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:

1. Enzyme-linked receptors (RTK)
2. G-protein coupled receptors (GPCR)
3. 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

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