abundant inner membrane protein ($12\%$), exchanging inorganic phosphate for photosynthetic products like 3PGA (glyceraldehyde-3-phosphate).
  • Glycolate Carrier: Transports glycolate (produced by Rubisco) to peroxisomes for photorespiration.

Peroxisomes

• Characteristics: Single-membrane enclosed, lack DNA and ribosomes. All proteins are nucleus-encoded.
• Key Enzymes:
  • Urate Oxidase.
  • Catalase: Uses hydrogen peroxide ($H_2O_2$) to oxidize substrates like alcohol. About $25\%$ of ethanol is oxidized to acetaldehyde in the liver via this peroxidation reaction.
  • Oxidation Reaction: $RH_2 + O_2 \rightarrow R + H_2O_2$
  • Catalase Detoxification: $2H_2O_2 \rightarrow 2H_2O + O_2$
• Protein Import:
  • Signal: C-terminal sequence (Ser-Lys-Leu) or an N-terminal sequence.
  • Receptor: Pex5 (peroxin) binds the C-terminal signal in the cytosol and accompanies the cargo into the lumen.
  • Mechanism: Driven by ATP hydrolysis. Unlike other organelles, peroxisomes can import fully folded oligomeric proteins. The translocator pore is "dynamic" in size.
  • Ubiquitylation: Pex5 undergoes ubiquitylation to be released back into the cytosol.

Endoplasmic Reticulum (ER)

• Structure: Network of branching tubules and flattened sacs (cisternae) enclosing the ER lumen. Its membrane is continuous with the outer nuclear membrane.
• Functions: Lipid and protein biosynthesis, and serves as an intracellular $Ca^{2+}$ store.
• Translocation Types:
  1. Co-translational Translocation: Ribosomes bind to the ER membrane during protein synthesis. Most common for transmembrane and water-soluble proteins.
  2. Post-translational Translocation: Synthesis is completed on cytosolic ribosomes before translocation occurs.
• SRP Mechanism:
  • Signal Recognition Particle (SRP): Binds the ER signal sequence as it emerges from the ribosome, causing a pause in translation.
  • SRP Receptor: Located in the ER membrane; binds the SRP-ribosome complex.
  • Translocator: Once the ribosome is engaged with the translocator, the SRP is released, and translation continues as the polypeptide is threaded into the lumen.
  • Signal Peptidase: Associated with the translocator, it clips off the signal sequence during translation.