Comprehensive Study Notes on Protein Sorting and Intracellular Transport

Foundations of Protein Localization and Sorting

• Protein localization is essential for maintaining cellular structure, function, and homeostasis. It ensures proteins perform roles in correct compartments; a protein in the incorrect compartment can be functionally absent or harmful.

• Sorting preserves biochemical compatibility, membrane topology, and signaling fidelity. Clinical diseases often result from failures in specific trafficking steps.

• The central logistics problem: Synthesis $\rightarrow$ Recognition of a sorting signal $\rightarrow$ Transport $\rightarrow$ Arrival and Retention.

• A protein's amino-acid sequence contains the information determining its cellular destination. A normal catalytic domain cannot compensate for delivery to the wrong compartment.

• Sorting Signals:

  • Signal Sequence: A continuous amino-acid stretch, typically at the N- or C-terminus. It is often cleaved after reaching the destination.

  • Signal Patch: A three-dimensional surface formed after the protein folds, where residues may be distant in the linear sequence but come together in the tertiary structure.

Mechanisms of Intracellular Transport

• Internal compartments are categorized by topological relationships. Topologically equivalent spaces include the ER lumen, Golgi lumen, lysosomes, and the cell exterior.

• Gated Transport:

  • Involves selective passage through nuclear pore complexes (NPCs).

  • Occurs between topologically equivalent spaces (cytosol and nucleus).

  • Cargo may remain folded during transport.

  • Example: Transcription factors.

• Transmembrane Transport:

  • Proteins cross a membrane through a membrane-bound protein translocator.

  • Transports proteins from the cytosol into a space that is topologically distinct.

  • Cargo usually must unfold to snake through the translocator.

  • Examples: Movement into the endoplasmic reticulum (ER) and mitochondria.

• Vesicular Transport:

  • Membrane-enclosed carriers (vesicles) move cargo between compartments.

  • Cargo does not cross a lipid bilayer; instead, vesicles bud from a donor compartment and fuse with a target compartment.

  • Occurs only between topologically equivalent compartments.

  • Example: ER to Golgi transport.

Gated Transport: Nuclear-Cytoplasmic Logistics

• Nuclear Pore Complex (NPC):

  • A selective gateway spanning the inner and outer nuclear membranes.

  • Built from nucleoporins; FG-repeat domains form the selective barrier.

  • Passive Diffusion: Small molecules and small proteins diffuse down concentration gradients.

  • Receptor-Mediated Transport: Large cargo requires importins or exportins and specific localization signals.

  • Traffic stats: Up to $500$ macromolecules per second, including histones, DNA polymerase, RNA polymerase, transcription factors, and splicing factors.

• Nuclear Import Mechanism:

  • The Nuclear Localization Signal (NLS) is exposed on the cargo.

  • Importin binds the cargo; the complex crosses the NPC.

  • Ran-GTP binds the importin, causing the release of the cargo into the nucleus.

  • Directionality is maintained by a gradient: High Ran-GTP in the nucleus and high Ran-GDP in the cytosol.

• Nuclear Export Mechanism:

  • Nuclear Export Signal (NES) cargo in the nucleus binds a complex of Exportin and Ran-GTP.

  • The complex crosses the NPC to the cytosol.

  • GTP hydrolysis occurs, releasing the cargo.

  • Comparison: Import cargo dissociates when Ran-GTP binds importin, whereas export cargo forms a stable complex with exportin and Ran-GTP inside the nucleus.

• Clinical Case (Nuclear Localization Defect):

  • In a child with recurrent severe infections, a mutation in a transcription factor prevents nuclear localization.

  • Mechanism: Altered NLS or importin recognition leads to impaired entry; the protein remains in the cytosol and cannot activate immune-response genes.

Mitochondrial Protein Import

• Mitochondria Structure:

  • Outer membrane: Freely permeable to small molecules.

  • Inner membrane: Contains complexes for electron transport and oxidative phosphorylation.

  • Matrix: Contains citric acid cycle enzymes.

  • Site of ATP production and apoptosis trigger (via cytochrome c release).

• Targeting Principles:

  • Most mitochondrial proteins are encoded by nuclear genes and translated on free cytosolic ribosomes.

  • Matrix-targeting presequences are typically N-terminal, amphipathic, positively charged, and cleaved after import.

• Matrix Import Steps:

  • Precursor proteins are kept unfolded by cytosolic chaperones (Hsp70).

  • TOM (Translocator of the Outer Membrane) recognizes the signal and facilitates outer-membrane passage.

  • TIM23 (Translocator of the Inner Membrane) facilitates crossing of the inner membrane.

  • Energy inputs: $ATP$ supports chaperone cycles; inner-membrane potential ($\Delta\psi$) electrophoretically favors the entry of the positively charged sequence.

  • Matrix Hsp70 pulls the protein into the matrix; the signal is then cleaved.

• Clinical Case (Mitochondrial Dysfunction):

  • Mutations weakening matrix-targeting presequences affect high-energy tissues first, such as the brain and skeletal muscle.

  • Symptoms include exercise intolerance, neurologic findings, and lactic acidosis due to impaired oxidative phosphorylation.

The Endoplasmic Reticulum (ER) and Secretory Pathway

• Proteins entering the ER include soluble secretory proteins (hormones, antibodies), lumenal resident proteins, and membrane proteins.

• Targeting via SRP (Signal Recognition Particle):

  • The ER signal peptide (often a stretch of hydrophobic amino acids preceded by basic residues like arginine) emerges from the ribosome.

  • SRP binds the peptide and ribosome, briefly pausing translation.

  • SRP receptor docks the complex at the ER; the ribosome transfers to a translocon (Sec61 complex).

  • Translation resumes as the growing chain enters the ER (co-translational translocation).

• Protein Processing and Quality Control:

  • N-linked Glycosylation: A preassembled oligosaccharide is transferred to selected asparagine residues.

  • Folding: Chaperones and disulfide-bond formation assist folding.

  • Quality-Control Checkpoint: Correctly folded proteins exit the ER; misfolded proteins undergo retrotranslocation and ubiquitin-proteasome degradation (ERAD).

  • Unfolded Protein Response (UPR): Activated by persistent stress.

• Clinical Case (Cystic Fibrosis):

  • Caused by the $CFTR \Delta F508$ mutation.

  • The protein misfolds and is retained by ER quality control, then degraded by ERAD. Even if potentially functional, the protein fails to reach the plasma membrane.

Vesicular Transport: Coats, Rabs, and SNAREs

• The Vesicle Cycle: Cargo selection $\rightarrow$ Coat assembly $\rightarrow$ Budding and Scission $\rightarrow$ Uncoating $\rightarrow$ Tethering and Docking $\rightarrow$ SNARE-mediated fusion.

• Coat Proteins defining routes:

  • COPII: Forward (anterograde) route from ER to Golgi.

  • COPI: Retrograde retrieval from Golgi to ER and transport within Golgi.

  • Clathrin: Trans-Golgi to endosomes and plasma membrane to endosomes.

• Specificity and Fusion:

  • Rab GTPases: Provide compartment identity and guide vesicles to the correct target membrane. Rab-GTP on the vesicle binds tethers on the target membrane.

  • SNARE Proteins: v-SNAREs (vesicle) and t-SNAREs (target) pair specifically to form a trans-SNARE complex. Zippering pulls bilayers together, opening a fusion pore.

Golgi Processing and Lysosomal Sorting

• Retrieval of Resident Proteins:

  • KDEL: Soluble ER proteins that escape to the Golgi bind KDEL receptors and return via COPI carriers.

  • KKXX: ER membrane proteins recruit COPI for retrograde transport.

• Golgi Transformations:

  • Organized into cis-, medial-, and trans-Golgi, plus the trans-Golgi network (TGN).

  • Functions: Glycan trimming/addition, O-linked glycosylation, proteolytic processing, and sorting.

  • Medical Application (ABO Blood Groups): Group A (N-acetylgalactosamine added), Group B (galactose added), Group O (inactive transferase leaves H antigen unmodified).

• Lysosomal Sorting (M6P Pathway):

  • Hydrolases receive a Mannose-6-phosphate (M6P) tag in the Golgi via GlcNAc phosphotransferase.

  • M6P receptors in the TGN bind the cargo; clathrin carriers transport them to endosomes.

  • Low pH in the endosome releases the enzyme; the receptor recycles.

• Clinical Case (I-cell Disease):

  • Defective GlcNAc phosphotransferase results in no M6P tag.

  • Acid hydrolases are secreted instead of being sent to lysosomes. Diagnostic logic: High plasma levels of lysosomal enzymes.

Endocytosis and Secretion

• Exocytosis:

  • Constitutive Secretion: Continuous delivery to the plasma membrane.

  • Regulated Secretion: Specialized cells (e.g., insulin in $\beta$-cells) store cargo in granules, released following a signal like ${Ca}^{2+}$ entry after membrane depolarization.

• Endocytosis Types:

  • Phagocytosis: Large particles (microbes) taken up by professional phagocytes.

  • Pinocytosis: Continuous uptake of extracellular fluid via clathrin-coated pits.

  • Receptor-Mediated Endocytosis: Selective concentrate of cargo (e.g., LDL).

• Clinical Case (Familial Hypercholesterolemia):

  • LDL receptor is a single-pass glycoprotein ($840$ amino acids).

  • Defects in the LDL receptor or clathrin-mediated uptake lead to high plasma LDL ($320\,mg/dL$) and premature atherosclerosis.