Cell Signaling Pathways and Cell Cycle Regulation

Signal Transduction Pathways and Receptors

• Signal Transduction Pathway Definition: A sequence of molecular events and chemical reactions leading to a cell's response following ligand binding to a receptor.
• Ligand-Receptor Interactions:
  • Receptor Proteins: Feature specific three-dimensional binding sites for chemical signaling molecules (ligands); binding induces a conformational change.
  • Agonists: Molecules that bind a receptor and mimic the cellular response of the natural ligand.
  • Antagonists: Molecules that bind a receptor to block the natural ligand without triggering a cellular response.
• Receptor Classification by Location:
  • Intracellular Receptors: Bind nonpolar signals capable of diffusing through the plasma membrane; frequently function as transcription factors that enter the nucleus to alter gene expression.
  • Membrane Receptors: Bind large or polar ligands unable to cross the plasma membrane.
• Classes of Membrane Receptors:
  • Ion Channel-Linked Receptors: Ligand binding alters channel conformation to open pores for ion flux (e.g., acetylcholine binds muscle receptors to open $Na^+$ channels, triggering contraction).
  • Protein Kinase Receptors: Receptors with catalytic activity that phosphorylate themselves or target proteins; protein phosphatases reverse this by removing phosphate groups. Phosphorylation specifically occurs on serine, threonine, and tyrosine residues.
  • Protein Kinase Cascades: Sequential phosphorylation where one kinase activates downstream kinases, amplifying the signal at each step and transmitting messages toward target proteins or the nucleus.

G Protein-Coupled Signaling and Second Messengers

• G Protein-Coupled Receptors (GPCRs):
  • Receptors that activate membrane-associated G proteins possessing three subunits.
  • G Protein State: Inactive when bound to guanosine diphosphate ($GDP$); active when bound to guanosine triphosphate ($GTP$).
  • Upon activation, a G protein subunit separates and diffuses laterally through the membrane to activate an effector protein.
• Second Messenger Systems:
  • Second messengers are small intracellular molecules that mediate, distribute, and amplify steps in a signaling cascade.
  • Adenylyl Cyclase Pathway: Active G protein stimulates adenylyl cyclase to synthesize cyclic AMP ($cAMP$) from $ATP$.
  • Phospholipase C Pathway: Effector phospholipase C hydrolyzes phosphatidylinositol-biphosphate ($PIP_2$) to produce diacylglycerol ($DAG$), which remains in the membrane, and inositol triphosphate ($IP_3$), which diffuses into the cytoplasm.
  • Calcium ($Ca^{2+}$): Operates as a second messenger; elevated concentrations assist in activating protein kinase C (PKC).

Cell Division Fundamentals and Prokaryotic Binary Fission

• Universal Events of Cell Division: Essential for reproduction, growth, and tissue regeneration:
  • Cell Division Signals: Internal or external stimuli that initiate division.
  • DNA Replication: Duplication of genetic material.
  • DNA Segregation: Distribution of replicated chromosomes equally to two daughter cells.
  • Cytokinesis: Physical division of the cytoplasm into two separate cells.
• Prokaryotic Binary Fission:
  • Division Signals: External conditions such as environmental status and nutrient concentrations.
  • Chromosomal Organization: Circular, folded DNA containing two critical regions: the origin of replication ($ori$) and the termination site ($ter$).
  • Process: Replication proceeds as DNA moves through a protein replication complex; $ori$ regions separate to opposite poles; cytokinesis finishes via cell membrane pinching and protein fiber ring constriction.

The Eukaryotic Cell Cycle and Molecular Regulation

• Cell Cycle Phases:
  • Interphase:
  • $G_1$ Phase: Each chromosome exists as a single, unreplicated DNA molecule. Non-dividing cells can arrest and exit into a resting phase designated $G_0$.
  • $S$ Phase: DNA replication duplicates chromosomes into paired sister chromatids.
  • $G_2$ Phase: Synthesis of structures required to segregate chromatids during division.
  • M Phase: Encompasses mitosis (nuclear division) and cytokinesis (cytoplasmic division).
• Regulation by Cyclin-CDK Complexes:
  • Cyclin-Dependent Kinases (CDKs): Kinases that regulate phase transitions; inactive until bound by a specific cyclin, which alters conformation to expose the catalytic active site.
  • Active CDKs phosphorylate target substrates that regulate cell cycle events.
  • $G_1/S$ Cyclin: Synthesized during $G_1$, binds and activates CDK, and is subsequently degraded to switch off kinase activity.
  • Restriction Point ($R$): Checkpoint at the $G_1$-to-$S$ transition controlled by a specific cyclin-CDK complex.
  • Retinoblastoma Protein ($RB$): An inhibitor that is active when unphosphorylated; $G_1/S$ cyclin-CDK phosphorylates and inactivates $RB$, allowing cell cycle progression past the restriction point. Inactivation or mutation of $RB$ occurs commonly across human cancers.