Chapter 11 Cell Communication

Cell communication signal cells communicate by direct contact or by secreting local regulators ex: growth factors, neurotransmitters

Cell signaling 3 Stages: 1) reception – target cell’s detection of signal molecule from outside of cell 2) transduction – conversion of signal to a form that can bring about a specific cellular response 3) response – specific cellular response to signal molecule

Reception ligand (signal molecule) & receptor are specific usu. binding of ligand to receptor is initial transduction signal receptors may be: intracellular – within membrane in cytosol or nucleus *signal molecule has to be hydrophobic - must cross membrane ex: testosterone (steroid) or small, as in nitric oxide (NO) membrane – bind to water soluble ligands

G protein coupled receptor membrane receptor that works with a G protein 1) G protein binds signaling molecule 2) conformational change in receptor, allowing binding to inactive G protein 3) GTP displaces GDP, activating G protein

Receptor tyrosine kinase 1)signal molecule binds to receptors forming a dimer, ea. tyrosine kinase adds a P from ATP 2)this activates receptor initiating a specific response for ea. tyrosine phosphorylated *single ligand activates multiple cellular responses

Structure & function of a tyrosine- kinase receptor

Ligand-gated ion channels on membranes regulate flow of specific ions

Transduction Signal transduction pathways often involve a phosphorylation cascade (multistep, allows amplification of signals) at ea. step, protein kinases phosphorylate & activate many proteins; greatly enhances signal producing large cellular response

Transduction many signaling pathways involve small, nonprotein water-soluble molecules or ions called second messengers ex: Ca+ and cAMP (common) activated 2 nd messengers can initiate phosphorylation cascade causing cellular response

Response leads to regulation of transcription or cytoplasmic activities usu. done by turning specific genes on/off in the nucleus (often the final signaling molecule in the pathway functions as a transcription factor) in cytoplasm, signaling pathways often regulate protein activity instead of synthesis ex: final step in the signaling pathway may affect the activity of enzymes or cause rearrangement of cytoskeleton

cAMP as a second messenger

Apoptosis (programmed cell death) integrates multiple signaling pathways cell systematically is dismantled & digested protects neighboring cells from damage that would occur if a dying cell merely leaked out its digestive enzymes triggered by signals that activate a cascade of “suicide” proteins in verts., normal part of development & is essential for a normal nervous system, immune system operations & morphogenesis of hands, feet in humans