Cell Signaling and Chemotaxis Read Chapter 15 of “Molecular Biology of the Cell”

Example for cell signaling in unicellular organisms: chemotaxis in bacteria (move cell optimally in environment),sexual mating in yeast (coordinate conjugation into cell with new assortment of genes)

Signaling cell releases signaling molecules S, target cell responds by means of receptors (usually on cell surface) that bind S and initiate a response in the target cell; in chemotaxis S is an environmental factor

Sexual Mating in Yeast (coordinate conjugation into cell with new assortment of genes)

When a haploid individual is ready to mate, it releases a peptide mating factor that signals cells of the opposite mating types to stop proliferating and prepare to conjugate; the subsequent fusion of two haploid cells of the opposite mating types produces two diploid cells, which then undergo meiosis and sporulate to generate haploid cells with a new assortment of genes (Alberts et al, Chpt. 15)

Cell Signaling and Chemotaxis Read Chapter 15 of “Molecular Biology of the Cell”

Example for cell signaling in unicellular organisms: chemotaxis in bacteria (move cell optimally in environment),sexual mating in yeast (coordinate conjugation into cell with new assortment of genes)

Signaling cell releases signaling molecules S, target cell responds by means of receptors (usually on cell surface) that bind S and initiate a response in the target cell; in chemotaxis S is an environmental factor

before after

John D. Scott and Tony PawsonScientific American

Action of Hormone Receptors

Action of Hormone Receptors

G-proteins are Signal Transducers

Malfunctioning G-proteins disturb the intracellular signaling pathways, altering normal cell functions.

Receptor AmplifierG-protein

Biological effect

Receptor Amplifier

No Biological effect

MissingG-protein

Cell membrane

Cytosol

signal signal

G-proteins transmit and modulate signals in cells. They can activate different cellular amplifier systems.

• smallest G-protein (189 residues, 21KDa mass) • acts as a molecular switch • cycles between an active (GTP-bound) and an inactive (GDP-bound) state• major conformational changes during the signaling cycle take place in the switch I and switch II regions • switching activity regulated by GAP and GEF proteins• activated forms of Ras genes are found in 30% of human tumors.

switch I

switch II

Ras GTP GDP + Pi + 7.3 kcal/mol

Signaling Cycle of Ras

Ras

GDP

Ras

GTP

GDP

GTP

GEF

OFF

Pi

GAP

ON

signal IN

signal OUT

Guanine nucleotideExchange Factor

GTPase ActivatingProtein

T-state

R-state

GTP hydrolysis

Exchange of GDP for GTP is catalyzed by GEF protein

GTP hydrolysis is induced by GAP protein

Conform

atio

nal

chan

ge

Mechanical Cycle of Ras/SpringRas

GTP

GDP

GTP

Pi

GTP hydrolysis

Ras

GDP

Ras

GDP R-state

~ 1ns

TR transiton

Ras/GDP

Ras/GDP

GAP

GAP

T-state

Ras/GTP hydrolysis, induced by GAP, leads to Ras/GDP in T-state

Ras/GDP evolves irreversibly and spontaneously from T-state to R-state

Ras separates from GAP, then exchanges GDP for GTP, and the reverse R-to-T transition takes place

What Happens After GTP Hydrolysis?

Switch I

RAS/GTPRAS/GDP

Switch IIstrong fluctuations

R-statesmall fluctuations

T-state

Switch II helix “melts” altering the contact area of RAS!

The Role of Modules in Signaling

John D. Scott and Tony PawsonScientific American

Scaffolds Speed Signal Transmission

John D. Scott and Tony PawsonScientific American

Neutrophils are our body's first line of defense against bacterial infections. After leaving nearby blood vessels, these cells recognize chemicals produced by bacteria in a cut or scratch and migrate "toward the smell". The above neutrophils were placed in a gradient of fMLP (n formyl methionine- leucine- phenylalanine), a peptide chain produced by some bacteria. The cells charge out like a "posse" after the bad guys.http://www.cellsalive.com/chemotx.htm

Phagocytosis:n Cell Eats Cell

Chemotaxis of neutrophil chasing a bacterium(http://www.hopkinsmedicine.org/cellbio/devreotes/movies.html)

This video is taken from a 16mm movie made in the 1950s by the late David Rogers at Vanderbilt University. It was given to me via Dr. Viktor Najjar, Professor Emeritus at Tufts University Medical School and a former colleague of Rogers. It depicts a human polymorphonuclear leukocyte (neutrophil) on a blood film, crawling among red blood cells, notable for their dark color and principally spherical shape. The neutrophil is "chasing" Staphylococcus aureus microorganisms, added to the film. The chemoattractant derived from the microbe is unclear, but may be complement fragment C5a, generated by the interaction of antibodies in the blood serum with the complement cascade. Blood platelets adherent to the underlying glass are also visible. Notable is the characteristic asymmetric shape of the crawling neutrophils with an organelle-excluding leading lamella and a narrowing at the opposite end culminating in a "tail" that the cell appears to drag along. Contraction waves are visible along the surface of the moving cell as it moves forward in a gliding fashion. As the neutrophil relentlessly pursues the microbe it ignores the red cells and platelets. However, its leading edge is sufficiently stiff (elastic) to deform and displace the red cells it bumps into. The internal contents of the neutrophil also move, and granule motion is particularly dynamic near the leading edge. These granules only approach the cell surface membrane when the cell changes direction and redistributes its peripheral "gel." After the neutrophil has engulfed the bacterium, note that the cell's movements become somewhat more jerky, and that it begins to extend more spherical surface projections. These bleb-like protruberances resemble the blebs that form constitutively in the M2 melanoma cells missing the actin filament crosslinking protein filamin-1 (ABP-280) and may be telling us something about the mechanism of membrane protrusion.

Written by Tom Stossel, June 22, 1999.

Bacterial Motility Typical of Flagellated Bacteria

Explain Tumbling Mechanism

Signaling and Adaptation in Chemotaxis

Receptor Signaling Complexes in Chemotaxis

http://www.genome.ad.jp/kegg/pathway/eco/eco02030.html

Genetics of Chemotactic Signaling System

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Model

Adaptation in Chemotaxis

slowslow

intermediate

fast

Adaptation

End

Signaling and Adaptation in Chemotaxis

Summary of the experiments of Lumsden and Davies showing chemotaxis between neural tissue (trigeminal ganglion) and itstarget (whisker pad). The chemoattraction is specific for (A) the target and (B) the epithelial cells of the target. Moreover, thechemotactic ability of the whisker pad is specific for the trigeminal neurons.