02.08.10Lecture 10:

Cell Communication II

GPCR signaling is inactivated by arrestins

Calcium transients trigger many cellular processes

• Many signals trigger Ca+2 release (not just GPCRs)

• Skeletal muscle contracts in response to calcium release

• Ca+2 triggers regulated secretion (I.e. in neurons)

• Sperm entry triggers a calcium wave during fertilization

Fertilization induces a rise in Ca+2 that starts embryogenesis

• Starfish egg loaded with a calcium-sensitive fluorescent dye

• Fertilized in vitro and monitored by fluorescence microscopy

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Fertilization induces a rise in Ca+2 that starts embryogenesis

The effects of calcium in the cytosol are mediated by calcium-binding proteins

• Protein kinase C: activated by calcium & DAG

• Calmodulin: activated by conformational change by calcium binding

• Ca+2 /calmodulin-dependent kinase (Cam-kinase): activated by Ca+2 -calmodulin

Activation of CaM-kinase

3 classes of cell surface receptors

Enzyme-linked receptors fall into 3 categories

1. Receptor tyrosine kinases

2. Cytokine receptors

3. TGF-β receptors

1. Receptor tyrosine kinases

• Ligands are soluble or membrane-bound peptide or protein hormones (I.e. insulin, growth factors)

• Some RTKs have been identified in studies of human cancers - mutant forms send proliferative signals to cells in absence of signal

Receptor tyrosine kinases autophosporylate themselves

• Phosphorylate tyrosine residues on target proteins and on themselves

• Activation of a receptor tyrosine kinase stimulates assembly of a signaling complex

Tyrosine receptor signaling complexes

• As many as 10 or 20 downstream signaling molecules - differ between receptors

• Components such as phospholipases, lipid kinases, other protein kinases, and Ras

• Complexes are disassembled by protein tyrosine phosphatases

Receptor tyrosine kinases activate the G protein Ras

GTP binding to Ras induces a conformational change

Ras activates a cascade of kinases called MAP-kinases

Scaffoldingprotein

RTKs can activate the PI-3-kinase-Akt pathway

Activated Akt serves as a survival signal for the cell and stimulates cell growth

2. Cytokine receptors

• Cytokines are small secreted proteins

• Control growth and differentiation of many types of tissues (I.e. induce formation of different types of blood cells, interferons)

Cytokine receptors signal to the nucleus in a direct pathway

3. TGFβ receptor signaling

• TGFβ - transforming growth factor β• A number of related extracellular signaling molecules

important during development

• Exert anti-proliferative signals to cells - loss of function can contribute to malignancy

• Mutations in this pathway are often associated with pancreatic cancers but also implicated in colon, liver, and gastric tumors

TGF-β receptors activate gene regulatory proteins at the membrane

Signaling pathways exhibit a high degree of interconnectedness

Some signaling proteins act to integrate incoming signals

Receptor signaling may be inactivated via different mechanisms