Lecture 3-14: Enzyme-Coupled Receptors

Enzyme-Coupled Receptors

• Enzyme-coupled receptors play a key role in cellular signaling.

• They either have intrinsic enzymatic activity or recruit other proteins that provide this function.

• Commonly involved in signalling pathways related to growth, differentiation, and metabolism.

Key Types of Enzyme-Coupled Receptors

Receptor Tyrosine Kinases (RTKs)

• Most prevalent type of enzyme-coupled receptors.

• Activated through ligand-induced dimerization which leads to autophosphorylation on tyrosine residues.

• Ligands for RTKs typically exist as dimers; their binding promotes receptor dimerization.

  • Dimerization facilitates proximity of kinase domains, allowing phosphorylation and creating specific binding sites for adapter proteins.

• Ligand-independent activation can occur through high levels of receptor expression due to mutations in regulatory DNA.

Serine/Threonine Kinases

• These kinases are another category of enzyme-coupled receptors, though less common than RTKs.

• They work similarly by promoting phosphorylation events when activated.

Signal Transduction through RTKs

1. Activation of Ras

• Ras is a monomeric G protein activated by RTKs through Ras-GEF.

• Ras-GEF exchanges GDP for GTP on Ras, activating the protein and allowing it to propagate the signal.

• The signal persists until Ras-GAP hydrolyzes GTP to GDP, turning Ras off.

2. MAP Kinase Cascades

• Activated Ras initiates MAP kinase cascades, crucial for promoting cell division (mitogenic signals).

• Typically involves a sequence of phosphorylation events:

  • MAPKKK (e.g., RAF) ➔ MAPKK (e.g., MEK) ➔ MAPK (e.g., ERK) ➔ effector proteins.

• Each kinase activates subsequent kinases, forming a phosphorylation cascade.

3. PI3-Kinase/Akt Pathway

• RTKs can also signal through the PI3K/Akt pathway promoting cell survival.

• Upon activation, PI3K converts PIP2 to PIP3, recruiting and activating Akt.

  • Akt, once activated, phosphorylates downstream targets, promoting cell growth and inhibiting apoptosis via Bcl2.

• This pathway can be terminated by the action of PTEN which converts PIP3 back to PIP2.

Role of Akt in Cell Survival and Growth

• Akt signaling helps inhibit apoptosis by activating Bcl2.

• In the absence of Akt, Bcl2 is sequestered by Bad, a pro-apoptotic factor.

• Akt phosphorylation of Bad releases Bcl2, promoting cell survival.

• Akt also activates Tor, promoting protein synthesis and cell growth.

  • Rapamycin inhibits Tor, leading to decreased cell growth and energy demands.

Shared Signaling Mechanisms

• Similar signaling events take place in both GPCR (G Protein-Coupled Receptors) and RTK networks.

• While mechanisms may differ, many cellular responses are integrated through common pathways.

• Components include primary signaling molecules, secondary messengers (like cAMP, Ca2+), and effector proteins that respond to these signals.

Techniques for Studying Signaling Complexes

Co-immunoprecipitation (Co-IP)

• A method to study protein interactions and complexes.

• Involves isolating specific proteins using antibodies; proteins that interact can be co-precipitated and analyzed via gel electrophoresis.

Functional Studies with Mutant Proteins

• Use mutations in signaling components to identify binding sites and deduce the order of protein interactions.

• Experiments can show relationships among proteins in a signaling pathway, such as Ras acting downstream of Protein X.

• Understanding these interactions is critical for developing targeted therapies.

Notch Signaling Pathway

• Functions through direct cell-to-cell contact, influencing tissue differentiation.

• As one cell becomes a neuron, it expresses the Delta ligand that affects neighboring cells through Notch receptor interaction, preventing their differentiation into neurons.

Hydrophobic Hormones and Their Receptors

• Examples include cortisol, estradiol, and testosterone, which bind intracellular receptors.

• Binding results in conformational changes that allow them to act as transcription factors by affecting gene expression in the nucleus.

Plant vs. Animal Signaling Mechanisms

• While sharing some kinase-based functions from a common ancestor, signaling pathways have diverged significantly between plants and animals.

• Ethylene signaling in plants provides a unique example, employing kinase pathways that inhibit transcription, distinct from animal signaling mechanisms.

Summary

• Understanding enzyme-coupled receptors and their signaling mechanisms is crucial in cell biology, especially RTKs which facilitate key cellular functions, integrating various signals to orchestrate complex biological processes.