Signal Transduction: Chemical Signaling

Key Concepts in Signal Transduction

Cellular communication relies on chemical signals that are detected and processed by cells through specialized pathways. These pathways involve receptors, ligands, and a variety of intracellular messengers, enabling cells to respond to their environment and coordinate complex behaviors.

• Receptors: Proteins on the cell surface or within cells that bind signaling molecules (ligands).

• Ligands: Molecules (e.g., hormones, neurotransmitters) that bind to receptors to initiate signaling.

• Agonists: Ligands that activate receptors.

• Antagonists: Ligands that block receptor activation.

• Second Messengers: Small molecules (e.g., cAMP, Ca2+) that relay signals inside the cell.

• Signal Amplification: A single ligand-receptor interaction can trigger a cascade, amplifying the response.

G Protein-Coupled Receptor (GPCR) Pathway

GPCRs are a large family of transmembrane receptors characterized by seven transmembrane helices. They mediate diverse cellular responses by activating heterotrimeric G proteins.

• GPCR Structure: Seven transmembrane domains; extracellular ligand-binding site.

• G Proteins: Composed of Gα, Gβ, and Gγ subunits. "On" when bound to GTP, "off" when bound to GDP.

• Activation: Ligand binding activates GPCR, which then activates G proteins.

• Downstream Signaling:

  • Gsα: Activates adenylyl cyclase → increases cAMP → activates protein kinase A (PKA).

  • Gqα: Activates phospholipase C (PLC) → produces IP3 and DAG. IP3 triggers Ca2+ release from ER, leading to calcium signaling.

Receptor Tyrosine Kinase (RTK) Pathway

RTKs are single-pass transmembrane proteins with intrinsic kinase activity. They regulate cell growth, differentiation, and metabolism.

• Structure: Extracellular ligand-binding domain, transmembrane region, intracellular tyrosine kinase domain.

• Activation: Ligand binding → receptor dimerization → autophosphorylation of tyrosine residues.

• Signaling Cascade: Recruitment of adaptor proteins → activation of Ras (small GTPase) → MAP kinase cascade → changes in gene expression.

• Alternative Pathways: RTK activation can also stimulate PLC and PI3K pathways.

Other Signaling Pathways

• Receptor Serine-Threonine Kinases: Example: TGFβ pathway activates Smad proteins.

• Signal Integration and Crosstalk: Multiple pathways can interact, allowing complex regulation.

• Short- vs. Long-Range Signals: Local signals (paracrine) vs. systemic signals (endocrine).

• Hormones and Nuclear Receptors: Lipid-soluble hormones (e.g., steroid hormones) bind intracellular receptors, directly affecting gene expression.

Cytoskeletal Systems

Types of Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, facilitates cell movement, and organizes intracellular components.

• Microtubules (MTs): Hollow tubes made of tubulin; involved in cell shape, transport, and division.

• Microfilaments (Actin Filaments): Thin filaments composed of actin; support cell shape and motility.

• Intermediate Filaments: Rope-like fibers; provide mechanical strength.

Microtubules

• Subunit: α- and β-tubulin dimers.

• Assembly: Requires GTP; exhibits polarity (plus and minus ends).

• Treadmilling: Simultaneous addition at one end and loss at the other.

• Dynamic Instability: Alternates between growth and shrinkage (catastrophe-rescue).

• Binding Molecules: Colchicine (inhibits assembly), Taxol (stabilizes MTs).

• Microtubule-Organizing Centers (MTOCs): Centrosomes and basal bodies; contain γ-tubulin.

• Microtubule-Binding Proteins: MAPs (stabilize MTs), +-TIPs (regulate MT dynamics).

Microfilaments (Actin Filaments)

• Subunit: Globular actin (G-actin).

• Assembly: Requires ATP; exhibits polarity.

• Structures: Stress fibers, gel-like networks, branched (lamellipodia), parallel bundles (filopodia).

• Branching Proteins: Arp2/3 complex, Rho family GTPases.

• Actin-Binding Molecules: Cytochalasin (inhibits polymerization), Profilin (promotes assembly), ADF/cofilin (disassembles filaments), Gelsolin (severs filaments).

Intermediate Filaments

• Subunit: Diverse proteins (e.g., keratins, lamins).

• Assembly: No energy requirement; non-polar.

• Function: Mechanical support, nuclear structure.

Cellular Movement

Motor Proteins and Cell Motility

Motor proteins convert chemical energy (ATP) into mechanical work, enabling movement of organelles, vesicles, and cells.

• Microtubule-Based Motors:

  • Kinesins: Move toward MT plus end.

  • Dyneins: Move toward MT minus end; drive cilia/flagella motion via sliding of MTs.

• Actin-Based Motors:

  • Myosins: Move along actin filaments; involved in muscle contraction and nonmuscle motility.

Muscle Contraction

• Thick/Thin Filaments: Myosin (thick) and actin (thin) filaments form the sarcomere.

• Sliding-Filament Model: Myosin heads bind actin and "walk" along filaments, causing contraction.

• Regulation: Troponin and tropomyosin control access to actin; Ca2+ triggers contraction.

Cell-Cell Junctions

Types of Cell Junctions

Cell junctions are specialized structures that connect cells to each other and to the extracellular matrix, maintaining tissue integrity and enabling communication.

• Adhesive Junctions: Include adherens junctions (cadherins, actin) and desmosomes (desmogleins, intermediate filaments).

• Tight Junctions: Seal spaces between cells; involve claudins and occludins.

• Gap Junctions: Allow direct communication via connexins.

• Plasmodesmata: Plant-specific channels for cell-cell communication.

Extracellular Matrix (ECM)

Components and Functions of ECM

The ECM is a complex network of macromolecules that provides structural support, regulates cell behavior, and mediates cell adhesion.

• Collagens: Provide tensile strength; assembled in a specific order.

• Elastins: Confer elasticity.

• Proteoglycans: Hydrated gel; resist compression.

• Fibronectins and Laminins: Mediate cell adhesion and migration.

Basal Lamina and Cell-ECM Adhesion

• Basal Lamina: Specialized ECM layer; supports epithelial cells.

• Focal Junctions: Link actin cytoskeleton to ECM via integrins.

• Hemidesmosomes: Link intermediate filaments to ECM.

DNA, DNA Packaging, and the Nucleus

Structure of DNA

DNA is the hereditary material, organized as a double helix with specific base pairing and antiparallel strands.

• B-form DNA: Right-handed helix; antiparallel strands (5' to 3').

• Base Pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

DNA Packaging

• Nucleosome: DNA wrapped around histone proteins.

• Chromatin Fiber: Nucleosomes organized into higher-order structures.

• Loops: Chromatin forms loops for further compaction.

• Heterochromatin: Densely packed, transcriptionally inactive.

• Euchromatin: Loosely packed, transcriptionally active.

• Chromosome: Fully condensed chromatin during cell division.

• Centromeres: Essential for chromosome segregation.

• Telomeres: Protect chromosome ends.

Nucleus and Nuclear Transport

• Nucleus: Contains DNA; site of transcription.

• Nuclear Transport: Proteins and RNA move in/out via nuclear pore complexes.

• NLS/NES: Nuclear localization/export signals direct transport.

• Importin/Exportin: Transport receptors; require energy (GTP).

Table: Comparison of Cytoskeletal Elements

Key Equations

• ATP Hydrolysis (for motor proteins):

• Base Pairing: