Cell SignalingThe 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)

Step 1: Reception

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

Step 2: Transduction

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

Step 3: Response