The 3 Steps

Intracellular Receptors
◦ Proteins in the cytoplasm or in the nucleus
◦ Example: transcription factors

Receptors in the Plasma Membrane
◦ Transmit information from extracellular
environment to inside by changing shape or
aggregating when bound by a ligand
◦ Examples: G-protein-linked-receptors (detect
neurotransmitters, hormones, etc.), tyrosine
kinases (catalyze transfer of phosphate groups),
and ion channel receptors (gates for ions)

Signal Transduction Pathways
◦ Signal is changed to a different form at each step –
usually conformational change of protein

Protein Phosphorylation and
Dephosphorylation
◦ Adding a phosphate group to activate enzymes in
the pathway, and removing them to deactivate them

Small Molecules and Ions as Second
Messengers
◦ Diffuse quickly through cytosol
◦ Examples: cAMP, Calcium ions

Cytoplasmic and Nuclear Responses
◦ Enzyme activity, etc.

Fine-Tuning of the Response
◦ Amplification of response by activating multiple
copies of the next component in a pathway
◦ Scaffolding proteins facilitate signal transduction by
holding several relay proteins close together
◦ When signal molecules leave their receptor, the
receptor reverts to its inactive form… and so do the
relay molecules
◦ http://www.youtube.com/watch?v=FkkK5lTmBYQ