Cell Signaling Pathways: G Proteins, Receptors, and Second Messengers

Autocrine signaling

A form of cell signaling where a cell releases signals that bind to receptors on its own surface, affecting itself.

Contact-dependent signaling

Signaling that requires direct physical contact between neighboring cells via membrane-bound molecules.

Endocrine signaling

Long-distance signaling where hormones are secreted into the bloodstream and affect target cells far from the source.

Paracrine signaling

Short-distance signaling where cells release factors that affect nearby cells in the local environment.

Feedback regulation

A control mechanism in which the output of a pathway regulates its own activity, often as negative or positive feedback.

GAP (GTPase-activating protein)

A protein that accelerates the GTP hydrolysis activity of G proteins, inactivating them.

GEF (Guanine nucleotide Exchange Factor)

A protein that promotes the exchange of GDP for GTP, activating G proteins.

G protein

A molecular switch that transmits signals from cell surface receptors to intracellular effectors; active when bound to GTP.

Monomeric GTPase

Small GTP-binding proteins (like Ras) that regulate signaling pathways and act as molecular switches.

G alpha subunit (Gα)

The part of heterotrimeric G proteins that binds GTP/GDP and regulates downstream effectors.

G beta subunit (Gβ)

Part of the G protein trimer that forms a complex with Gγ to regulate effectors and stabilize Gα.

G gamma subunit (Gγ)

Works with Gβ to anchor the G protein to the membrane and regulate signaling.

GPCR (G-protein-coupled receptor)

A receptor that activates heterotrimeric G proteins upon ligand binding.

cAMP (cyclic AMP)

A small molecule second messenger that activates protein kinase A (PKA) in response to signaling.

AMP (adenosine monophosphate)

A nucleotide involved in energy metabolism and signaling.

AMP phosphodiesterase

An enzyme that degrades cAMP to AMP, terminating the signal.

Adenylyl cyclase

An enzyme that converts ATP into cAMP, generating a second messenger.

Second messenger

Small intracellular molecules (like cAMP, IP₃, DAG, NO) that relay signals from receptors to effectors.

DAG (diacylglycerol)

A lipid-derived second messenger that activates protein kinase C (PKC).

IP₃ (inositol 1,4,5-trisphosphate)

A second messenger that triggers calcium release from the endoplasmic reticulum.

Calmodulin

A calcium-binding protein that mediates calcium-dependent signaling.

Nitric oxide (NO)

A gaseous second messenger involved in vasodilation and signaling.

Nitric oxide synthase (NOS)

An enzyme that produces NO from arginine.

Protein kinases

Enzymes that transfer phosphate groups from ATP to target proteins, modulating their activity.

Protein phosphatase

Enzymes that remove phosphate groups from proteins, reversing kinase activity.

Protein kinase A (PKA)

A cAMP-activated kinase that phosphorylates target proteins.

Protein kinase C (PKC)

A kinase activated by DAG and calcium, phosphorylating serine/threonine residues.

PI 3-kinase (Phosphoinositide 3-kinase)

A kinase that phosphorylates phosphoinositides, activating signaling pathways like Akt/PKB.

PKB (Protein kinase B / Akt)

A kinase involved in survival, growth, and metabolism downstream of PI3K.

Cholera toxin

Bacterial toxin that locks Gαs in the active state, causing excessive cAMP production.

Pertussis toxin

Bacterial toxin that inactivates Gαi, preventing inhibition of adenylyl cyclase.

Enzyme-coupled receptor

Cell surface receptors with intrinsic enzymatic activity or associated enzymes (e.g., RTKs).

RTK (Receptor tyrosine kinase)

A receptor that phosphorylates tyrosine residues upon ligand binding, initiating signaling cascades.

Notch

A receptor involved in contact-dependent signaling that influences cell fate decisions.

MAP kinase pathway

A kinase cascade (Raf → MEK → ERK) that transmits signals from RTKs to regulate gene expression.

Ras

A small GTPase that links RTKs to the MAP kinase pathway.

Tor (Target of rapamycin)

A kinase that regulates cell growth, metabolism, and nutrient sensing.

Bad

A pro-apoptotic protein regulated by phosphorylation in survival pathways.

Phospholipase C (PLC)

An enzyme that cleaves PIP₂ into DAG and IP₃, activating downstream signaling.

Ethylene

A plant hormone that regulates growth, fruit ripening, and stress responses.