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Cell Communication & Signaling Cheat Sheet

Over 1400+ words covering the absolute core of Cell Signaling. Master GPCRs, Receptor Tyrosine Kinases, Second Messengers, Apoptosis pathways, and Bacterial Toxins to secure guaranteed marks in the DBT-BET JRF exam.

1. Fundamentals of Cell Communication

Cells communicate through chemical signals (ligands) that bind to specific receptors, initiating an intracellular cascade. The distance the signal travels dictates the type of signaling:

• Endocrine Signaling: Hormones are secreted into the bloodstream and travel long distances to reach target cells. (e.g., Insulin, Glucagon, Thyroid hormones).
• Paracrine Signaling: Signals act locally on neighboring cells in the immediate environment. The signaling molecules are quickly degraded by local enzymes to prevent them from entering the blood. (e.g., Neurotransmitters at a synapse, Somatostatin inhibiting insulin release locally).
• Autocrine Signaling: A cell secretes a ligand that binds to receptors on its own surface. Crucial in embryonic development, immune responses (e.g., T-cells secreting IL-2 to stimulate their own proliferation), and cancer cell metastasis.
• Juxtacrine Signaling (Contact-Dependent): Requires direct physical contact between the signaling and responding cell. The ligand is bound to the membrane of one cell, and the receptor is on the adjacent cell. (e.g., Notch/Delta signaling, Gap junctions).

2. G-Protein Coupled Receptors (GPCRs)

GPCRs form the largest family of cell-surface receptors. They are completely integral to sensory perception (vision, smell, taste) and hormonal responses. A defining hallmark of all GPCRs is their structure: a single polypeptide chain that threads back and forth across the lipid bilayer exactly seven times. Hence, they are also called 7-Transmembrane (7-TM) Receptors or Serpentine Receptors.

Figure 1: Activation of a GPCR. Ligand binding induces a conformational change, allowing the receptor to act as a GEF, exchanging GDP for GTP on the Gα subunit, which then dissociates to activate an effector.

The Heterotrimeric G-Protein Cycle

The G-protein consists of three subunits: α (alpha), β (beta), and γ (gamma). The α and γ subunits are covalently attached to the lipid membrane via lipid anchors (myristoylation/prenylation).

1. Resting State: The Gα subunit is bound to GDP (Guanosine Diphosphate). The αβγ complex is intact and inactive.
2. Activation: Ligand binds to the GPCR. The GPCR changes shape and binds the G-protein. The GPCR acts as a GEF (Guanine nucleotide Exchange Factor), causing the Gα subunit to release GDP and bind a fresh molecule of GTP.
3. Dissociation: Binding of GTP causes a conformational change. The Gα-GTP subunit dissociates from the Gβγ dimer. Both active components can now independently activate downstream effector enzymes or ion channels.
4. Termination: The Gα subunit has intrinsic GTPase activity. It hydrolyzes its own bound GTP back to GDP + Pi. This is the built-in timer. Once GTP becomes GDP, Gα re-associates with Gβγ, returning to the resting state. This hydrolysis is greatly accelerated by RGS proteins (Regulators of G-protein Signaling), which act as GAPs (GTPase Activating Proteins).

Key Gα Subunit Families

Subunit Type	Target Effector Enzyme	Effect on Second Messenger	Physiological Example
Gαs (Stimulatory)	Adenylyl Cyclase (Activates)	Increases cAMP → Activates PKA	Epinephrine binding to β-adrenergic receptors (Heart rate increase).
Gαi (Inhibitory)	Adenylyl Cyclase (Inhibits)	Decreases cAMP → Inactivates PKA	Epinephrine binding to α2-adrenergic receptors.
Gαq	Phospholipase C-β (PLCβ)	Cleaves PIP2 into IP3 and DAG. IP3 releases Ca2+.	Vasopressin, Smooth muscle contraction, Acetylcholine (M1, M3 receptors).
Gαt (Transducin)	cGMP Phosphodiesterase	Decreases cGMP (closes Na+ channels)	Phototransduction in rod cells of the retina (activated by Rhodopsin).

3. Enzyme-Linked Receptors: RTKs & JAK/STAT

Unlike GPCRs, these receptors have only one transmembrane domain. Their intracellular domains either possess intrinsic enzymatic activity or associate directly with an enzyme.

Receptor Tyrosine Kinases (RTKs)

RTKs mediate responses to most growth factors (EGF, PDGF, FGF).
Mechanism: Ligand binding causes two receptor monomers to physically associate (Dimerization). This brings their intracellular kinase domains close together, allowing them to cross-phosphorylate each other on specific Tyrosine residues (Autophosphorylation).

These newly created phosphotyrosines act as high-affinity docking sites for intracellular signaling proteins containing SH2 (Src Homology 2) or PTB (Phosphotyrosine Binding) domains.

The Exception to the Rule: The Insulin Receptor. Unlike other RTKs, the insulin receptor exists as a pre-formed dimer (α₂β₂ tetramer linked by disulfide bonds) even in the absence of insulin. Ligand binding simply rearranges the pre-existing dimer to induce autophosphorylation.

Cytokine Receptors and the JAK/STAT Pathway

Receptors for cytokines (e.g., Interferons, Interleukins, Prolactin) completely lack intrinsic kinase activity. Instead, they are non-covalently associated with cytoplasmic tyrosine kinases called JAKs (Janus Kinases).

• Cytokine binds → Receptors dimerize → JAKs are brought together and phosphorylate each other → JAKs then phosphorylate the receptor.
• STAT proteins (Signal Transducers and Activators of Transcription) dock onto these phosphotyrosines via their SH2 domains.
• JAKs phosphorylate the STATs. The STATs dimerize, translocate straight into the nucleus, and directly activate target gene transcription. It is a very rapid, direct pathway to the nucleus.

4. Second Messengers: Amplifying the Signal

Second messengers are small, non-protein, highly diffusible molecules that rapidly broadcast the signal throughout the cell, amplifying the original message.

Second Messenger	Synthesizing Enzyme	Primary Intracellular Target
cAMP (Cyclic AMP)	Adenylyl Cyclase (uses ATP)	Protein Kinase A (PKA). (cAMP binds the regulatory subunits, freeing the catalytic subunits).
cGMP (Cyclic GMP)	Guanylyl Cyclase (uses GTP)	Protein Kinase G (PKG), or directly opens/closes cation channels (like in vision).
IP3 (Inositol trisphosphate)	Phospholipase C (cleaves PIP2)	Diffuses to the Endoplasmic Reticulum (ER) to open ligand-gated Calcium channels.
DAG (Diacylglycerol)	Phospholipase C (cleaves PIP2)	Remains embedded in the plasma membrane. Activates Protein Kinase C (PKC) along with Calcium.
Nitric Oxide (NO)	NO Synthase (from Arginine)	A gas! It diffuses across membranes to activate soluble Guanylyl Cyclase in neighboring cells (causes vasodilation).

Calcium (Ca²⁺): Cytosolic calcium levels are normally kept extremely low (10^-7 M). When released from the ER via IP₃, calcium binds to a universal calcium-binding protein called Calmodulin. The Ca²⁺/Calmodulin complex wraps around and activates various target proteins, notably CaM-Kinases.

Guaranteed Exam Hits

PYQ Direct Statements (High Yield Facts)

• Bacterial Toxins & G-Proteins (Crucial PYQ!):
  • Cholera Toxin: Adds an ADP-ribose group to the Gα_s subunit. This permanently abolishes its GTPase activity. Result: Gα_s is locked in the "ON" state, leading to massive, relentless cAMP production in intestinal cells, causing severe watery diarrhea.
  • Pertussis Toxin (Whooping Cough): ADP-ribosylates the Gα_i subunit. This prevents it from interacting with the receptor. Result: Gα_i is locked in the "OFF" state (bound to GDP). Since the inhibitory protein is disabled, adenylyl cyclase remains highly active, again resulting in elevated cAMP levels.
• Wnt/β-Catenin Pathway: In the absence of Wnt, β-catenin is constantly targeted for ubiquitin-mediated proteasomal degradation by a destruction complex (containing APC, Axin, and GSK3). When Wnt binds to its receptor (Frizzled), the destruction complex is dismantled. β-catenin accumulates, enters the nucleus, and activates transcription of target genes. Mutations in the APC gene lead to colon cancer.
• Intracellular Receptors: Receptors for steroid hormones (Cortisol, Estrogen, Testosterone), Thyroid hormone, and Retinoic Acid are located strictly inside the cell (cytoplasm or nucleus). The ligands are small and hydrophobic, easily passing through the lipid bilayer. These receptors function directly as ligand-dependent transcription factors. They bind to DNA using Zinc-Finger motifs.
• Signal Termination by Phosphodiesterases (PDEs): Secondary messengers like cAMP and cGMP are rapidly destroyed by PDEs, which convert them into regular 5'-AMP and 5'-GMP. Caffeine and Theophylline act as non-specific PDE inhibitors, leading to artificially sustained high cAMP levels (which keeps you awake).
• Ras and Cancer: Ras is a monomeric, small G-protein (distinct from heterotrimeric G-proteins) activated by RTKs. Approximately 30% of all human cancers involve a mutation in the Ras gene that destroys its intrinsic GTPase activity, locking it in the ON state and driving uncontrolled cellular proliferation via the MAPK cascade.

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