Cell Signaling Cells communicate in various ways. – The type of communication used by each cell is based on the type of information that needs to be passed. – The signal transduction pathway is a series of steps where the surface recognition of a message (molecule, gas, hormone, ion,...) is converted into a specific cellular response.

Examples of cell to cell signaling Cell junctions - allow molecules to pass freely between cells – animal - Gap – plants - Plasmodesmata Paracrine signaling - messenger molecules – secreted by signaling cell – short distance – growth factors Synaptic signaling - vesicles – electrical signal triggers vesicle formation – vesicle fuses with nearby cell releasing the neurotransmitter Endocrine signaling - hormones – long distance – dumped into the blood stream for widespread delivery

3 Basic Steps Reception Transduction Response

Reception – cell signal (ligand) binds to a surface receptor on the outside (or inside) of the cell – large molecules and hydrophilic molecules must bind to a surface receptor (3-types) G-proteins - associated with GTP – protein receptor binds ligand & causes a change in G-protein from GDP to GTP – activated complex travels to an enzyme (still in the membrane) and activates it causing a cellular response in the cytosol – Some signal molecules are not proteins but are considered "secondary messangers" » cAMP is the secondary messenger of a G-protein

Reception Tyrosine Kinases – transmembrane proteins have binding sites and multiple tyrosine (Tyr) sites in the cytosol – protein receptor binds the ligands and causes 2 complexes to move together – the phosphorylation of the active Tyr by ATP » the active sites then activate relay proteins to cause a cellular response » 1 tyrosine kinase may activate up to ten different processes

Reception Ligand-gated Ion channels – binding of the ligand causes a conformation change and opening of a gate to ions of a specific charge hydrophobic molecules (steroids) and small molecules (NO) can pass directly through the plasma membrane to bind with receptors in the cytosol

Transduction after binding the surface protein changes to a form that brings about cellular change – change may be in the cytoplasm or in the nucleus – involves the change in inactive compounds to their active conformations – efficiency may be increased by the presence of scaffolding proteins protein complex with multiple relay protein receptor sites that carry out a cascade of reactions

Response cellular change brought about by the transduction of the signal involves signal amplification – activated complex during transduction may activate many molecules before becoming inactive again all different cells are specific in their response – they all have differing amounts and types of protein complexes and receptors inactivation of the receptor occurs as the product is produced and the molecule departs from its receptor