Signaling molecules that function within anorganism to control metabolic processes withincells, the growth and differentiation of tissues,the synthesis and secretion of proteins, and thecomposition of intracellular and extracellularfluids.
Extracellular signaling molecules aresynthesized and released by signaling cells andproduce a specific response only in target cells thathave receptors for the signaling molecules.
The overall process of converting signals intocellular responses, as well as the individualsteps in this process, is termed signaltransduction.
Signal transduction pathways may involverelatively few or many components.
Communication by extracellular signalsusually involves the following steps:
(1) synthesis and
(2) release of the signaling molecule by thesignaling cell;
(3) transport of the signal to the target cell;
(4) binding of the signal by a specific receptorprotein leading to its activation;
(5) initiation of one or more intracellular signal-transduction pathways by the activated receptor;
(6) specific changes in cellular function,metabolism, or development;
(7) removal of the signal, which often terminatesthe cellular response
Endocrine, Hormones
Paracrine, Growth factors
Autocrine,
Receptor Proteins Exhibit Ligand-Bindingand Effector Specificity.
Intracellular Signal Transduction
Many Conserved Intracellular ProteinsFunction in Signal Transduction
GTPase Switch Proteins
In mammals, twenty distinct genes for RGS proteinshave been identified.
At last count the human genome encodes 500protein kinases and 100 different phosphatases.
In some signaling pathways, the receptor itselfpossesses intrinsic kinase or phosphataseactivity; in other pathways, the receptorinteracts with cytosolic or membraneassociated kinases.
Hormone binding to receptors coupled toeither a Go or a Gq protein (see Table 13-1)induces activation of the isoform ofphospholipase C (PLC).
The IP3-mediated rise in the cytosolic Ca2+level is only transient because Ca2+ ATPaseslocated in the plasma membrane and ERmembrane actively pump Ca2+ from the cytosolto the cell exterior and ER lumen, respectively.Furthermore, within a second of its generation,one specific phosphate on IP3 is hydrolyzed,yielding inositol 1,4-bisphosphate, which doesnot stimulate Ca2+ release from the ER.
The activation of protein kinase C in different cellsresults in a varied array of cellular responses,indicating that it plays a key role in many aspectsof cellular growth and metabolism.
In liver cells, for instance, protein kinase C helpsregulate glycogen metabolism by phosphorylatingand thus inhibiting glycogen synthase.
Protein kinase C also phosphorylates varioustranscription factors; depending on the cell type;these induce synthesis of mRNAs that trigger cellproliferation.
