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Cellular Communication Notes

Signal Transduction Pathways

Chemical messages which elicit a response in cells server as a form of communication between cells

Found in all cells

Extremely conserved (similar) in widely different organisms (such as humans and yeast) leads one

to believe that this evolved very early in the history of life

Local Communication in Animal Cells

Used by cells to communicate to their immediate neighbors

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One cell secretes a signal molecule into the extracellular fluid which is picked up by the target cells

One example of this is at the synapse of two neurons

Hormonal Signaling in Plants and Animals

Used by cells to communicate to other cells a great distance away (but still in the same organism)

One cell secrets a signal molecule (hormone) into the blood system (if an animal) or into the

extracellular fluid (if a plant)

he signal molecules travels throughout the body! most likely contacting nearly all cells in the

organism

Only the target cells! however! will have the receptors necessary to elicit the response

The Three Stages of Cell Signaling - Reception, Transduction, Response

"eception

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o # chemical message binds to a protein on the cell surface

ransduction

o he binding of the signal molecule alters the receptor protein in some way$

o he signal usually starts a cascade of reactions known as a signal transduction pathway

"esponse

o he transduction pathway finally triggers a response

o he responses can vary from turning on a gene! activating an en%yme! rearranging the

cytoskeleton

o here is usually an amplification of the signal (one hormone can elicit the response of over

&'molecules

-Protein-Lin!ed Receptor Se"uences

*protein*linked receptor is bound to the plasma membrane$

o #ll *protein*linked receptors have similar structure regardless of the organism in which

they are found

o +even alpha*helices integrate the *protein*linked receptor to the membrane

o +ignal*binding site on outside of cell

o *protein*interacting site on inside of cell

,hen signal molecules binds to *protein*linked receptor! the receptor is activated

#ltered *protein*linked receptor activates a nearby *protein

o *protein * molecule in signal transduction se-uence which has a bound ./ (guanine

diphosphate! a relative of #./ and #/)

he activation occurs when a / displaces the ./ bound to the the *protein$

he activated *protein then binds to another protein! usually an en%yme! and alters itsactivity

his activation is usually temporary as the activated *protein soon hydroly%es the terminal

phosphate on the bound /! forming ./! thereby deactivating the *protein

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he deactivated *protein is available for reactivation if the *protein*linked receptor becomes

activated again

#ll three molecules! the *protein*linked receptor! the *protein! and the target en%yme! remain

bound to the plasma membrane

*protein signal transduction se-uences are extremely common in animal systems

o embryonic development

o human vision and smell

o over 0'1 of all medications used today exert their effects by influencing *protein

pathways

Tyrosine-#inase Receptors - Another $%ample of a Signal Transduction Pathway

yrosine*2inase "eceptors often have a structure similar to the diagram below3

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/art of the receptor on the cytoplasmic side serves as an en%yme which cataly%es the transfer of

phosphate groups from #/ to the amino acid yrosine on a substrate protein

he activation of a yrosine*2inase "eceptor occurs as follows3

o wo signal molecule binds to two nearby yrosine*2inase "eceptors! causing them to

aggregate! forming a dimer

o he formation of a dimer activated the yrosine*2inase portion of each polypeptide

o he activated yrosine*2inases phosphorylate the yrosine residues on the protein

he activated receptor protein is now recogni%ed by specific relay proteins

hey bind to the phosphorylated tyrosines! which cause a conformation change$

he activated relay protein can then trigger a cellular response

o One activated yrosine*2inase dimer can activate over ten different relay proteins! each

which triggers a different response

o he ability of one ligand binding event to elicit so many response pathways is a key

difference between these receptors and *protein*linked receptors (that! and the absence

of * proteins of course$$$)

o #bnormal yrosine*2inases that aggregate without the binding of a ligand have been linked

with some forms of cancer

+ignal ransduction /athways are often complex! having many! many intermediates participating in the

cascade$