Cell Signaling

Three stages of cell signaling

1. Reception - Ligand binds to receptor

2. Transduction - Signal is converted

3. Response - Cell process is altered

Reception

The detection and receiving of a ligand by a receptor in the target cell

Receptor

Macromolecule that binds to a signal molecule (ligand)

Binding between ligand and receptor

Highly specific

Reception Process

1. Ligand binds to receptor, activating it

2. Receptor interacts with other cellular molecules

3. Initiates transduction signal

Receptor location

Plasma membrane or intracellular

Plasma Membrane Receptors

• Most common

• Binds to ligands that are polar, water-soluble, and large (cannot pass easily)

Example of Plasma Membrane Receptors

• G-Protein coupled receptors (GPCRs)

• Ligand-gated ion channels

Intracellular Receptors

• Found in cytoplasm or nucleus of target cell

• Binds to ligands that can pass through plasma membrane (hydrophobic)

Examples of hydrophobic molecules that use intracellular receptors

• Steroid and thyroid hormones

• Gasses like nitric oxide

Transduction

The conversion of an extracellular signal to an intracellular signal that will bring about a cellular response

Signal transduction pathway

A sequence of changes in a series of molecules that will bring about a cellular response

Signal Transduction Pathway protein regulation

• Phosphorylation by enzyme protein kinase

• Dephosphorylation by enzyme protein phosphatase

Protein kinase

Relays signal inside cell

Protein phosphatase

Shuts off pathways

Signal in transduction

Amplified

Second messengers

Small, non-protein molecules and ions help relay the message and amplify the response

Common second messenger

Cyclic AMP (cAMP)

Response

The final molecule in the signaling pathway converts the signal to a response that will alter a cellular process

Examples of response

A. Protein that can alter membrane permeability

B. Enzyme that will change a metabolic process

C. Protein that turns genes on or off

Changes by signal transduction pathways

• Can influence how cell responds to environment

• Can result in changes in gene expression and cell function

• Can alter phenotypes or result in cell death

Mutations to receptor proteins or signaling pathway

Results in change to transduction of signal

Two main cell membrane receptors

• G protein coupled receptors

• Ion channels

G protein coupled receptors (GPCRs)

• Largest category of cell surface receptors

• Important in animal sensory systems

• Binds to a G-Protein that can bind to GTP, which is an energy molecule similar to ATP

status of GPCR, enzyme, and G protein

Inactive until ligand binds to GPCR on the extracellular side

Ligand binding to GCPR

• Causes cytoplasmic side to change shape

• GPCR and G protein bind b/c they are activated

• GDP replaced by GTP

Activated G protein

• Binds to enzyme and activates it

• Amplifies signal and leads to cellular response

Ligand gated ion channels

• Located in plasma membrane

• Important in nervous system

• Act as a “gate” for ions

Ligand-Receptor binding in ligand gated ion channels

Receptor (gate) opens or closes, allowing diffusion of specific ions which initiates a series of events that lead to a cellular response