Cell Signaling: Receptors

Categories of Receptors

Ion channels receptors
Ligand gated ion channels
Voltage gated ion channels
Receptors that are linked to cytoplasmic enzymes
Cytokine receptors
Receptors that have intrinsic enzymatic activity (they are enzymes)
Receptor tyrosine kinases
Receptors that activate intracellular second messengers
G protein coupled receptors
Cell adhesion receptors

Cell Surface Receptors

Three kinds:

Enzyme-linked receptors (RTK)
G-protein coupled receptors (GPCR)
Ligand-gated ion channels

Enzyme-linked Receptors

Found in all living species
Extracellular domain binds signal
Intracellular domain becomes functional catalyst
Most are protein kinases
Many receptors have intracellular domains with enzyme function
Most are receptor tyrosine-kinase
They phosphorylate tyrosine residues in selected intracellular proteins
These receptors are activated by growth factors, thus being important in cell proliferation

Receptor Tyrosine Kinases

Receptor tyrosine kinases: category of enzyme-linked receptors found in animals
Recognize various types of signaling molecules
- Growth factor – hormone that acts to stimulate cell growth or division
example: Epidermal Growth Factor (EGF)
- One function is to stimulate epidermal cells to divide
- Functions in many different contexts

G-Protein Couples Receptors

Found in all eukaryotes, common in animals
7 transmembrane segments
Activated receptor activates G protein
Releases GDP and binds GTP instead
GTP causes G protein to dissociate
α subunit and β/γ dimer interact with other proteins in a signaling pathway
Signals binding to cell surface are “first messenger”

Intracellular Receptors

Intracellular receptors: found in the cytosol or nucleus of target cells
Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors
Examples of hydrophobic messengers are the steroid and thyroid hormones of animals
- An activated hormone-receptor complex can act as a transcription factor, turning on specific genes