AE

Comprehensive Notes: Cellular Membrane Structure, Trafficking, and Cytoskeletal Organization

Major Topics

  • Phospholipid bilayers and phospholipid biochemistry
  • Composition of the plasma membrane: lipids and proteins
  • Structures, functions, and synthesis of membrane proteins
  • Organization and membrane structure of major intracellular organelles: nucleus, mitochondria, lysosomes
  • Structure and function of the cytoskeleton: intermediate filaments, microtubules, actin thin filaments, myosin thick filaments
  • Roles of the endoplasmic reticulum and Golgi apparatus in synthesis and trafficking of membrane and their constituent proteins
  • Mechanisms of exocytosis and endocytosis
  • Special organizational requirements of polarized epithelial tissues

Structure of Lipids and Bilayers

  • A phosphatidylethanolamine and B phospholipid icon are used in the text to represent phospholipid molecules
  • Monolayer vs bilayer formation
    • In an aqueous environment, polar hydrophilic head groups orient toward water
    • Nonpolar hydrophobic tails orient away from water
    • Result: formation of a phospholipid bilayer
  • Components shown in structural figures include glycerol backbone, phosphate group, fatty acid tails, and headgroups (e.g., ethanolamine)
  • Denotation and labeling in figures: C MONOLAYER, D PHOSPHOLIPID BILAYER
  • Diagrammatic note: RI, R2, and hydrophobic vs hydrophilic regions illustrate bilayer assembly

Major Types of Phospholipids, Sphingolipids, and Cholesterol

  • Major types of phospholipids and other membrane lipids (including sphingolipids and cholesterol)
  • Important roles for phospholipid headgroups in determining electrostatic surface charge of biological membranes (negatively vs positively charged headgroups)

Lipid Mobility and Asymmetry

  • Mobility of phospholipids and cholesterol within membrane lipid bilayers
  • Plasma membrane maintains asymmetric phospholipid composition in each leaflet
    • Example: phosphatidylcholine (PC) enriched in outer (extracellular) leaflet
    • Phosphatidylserine (PS) enriched in inner (intracellular) leaflet

Structure, Function, and Synthesis of Membrane Proteins (Overview)

  • Peripheral versus integral (intrinsic) membrane proteins
  • Four major structural subtypes of integral membrane proteins
  • Five major functions of membrane proteins:
    • Receptors
    • Adhesion proteins (2 types)
    • Transport proteins
    • Enzymes
  • Synthesis of membrane proteins in the endoplasmic reticulum (ER)
  • Maturation and “trafficking” of membrane proteins in the Golgi apparatus

Peripheral vs Integral Membrane Proteins

  • Peripheral proteins are located in the extracellular space and are noncovalently bonded with integral proteins
  • Most integral membrane proteins have membrane-spanning α-helical domains of about 20 amino acids
  • Some integral proteins have multiple membrane-spanning domains
  • Some proteins are linked to membrane phospholipids via an oligosaccharide (glycophosphatidylinositol anchor, GPI)
  • Some proteins are linked directly to fatty acids or prenyl groups
  • Role of transmembrane-spanning domains: usually α-helices but sometimes β-sheets

Functions of Plasma Membrane Proteins: Receptors and Adhesion Molecules

  • Receptors: bind signaling molecules and trigger intracellular responses
  • Adhesion molecules: two subtypes
    • Bind to extracellular matrix
    • Bind to adjacent cells (via adhesion proteins)

Transport Functions of Membrane Proteins

  • Major role: transport ions, metabolites, and cellular waste products
  • Three subtypes of membrane transport proteins:
    • Channels
    • Carriers
    • ATP-driven ion pumps

Interaction with the Submembrane Cytoskeleton

  • Membrane protein–cytoskeleton interactions regulate mobility and localization of membrane proteins

Nuclear, Mitochondrial, and Lysosomal Membrane Organization

  • Nucleus and mitochondria have double membrane bilayers
  • Nuclear pores facilitate transport of RNA and bidirectional transport of soluble proteins between cytosol and nucleus
  • Mitochondria generate and maintain a large proton gradient (inside basic) for ATP synthesis
  • Lysosomes generate and maintain a large proton gradient (inside acidic) to degrade proteins and other macromolecules

Organization of Nuclear vs Mitochondrial Membranes (Illustrative Elements)

  • Smooth endoplasmic reticulum (ER) and Rough ER
  • Intermembrane space (mitochondria)
  • Animal cell components: centrioles, mitochondrion (outer and inner membranes), matrix, ribosomes, Golgi apparatus, lysosomes (endosomes, peroxisomes, transport vesicles), nucleolus, chromatin, nuclear lamina, nuclear pore complex, various transporter subunits
  • Nuclear envelope: outer and inner membranes with nuclear pore complexes

Cytoskeleton: Components and Basic Dimensions

  • Subunits and diameters:
    • Intermediate filaments: ext{diameter} = 8-10 ext{ nm}
    • Microtubules: ext{diameter} = 25 ext{ nm}
    • Thin (actin) filaments: ext{diameter} = 5 ext{ nm}
    • Thick (myosin) filaments: ext{diameter} = 10 ext{ nm}
  • Components of the cytoskeleton

Microtubules: Structure and Function

  • Tubulin is the major filament protein
  • Kinesin and dynein act as molecular motors that move along microtubules
  • Microtubules are essential for intracellular transport and organelle positioning

Actin-based Thin Filaments: Structure and Synthesis

  • Formation of F-actin from G-actin
    • ATP-bound G-actin polymers to form F-actin
    • Activation and nucleation steps lead to filament formation
  • Treadmilling dynamics:
    • At the assembly (barbed) end, polymerization occurs with ATP-bound actin
    • At the disassembly (pointed) end, ADP-actin disassembles
  • Schematic sequence involves nucleation, formation of a stable actin oligomer, ATP-actin incorporation, and eventual conversion to ADP-actin at the shrinking end
  • Key terms: ATP-bound G-actin, ATP-actin, F-actin, end-growth vs end-shrinkage

Myosin-based Thick Filaments

  • Myosin thick filaments participate in cyclic interactions with actin filaments
  • Critical for contraction in skeletal, cardiac, and smooth muscle cells
  • also contribute to movements of many non-muscle cells

Actin-Myosin Interaction in Non-Muscle Motility

  • Example: Movement of microvilli in the brush border of intestinal epithelial cells
  • Key players around the terminal web include dense plaque material, fimbrin, villin, and myosin I; associated networks include actin filaments, fodrin (spectrin family), intermediate filaments, and cytoskeletal linkages

Synthesis, Processing, and Trafficking of Membrane and Secreted Proteins in the Rough ER

  • Interaction of ribosomes and ER membranes is essential
  • Critical roles for:
    • Signal sequences
    • Signal recognition particle (SRP)
    • SRP receptor
    • Translocon
    • Stop-transfer sequences
  • Post-translational modification and protein folding within the ER
  • Exit from the ER to the Golgi

Ribosome–ER Membrane Interaction

  • Roles for recognition and transport of nascent proteins across the ER membrane bilayer

Synthesis of Integral Membrane Proteins with Membrane-Spanning Alpha Helices

  • A single membrane-spanning segment with a cytoplasmic C-terminus
  • Key elements in the translocation process:
    • Signal sequence
    • Signal peptidase
    • ER lumen exposure
    • Dissociation of the translocon
    • Stop-transfer sequence
  • Important roles for both signal sequences and stop-transfer sequences

Processing inside the ER and Post-Translational Modifications

  • Processing enzymes within the ER lumen modify proteins
  • Synthesis of integral membrane proteins with glycosylation sites or GPI anchors

Trafficking of Membrane and Secreted Proteins to the Plasma Membrane

  • Step 1: Trafficking from the ER to the Golgi via membrane carrier vesicles
  • Step 2: Maturation and post-translational modification within the Golgi
  • Step 3: Trafficking from the Golgi to the plasma membrane via vesicles
  • Step 4a: Immediate fusion with the plasma membrane → Constitutive exocytosis/secretion
  • Step 4b: Delayed fusion until an appropriate secretion signal → Regulated exocytosis/secretion

Constitutive vs Regulated Secretion and Exocytosis

  • Common pathway up to the last step
  • Constitutive secretion: continuous and unregulated
  • Regulated secretion: directed by hormonal or neural signals
  • Key components include rough ER, cis/trans-Golgi network, medial Golgi, and trans-Golgi

Mechanisms for Formation of Secretory Vesicles and Fusion

  • Critical roles for clathrin, SNARE proteins, SNAP proteins, and Rab-family GTPases

Endocytosis and Internalization

  • Fluid phase endocytosis vs receptor-mediated endocytosis
  • Some of the same players as exocytosis (e.g., clathrin and Rab GTPases)
  • Endocytosis represents the reverse process: uptake of extracellular molecules into cells

Epithelial Tissue: Structure and Function

  • Special organizational requirements of epithelial cells
  • Barrier function vs efficient transepithelial transport
  • Two different plasma membranes in one cell: apical membrane vs basolateral membrane
  • Polarized trafficking of membrane proteins and secreted proteins to apical vs basolateral membranes
  • Barrier function requires special intercellular junctions: tight junctions, adherens junctions, gap junctions
  • Polarized trafficking underpins transepithelial transport across the two membrane domains

Epithelial Cell Junctions

  • Epithelial cells have two physically distinct components of plasma membrane with different complements of membrane transport proteins
  • Junctional complexes include:
    • Tight junctions (claudins involved)
    • Adhering junctions (cadherins involved)
    • Gap junctions (connexons; connexins involved)
  • Junctional architecture includes grooves and ridges, basal basement membrane, actin filaments in the cytoskeleton, and intermediate filaments

Key Junctional Components and Their Roles

  • Claudins: critical for tight junctions and barrier function
  • Cadherins: essential for adherens junctions and cell–cell adhesion
  • Connexins: form gap junction channels for intercellular communication

Miscellaneous Notes

  • The content covers both structural biology (membrane architecture) and functional biology (trafficking, signaling, and epithelial polarity)
  • The material emphasizes integration: how lipid composition, protein organization, and trafficking pathways contribute to cellular homeostasis
  • Ethical/philosophical/practical implications: understanding precise trafficking and membrane organization is essential for insights into disease mechanisms (e.g., lysosomal storage diseases, epithelial barrier dysfunction) and for pharmacological targeting (e.g., receptor trafficking, SNARE/Rab pathways)

Quick Summary of Core Concepts

  • Lipid bilayers form spontaneously in water due to hydrophilic head interactions with water and hydrophobic tails avoiding water
  • Membrane asymmetry is biologically important for signaling and transport
  • Membrane proteins perform receptor signaling, adhesion, transport, and enzymatic roles
  • The ER and Golgi coordinate synthesis, processing, and trafficking of membrane and secreted proteins, with distinct pathways for constitutive vs regulated secretion
  • The cytoskeleton provides structural support and actively participates in motor-driven transport and cell movement
  • Epithelial tissues are highly polarized with specialized junctions and trafficking requirements to sustain barrier and transepithelial transport functions

Equations and Numerical Details

  • Cytoskeletal diameters (nm):
    • Intermediate filaments: 8-10 ext{ nm}
    • Microtubules: 25 ext{ nm}
    • Thin actin filaments: 5 ext{ nm}
    • Thick myosin filaments: 10 ext{ nm}
  • Actin polymerization dynamics (conceptual notation):
    • ext{G-actin} + ext{ATP}
      ightarrow ext{ATP-G-actin}
      ightarrow ext{F-actin (polymerized)}
    • Plus end growth vs minus end disassembly (treadmilling) with ATP vs ADP states

Connections to Core Principles

  • The material bridges molecular structure (lipids, proteins) with cellular function (trafficking, signaling, motility)
  • Emphasizes homeostasis at the cellular level through membrane organization and organelle function
  • Highlights how intracellular trafficking relies on specific proteins (SRP, translocon, SNAREs, SNAPs, Rab GTPases) and post-translational modifications (glycosylation, GPI anchors)