Post on 28-Dec-2015
transcript
Cell Signaling&
Communication
Cellular Signaling•cells respond to various types of signals
•signals provide information about a cell’s environment
signal molecules are
chemically
diverse
gaseous hydrocarbon
steroid
catecholamine
peptide
Signal Receptors•cells respond to signals only if they have the right signal receptors–receptors bind or absorb specific signals
–cells without appropriate receptors “ignore” specific signals
some signals communicate local information
some signals communicate over long distanceFigure 15.1
•signaling evokes specific responses from specific cells–a signal does not specify a cell’s response
–a cell’s response is determined by the cell
Cell Responses
•The “Fight or Flight” Response–epinephrine (adrenaline) is released into the blood stream
–receptors in different tissues bind epinephrine•heart: beats faster, more strongly•digestive system: blood vessels constrict•liver: cleave glycogen; release glucose•adipose tissue: breaks down triglycerides
Cell Responses
•cell responses involve three components–receptor–transduction mechanism (amplifier)
–effect
Cell Responses
change in the environment
signal: solute in intermembrane spacereceptor: EnvZFigure 15.2
transduction: >autophosphorylate EnvZ >phosphorylate OmpR, >activate OmpC amplification: one gene, many proteinseffect: block pores
Receptors •each cell makes a specific group of receptors so it can respond to a specific set of signals–many in the plasma membrane –some in the cytoplasm or the nucleoplasm
•a receptor has a binding site for its ligand, the signal molecule
•ligand binding causes a conformational change in the receptor
Human growth hormone
Human growth Hormone receptor
Figure 15.3
ligand binding causes a
conformational change in the
receptorFigure 15.4
Receptors
•different types of receptors react to signals differently–gated ion channels•regulate passage of Na+, K+ Ca2+, Cl-
•ligand binding causes the channel to open
acetylcholine receptor responds to acetylcholine
Figure 15.5
Receptors•different classes of receptors react to signals differently–receptor protein kinases•ligand binding activates a cytoplasmic kinase domain–dimerizaton often occurs–autophosphorylation further activates the receptor–phosphorylation of cellular targets begins signal transduction
insulin recepto
r is a
protein kinaseFigure 15.6
Receptors•different classes of receptors react to signals differently–G protein-linked receptors•ligand binding causes the receptor to bind an inactive G protein-GDP•G protein is activated to G protein-GTP•GTP-bearing subunit diffuses to effector•effector initiates cell response–G protein may activate or inhibit effector
signal binding & G-protein activationFigure 15.7
Receptors•different classes of receptors react to signals differently–cytoplasmic receptors bind nonpolar ligands•ligand binding lets the receptor enter the nucleus & activate transcription
–nuclear receptors bind ligands in the nucleus•receptors without bound ligands repress transcription•ligand binding activates transcription
Figure 15.8
Transducers
•signal transduction–converting the information “signal X has arrived” into a cellular response
–may be “direct” by the activated receptor
–may be “indirect” by a second messenger
–enzymatic steps in signal transduction pathways amplify signal strength
transduction of a growth
factor signal
amplifies the signal at
several stepsFigure 15.9
Transducers •second messengers can trigger multiple responses one signal–cAMP•synthesized by a G protein-activated membrane-bound adenylyl cyclase•binds to ion channels in some cells•binds to protein kinases in other cells•may do both in some cells
cAMP synthesis
from ATP
Figure 15.10
Transducers •second messengers can trigger multiple responses one signal–receptor activates G protein activates effector, phospholipase C
–phospholipase C cleaves PTI into inositol triphosphate (IP3) and diacylglycerol (DAG)•DAG activates a membrane-bound, Ca2+-dependent protein kinase C (PKC)•IP3 opens a Ca2+ channel in ER membrane•Ca2+ activates PKC•PKC phosphorylates many cellular target molecules
phosphatidyl inositol bisphosphat
e
phospholipase C
IP3 + DAG
glycerol
phosphate
DAG
IP3
second messengers IP3 & DAG activate PKCFigure 15.11
Transducers•second messengers can trigger multiple responses one signal–Ca2+ is a common second messenger•a steep Ca2+ gradient exists across ER & plasma membranes•opening gated Ca2+ channels raises cytoplasmic [Ca2+]•Ca2+ activates many cellular targets•Ca2+ activation often involves calmodulin
Transducers•second messengers can trigger multiple responses one signal–nitric oxide (NO) is a gas–NO synthase is activated by Ca2+ in response to IP3 after an acetylcholine receptor binds its ligand
–NO diffuses to a neighboring smooth muscle cell and activates an enzyme causing relaxation
smooth muscle relaxation response to acetylcholine
signa
Relax!!
Figure 15.13
Regulation of Signal Transduction
•activation of signal transduction is opposed by inactivating factors–NO breaks down very rapidly–Ca2+ channels open very briefly & Ca2+ pumps remove Ca2+ immediately
–protein phophatases inactivate P-enzymes
–GTPases return G proteins to inactive form
–cAMP is converted to AMP•members of different types of pathways interact in regulatory roles
Effects •cell responses include –opening membrane channels•important in sensory cells –odorant receptors send nerve impulses to the brain
activated G protein activates adenylyl cyclase
cAMP opens ion channels to signal the brain
1. odorant receptors are displayed on the surface of nasal epithelial cells;
2. each receptor binds a particular odorant molecule;
3. odorant binding activates a G proteinFigure 15.14
Effects •cell responses include –opening of membrane channels–alteration of enzyme activities•covalent modification (phosphorylation) or allosteric modification (cAMP) cause expose active sites–glycogen metabolism in the liver is regulated by a protein kinase cascade in response to epinephrine
epinephrine
causes glucose release from
glycogenstores in the liverFigure 15.15
Effects
•cell responses include –opening of membrane channels–alteration of enzyme activities
–changes in gene transcription•a common response is new protein synthesis–the Ras pathway stimulates cell division in response to growth factors
a signal to divide & its transduction pathway
Figure 15.9
Direct Intercellular Communication
•animal cells communicate directly, through gap junctions–small molecules diffuse between cells•ATP•second messengers•waste or nutrient molecules
–tissue function can be coordinated
gap junctions between
adjacent animal cells allow direct
communicationFigure 15.16
~ 1 nm
Direct Intercellular Communication
•plant cells communicate through plasmodesmata–lined by plasma membrane–occupied by desmotubules–permit rapid exchange of small molecules
plant cells communicate via plasmodesmataFigure 15.17