Which of the following statements best describes the mechanism of action of insulin on target cells?

Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor.

Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.

Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.

Insulin enters the cell and causes the release of calcium ions from intracellular stores.

Which of the following statements accurately describes G proteins?

Are associated with cellular membranes and play a crucial role in signal transduction.

Regulate second messengers like cyclic adenosine monophosphate (cAMP).

Play a role in the amplification of hormonal signals.

Consist of three subunits: alpha, beta, and gamma.

Mediators of post-translational assembly of protein complexes

Protein Domains and Motifs - Free Indian Medical PG Review

Protein Domains and Motifs - Protein Lego Land

Protein Domain:
- A stable, independently folding unit within a protein.
- Has a specific function (e.g., binding, catalysis).
- Fundamental unit of structure, function, and evolution.
Protein Motif (Supersecondary Structure):
- A short, conserved sequence/structural arrangement.
- Often part of a domain; may not be stable or functional alone.
- Examples: β-α-β unit, Greek key, helix-loop-helix.
Structural Hierarchy:
- Primary (Sequence) → Secondary (α-helix, β-sheet) → Motifs → Tertiary (Domains, 3D shape) → Quaternary.
Domain vs. Motif:
- Domain: Independent functional & structural unit.
- Motif: Structural pattern; function often tied to its domain context.
Importance:
- Modularity: Proteins are like "Lego" structures built from domains.
- Evolution: New functions via domain shuffling.
- Prediction: Domains/motifs aid in predicting protein roles.

Protein domain structure and topology diagram

⭐ Protein domains are often considered the fundamental units of protein evolution.

Protein Domains and Motifs - Functional Hotspots

Protein domains are conserved, independently folding functional units within a protein. Motifs are shorter, recurring sequence patterns. These are crucial for protein interactions and cellular signaling.

Domain Name	Recognized Ligand/Site	Key Function(s)	Clinical Relevance Example
SH2 (Src Homology 2)	Phospho-Tyrosine (pY)	Signal transduction, protein-protein interaction	Chronic Myeloid Leukemia (BCR-ABL signaling)
SH3 (Src Homology 3)	Proline-rich sequences	Signal transduction, cytoskeletal organization	Adapter proteins in signaling (e.g., Grb2)
Pleckstrin Homology (PH)	Phosphoinositides (e.g., PIP3)	Membrane targeting, signal transduction	Akt/PKB signaling in cancer
Zinc Finger	DNA/RNA	Gene transcription, DNA repair, RNA binding	Transcription factor defects (e.g., steroid hormone receptors)
Kinase Domain	ATP, substrate protein	Catalyzes phosphorylation, signal amplification	Kinase inhibitors in cancer therapy (e.g., Imatinib)

⭐ Many signaling proteins utilize SH2 domains to interact with activated receptor tyrosine kinases.

SH2 domain binding to phosphorylated tyrosine and PIPs

Protein Domains and Motifs - Pattern Play

Motifs (Supersecondary Structures): Small, conserved structural units built from specific arrangements of secondary structure elements ($\alpha$-helices, $\beta$-strands, and turns). They are smaller than domains and often represent functional sites or form part of a larger domain.

Motif Name	Structural Description	Common Role(s)	Example Protein(s)
Helix-turn-helix	Two $\alpha$-helices connected by a short amino acid sequence (turn)	DNA binding, gene regulation	Lac repressor, homeodomain proteins
Leucine zipper	Two amphipathic $\alpha$-helices with leucine at every 7th residue, forming a coiled-coil	Dimerization of transcription factors, DNA binding	c-Fos/c-Jun, CREB
Zinc finger	One or more Zn²⁺ ions coordinate with Cys/His residues, stabilizing a small fold	DNA/RNA binding, protein-protein interactions	TFIIIA, GATA factors, steroid receptors
EF hand	A helix-loop-helix structure; the loop binds Ca²⁺ ions	Calcium sensing and binding	Calmodulin, parvalbumin, troponin C
Beta-alpha-beta unit	Two parallel $\beta$-strands linked by an intervening $\alpha$-helix	Forms core of many nucleotide-binding domains, enzymes	Rossmann fold, dehydrogenases, kinases

⭐ The helix-turn-helix motif is a common DNA-binding motif found in many prokaryotic and eukaryotic transcription factors.

High‑Yield Points - ⚡ Biggest Takeaways

Protein domains are stable, independently folding globular units within a polypeptide, often associated with a specific function.

Motifs (or supersecondary structures) are short, conserved sequence patterns or structural arrangements, like the helix-turn-helix or beta-alpha-beta unit.

A single domain can comprise multiple motifs; motifs themselves typically do not fold independently.

Modular evolution: Proteins often evolve by shuffling or combining existing domains.

Key examples: Zinc finger (DNA binding), Leucine zipper (dimerization), EF hand (Ca²⁺ binding).

Mutations within critical domains can impair protein function, leading to disease states (e.g., kinase domain mutations in cancer).