Lec 13 Cytoplasmic Membrane Systems

Cytoplasmic Membrane Systems — Comprehensive Study Notes

Overview: Key membrane trafficking themes

• The cell uses coordinated vesicle transport to move proteins and lipids between organelles, especially between the ER and Golgi, the Golgi network, endosomes, lysosomes, and the plasma membrane.
• Major processes include exocytosis (secretion), endocytosis (uptake from the plasma membrane), and the endocytic pathway that routes material to lysosomes or back to the cell surface.
• Vesicle transport is controlled by coats, adaptors, motors, tethering factors, and SNAREs; coating influences budding and uncoating, targeting determines destination, and fusion requires Rabs and SNAREs.
• Three well-understood sorting pathways at the trans-Golgi network (TGN) organize protein routing: constitutive secretory, regulated secretory, and the endocytic pathway via lysosome targeting (M6P-mediated).

Three best-understood protein sorting pathways at the TGN

• Exocytosis pathways
  • Constitutive secretory pathway: continuous, unregulated delivery of proteins and lipids to the plasma membrane or exterior; vesicles bud from the TGN and fuse with the plasma membrane immediately.
  • Regulated secretory pathway: secretory vesicles accumulate and fuse with the plasma membrane only in response to specific signals (e.g., Ca2+). Often involves maturation processes such as condensation and proteolytic processing.
  • Secretory granules/secretory vesicles: storage forms of regulated secretory products (e.g., digestive enzymes, zymogens, insulin in pancreatic cells).
  • Polarized exocytosis: secretion targeted to a specific plasma membrane domain (e.g., neurotransmitter release at synapses, digestive enzyme release into the intestinal lumen).
  • Key idea: sorting signals and maturation steps determine whether cargo follows constitutive or regulated routes after TGN sorting.
• Endocytosis pathway
  • Endocytosis (internalization) moves material from the plasma membrane into the cell, beginning the endocytic pathway.
  • Internalized cargo enters early endosomes and may be sorted for recycling, degradation, or further trafficking along the pathway toward late endosomes and lysosomes.
• Secretory and endocytic integration at the TGN
  • Maturation and routing decisions depend on signals and receptor–cargo interactions; the M6P receptor (see below) exemplifies how lysosomal enzyme sorting couples with receptor recycling.
  • The Golgi complex, trans-Golgi network, and cis/trans cisternae coordinate cargo sorting via a network that includes cis, medial, and trans compartments, with cargo traffic driven by coat proteins (Clathrin, COPI, COPII).

Mannose-6-phosphate (M6P) traffic to lysosomes: sorting and receptor cycling

• Lysosomal hydrolases receive the M6P tag added in the cis-Golgi; this is the “postal code” for lysosomal delivery.
• Sorting steps (M6P pathway)
  • Lysosomal hydrolases acquire the M6P tag in the cis-Golgi:
  • Enzyme: addition of a mannose-6-phosphate group to lysosomal hydrolases.
  • Enzyme involved in tagging: phosphotransferase adds P-GlcNAc as a recognition tag; later processing yields the final M6P signal.
  • The M6P receptor in the trans-Golgi recognizes hydrolases bearing the M6P tag and directs them into clathrin-coated vesicles destined for late endosomes.
  • The M6P receptor–ligand complex travels in clathrin-coated vesicles to late endosomes.
  • In the late endosome, acidic pH prompts dissociation of ligand from the receptor. This involves:
  • Uncovering of H+ (protonation effects) and removal of the phosphate group from M6P signaling, leading to release of hydrolase from the receptor.
  • Receptor retrieval: after ligand release, the receptor is retrieved back to the Golgi by a retromer coat for reuse (receptor recycling).
• The endosome-to-Golgi retrieval route ensures lysosomal enzymes reach their target while receptors are recycled for continued use.
• Consequences and significance:
  • This pathway ensures lysosomal enzymes are delivered to lysosomes for degradation and recycling of the receptor maintains efficient cargo sorting.
  • Defects in this system lead to lysosomal storage diseases among others.
• Visual cues from the lectures (conceptual): M6P receptor binds in the trans-Golgi, follows clathrin-coated transport to late endosome, release occurs in acidic endosomal lumen, receptor retrieved by retromer for reuse, while hydrolases proceed to lysosomes.

Lysosomal storage diseases (examples)

• Table highlights key diseases and substrates that accumulate when lysosomal function is impaired:
  • Sphingolipidosis: GM, β-galactosidase deficiency → accumulation of GM/glycolipids; example: Tay-Sachs disease (GM2).
  • Gaucher's disease: β-glucocerebrosidase deficiency → accumulation of glucosylceramide.
  • Niemann-Pick disease: sphingomyelinase deficiency → sphingomyelin accumulation.
  • Metachromatic leukodystrophy: arylsulfatase A deficiency → sulfated lipids (sulfatides) accumulate.
  • Inclusion cell disease (I-cell disease, mucolipidosis II): multiple lysosomal enzymes absent from lysosomes → accumulation of various glycolipids, glycoproteins, and sialyloligosaccharides.
• Source: Table 16.4 (The Human Perspective pages 342-343).

Endocytosis and the endocytic pathway: overview and types

• Endocytosis vs exocytosis balance
  • The steady-state plasma membrane composition results from a balance between exocytosis (adding material) and endocytosis (removing material).
• Endocytic process types
  • Bulk-Phase Endocytosis (fluid-phase, clathrin-independent): non-specific uptake of extracellular fluid; material internalized reflects extracellular composition; relatively constant rate; compensates for membrane turnover.
  • Receptor-Mediated Endocytosis (clathrin-dependent): selective uptake of ligands via receptors; high specificity; used for hormones, growth factors, enzymes, cholesterol (LDL), antibodies, etc.
  • Phagocytosis: ingestion of large particles (>500 nm), such as cells or debris; prominent in certain unicellular organisms and immune cells (e.g., neutrophils, macrophages).
• Endocytic pathway organization
  • Endocytic vesicle: budding vesicles from the plasma membrane due to endocytosis.
  • Early endosome: primary sorting station; vesicles fuse with early endosome; cargo is recycled to membrane or sent toward degradation.
  • Late endosome: contains full complement of acid hydrolases but lumen not yet fully acidic; site of intraluminal vesicle formation and receptor downregulation.
  • Lysosome: digestive organelle with hydrolases and acidic pH; final degradation site.
  • Maturation routes: two routes exist for late endosome maturation into lysosome – (1) ATPase-driven acidification to pH 4.0–5.0 activates hydrolases; (2) fusion with an existing lysosome.
• Receptor recycling and degradation (LDL example)
  • Housekeeping receptors (e.g., LDL receptor) internalize material for cellular use and recycle back to the surface.
  • Signaling receptors (e.g., insulin, growth factor receptors) are often downregulated after endocytosis, reducing sensitivity to further stimulation.

Clathrin-coated vesicles: structure and mechanism

• Formation and cargo selection
  • Specific ligand–receptor complexes bind to receptors on the plasma membrane.
  • Receptors cluster in coated pits, sites enriched in receptors (10–20× the level in the surrounding membrane).
  • Accumulation triggers recruitment of clathrin triskelions via adaptor proteins; adaptor AP2 promotes coat assembly and receptor recruitment.
  • Phosphoinositide PI(4,5)P2 binding to AP2 alters conformation and exposes cargo-binding sites.
• Coat assembly and cargo selection diversity
  • Various adaptors partner with clathrin to support vesicles budding from the plasma membrane or the trans-Golgi network; multiple accessory proteins participate (two dozen or more).
• Vesicle fission and uncoating
  • Dynamin, a cytosolic GTPase, wraps around the neck of the budding vesicle and, upon GTP hydrolysis, constricts to sever the vesicle from the plasma membrane.
  • COPII-coated vesicles bud from the ER, while COPI vesicles mediate Golgi-to-ER retrograde transport; clathrin-coated vesicles can bud from the plasma membrane or the trans-Golgi network.
• Key terms
  • Adaptor Protein 2 (AP2): promotes assembly of clathrin cage and recruitment of receptors to the budding vesicle.
  • PI(4,5)P2: lipid that regulates AP2 conformation and cargo binding.
  • Dynamin: GTPase required for vesicle scission; mutations can block fission (e.g., GTPγS mutant blocks GTP hydrolysis).
• Difference from COPII vesicles
  • Clathrin-coated vesicles rely on clathrin and adaptors; COPII vesicles rely on coat proteins such as Sec23/24 and Sec13/31 without clathrin involvement.

Receptor-mediated endocytosis of LDL: a detailed example

• LDL particle structure
  • LDL consists of an amphipathic shell (phospholipid monolayer, cholesterol, ApoB) and a hydrophobic core rich in cholesterol esters and triglycerides.
• LDL receptor and NPXY sorting signal
  • Familial hypercholesterolemia (FH) study revealed multiple LDL receptor mutations causing high plasma LDL and early cardiovascular disease.
  • A crucial motif in the cytosolic domain is NPXY (Asn-Pro-X-Tyr); this sorting signal binds to the AP2 complex to drive clathrin-coated pit formation.
• Uptake steps (LDL endocytosis pathway)
  1) LDL particles bind to LDL receptors at the cell surface; NPXY motif in the receptor cytosolic tail interacts with AP2, promoting endocytosis.
  2) Clathrin-coated pits invaginate and are pinched off by dynamin to form coated vesicles.
  3) After uncoating, vesicles fuse with late endosomes; LDL is delivered toward lysosomes.
  4) Late endosome fuses with lysosome where LDL is degraded; receptors recycle to the surface for reuse.
• pH-dependent ligand release and receptor recycling
  • In the late endosome, acidic pH triggers release of LDL from the receptor via protonation events in the b-propeller domain and interactions with the ligand-binding arm.
  • Histidine residues protonate at low pH, altering affinity and enabling disassociation; receptor returns to the plasma membrane for another round of uptake.
• Implication for disease and biology
  • The LDL receptor–mediated endocytosis pathway illustrates how receptor signaling and cargo uptake are coupled to receptor recycling and lysosomal degradation of cargo.

Endocytosis: receptor vs housekeeping receptors and signaling receptors

• Housekeeping receptors (e.g., LDL receptor)
  • Mediate uptake of essential materials (e.g., iron via transferrin receptor, cholesterol via LDL receptor).
  • Receptors deliver cargo to early endosomes, where low-affinity interactions at higher acidity release cargo; receptor then recycles to the surface.
• Signaling receptors
  • Bind extracellular messengers (insulin, growth factors) and alter intracellular signaling; often downregulated upon endocytosis to reduce sensitivity to further stimuli.
  • Downregulation serves as a mechanism for adjusting cell responsiveness.

Endocytic pathway diagrams and concept maps (core components)

• Endocytic vesicle: budding vesicle from the plasma membrane due to bulk-phase or receptor-mediated endocytosis.
• Early endosome: primary sorting station; cargo either recycled to membrane or targeted for degradation.
• Late endosome: contains acid hydrolases; lumen becomes acidic; site of intraluminal vesicle formation for receptor downregulation.
• Lysosome: digestive organelle with hydrolases functioning at low pH to degrade cargo.
• The endocytic pathway intersects with the Golgi network and lysosome through retrograde transport and receptor recycling mechanisms.
• Core concept: maturation from early to late endosome involves acidification and cargo sorting; fusion events with lysosomes complete degradation.

Polarized and general secretory pathways: practical distinctions

• Constitutive secretion
  • Continuous, unregulated delivery of proteins to the plasma membrane or exterior; not restricted to specific stimuli.
  • Historically viewed as the default for proteins synthesized in the rough ER, but evidence suggests some sorting requirements exist to limit constitutive loss of cargo.
• Regulated secretion
  • Cargo stored in mature secretory vesicles until a specific signal triggers fusion with the plasma membrane (e.g., Ca2+ signals, hormonal cues).
  • Examples include zymogen and insulin release from pancreatic cells.
  • Maturation steps include condensation (concentration) and sometimes proteolytic processing of cargo.
• Polarized secretion
  • Cargo delivered to a specific domain of the plasma membrane (apical vs basolateral) to create functional asymmetry (e.g., neurotransmitter release at synapses, digestive enzymes released to intestinal lumen).
• Key takeaway: cargo routing after the TGN determines whether a protein participates in constitutive, regulated, or polarized secretion.

Connections to foundational principles and real-world relevance

• Coat proteins and adaptors shape vesicle formation and cargo selection, illustrating how molecular recognition guides organelle biogenesis.
• The M6P sorting system connects ER/Golgi processing with lysosomal degradation, highlighting quality control and lysosome biogenesis as essential cellular housekeeping.
• Endocytosis and receptor recycling regulate cellular responsiveness to external signals, linking membrane traffic to metabolic control and signaling networks.
• Defects in vesicle trafficking are linked to human disease (e.g., lysosomal storage diseases, FH with LDL receptor defects), underscoring the clinical significance of membrane transport biology.

Key quantitative and terminological notes (LaTeX-ready)

• Clathrin-coated vesicle size context: typical vesicle diameters on the order of tens to ~100 nm; scale bar example: 0.1 μm is used in electron micrographs ($0.1 \,\mu\mathrm{m}$).
• Adaptor protein complex AP2 involvement in clathrin coat assembly and cargo selection; clathrin lattice forms a polyhedral cage around budding vesicles.
• Receptor-mediated endocytosis parameters:
  • Local receptor concentration in coated pits can be ~10–20× higher than surrounding membrane regions; high local concentration promotes efficient vesicle budding.
  • Entry sites and coat dynamics are regulated by PI(4,5)P2 binding and conformational changes in AP2.
• Endosomal pH and maturation:
  • Late endosomes achieve pH around $pH \,\approx\, 4.0\text{-}5.0$, sufficient to activate lysosomal hydrolases and facilitate receptor-ligand dissociation.

Summary: core principles to memorize

• Vesicle coats (clathrin, COPI, COPII) direct budding, cargo selection, and trafficking routes; adaptors like AP2 and coats modulate receptor recruitment and vesicle formation.
• The M6P tagging system targets lysosomal hydrolases to lysosomes; M6P receptor binds cargo in the TGN, traffics to endosomes, releases cargo in acidic environments, and recycles.
• Endocytosis includes bulk-phase (non-specific), receptor-mediated (specific, clathrin-dependent), and phagocytosis (large particle uptake); early and late endosomes coordinate cargo sorting and degradation with lysosomes.
• LDL receptor-mediated endocytosis is a key model: NPXY motif drives clathrin-mediated uptake; receptor recycling and pH-dependent ligand release ensure efficient cholesterol uptake.
• Secretion pathways are constitutive, regulated, or polarized; trafficking decisions influence secretion timing, site, and cargo processing.
• Lysosomal storage diseases illustrate the clinical consequences of trafficking and enzymatic deficiencies.

Quick reference: glossary of core terms

• M6P receptor: receptor recognizing mannose-6-phosphate on lysosomal enzymes in the Golgi to route them to lysosomes.
• NPXY sorting signal: cytosolic motif in LDL receptor that binds AP2 for endocytosis.
• Retromer: coat complex that retrieves receptors from endosomes back to the Golgi.
• Dynamin: GTPase essential for pinching off vesicles during clathrin-mediated endocytosis.
• AP2: adaptor protein complex that recruits clathrin and cargo receptors to budding pits.
• PI(4,5)P2: phosphoinositide that regulates adaptor conformation and cargo binding.
• Rab/SNAREs: small GTPases and membrane fusion proteins that guide vesicle targeting and fusion.
• pH 4.0–5.0: endosomal/lysosomal lumen pH range necessary for hydrolase activity and receptor–ligand dissociation.

End of notes