Cell Communication Notes
Ex. Mating in Sacchromyces cerevisiae yeast

Two types exist: a and α
o
a type make a factor signaling molecules, which bind to specific receptor proteins on
nearby α cells while α make α factors that bind to receptors on a cells
o
Each cell responds to this by growing toward each other until the fusion (or mating) of
two cells of opposite type occurs
Ex. Quorem Sensing in bacteria

Cells secrete small molecules that can be detected by other bacterial cells

Allows them to monitor the density of cells around them = quorem sensing
o
Bacteria use it to coordinate behavior so they can perform activities that are only
productive when performed by several cells in synchrony

Ex. Biofilm = an aggregation of bacterial cells adhered to a surface for feeding

Ex. Slimy coating on teeth in morning  responsible for cavities
LOCAL & LONG-DISTANCE SIGNALING
Local Signaling includes:
1) Cell Junctions (Direct Contact)

Signaling molecules dissolved in the cytosol can pass freely between adjacent cells

Gap Junctions (animals) and Plasmodesmata (plants)
2) Cell-Cell Recognition (Direct Contact via membrane-bound surface molecules)

Used in embryonic development and the immune response
3) Local Regulators

Messenger molecules are secreted by the signaling cell

Travel short distances and influence cells in the vicinity
o
Ex. Growth Factors in Animals = compounds that stimulate nearby target cells to
grow and divide

Numerous cells can simultaneously receive and respond to the molecules of growth factor
produced by a single cell in their vicinity = paracrine signaling
o
Ex. Animal Nervous System

Synaptic Signaling = an electrical signal along a nerve cell triggers the
secretion of neurotransmitter molecules carrying a chemical signal

These diffuse across the synapse (the narrow space between the nerve
cell and its target cell), triggering a response in the target cell
Long-Distance Signaling includes:
1) Hormones = chemical messenger molecules that are released by one cell that trigger a response in
another (target) cell

Ex. Ethylene = gaseous plant hormone that promotes fruit ripening
o
Why “one bad apple…”
2) Nervous System

Electrical signal travels the length of a nerve cell and is converted back to a chemical
signal when a signaling molecule is released and crosses the synapse to another nerve cell

Converted back into an electrical signal, etc.
THREE STAGES OF CELL SIGNALING
The “Fight or Flight” Response

Sutherland (1971)

Investigated how epinephrine (a.k.a. adrenaline) stimulates the breaks down glycogen within liver
and muscle cells
o
Glycogen  glucose-1-phosphate  glucose-6-phosphate (which can enter glycolysis
and produce more ATP)
o

OR it can continue on to be converted into glucose  fuels cells throughout the body
Discovered that epinephrine stimulates glycogen breakdown by somehow activating Glycogen
Phosphorylase
o
Adding epinephrine to a test tube of glycogen + cytosol  no breakdown occurs
o
Adding epinephrine to a test tube of glycogen + cytosol + membrane fragments 
breakdown occurs

Therefore, (1) epinephrine does NOT interact with the enzyme directly and (2)
the membrane has some role in the process
1) RECEPTION = the target cell detects a signaling molecule coming from outside the cell

Signaling molecule binds to a receptor protein located at the cell’s surface or inside the
cell
o
Signaling molecule is complementary in shape to a specific site on the receptor
molecule
o
Signaling molecule is the Ligand = a molecule that specifically binds to another
molecule
o
Usually causes it to undergo a change in shape, which activates the receptor,
enabling it to interact with other cellular molecules

Plasma Membrane Receptors:
o
Most water-soluble (polar or ionic) ligands
o
Many diseases are due to malfunctions of these
a) G-Protein Coupled Receptors (GPCR)

Nearly 1000 exist in humans

Works with G protein

G protein is “off” when bound to GDP and “on” when bound to GTP
o

(See Diagram for details)
Used by yeast mating factors, epinephrine responses, neurotransmitters,
and many hormones

Have roles in embryonic development and sensory reception

Malfunctions responsible for illnesses, like cholera, pertussis, and
botulism
b) Receptor Tyrosine Kinases (RTKs)

Kinase = enzyme that transfers a phosphate group from one molecule to
another

o
(See Diagram for details)
Abnormal function of these is related to many types of cancers
c) Ligand-Gated Ion Channel Receptors

Changes shape when ligand attaches, either opening or closing an ion
channel

Voltage-gated ion channels use electrical signals instead of ligands
o
Useful in nervous systems

Intracellular Receptors:
o
Found in either the cytoplasm or nucleus of target cells
o
Usually signaling molecules for these are small and/or hydrophobic (can cross the
hydrophobic barrier of the plasma membrane)

Ex. Steroid hormones and thyroid hormones

Ex. Testosterone only impacts cells with receptors for testosterone  activates
transcription factors for growth hormones that turn on specific genes for male sex
characteristics in development
2) TRANSDUCTION = cascades of molecular interactions relay signals from receptors to target
molecules in the cell  Signal Transduction Pathway

Usually includes activation of proteins by addition or removal of phosphate groups or
release of other small molecules or ions that act as messengers


Benefits include:
o
Ability to amplify a signal very quickly
o
Many opportunities to coordinate and regulate the pathway
Many of the relay molecules in signal transduction pathways are protein kinases 
transfer phosphates from ATP to a protein
o
About 2% of our genes code for protein kinases
o
Most cytoplasmic protein kinases phosphorylate either of two other amino acids:
serine or threonine (instead of tyrosine)
o
May create a “phosphorylation cascade”  each phosphorylation event creates a
change in shape of a molecule, which leads to either activation or deactivation of
a step in the pathway


Abnormal function of these can lead to cancer
Equally important are protein phosphatases = enzymes that rapidly remove phosphate
groups from proteins  dephosphorylation
o
Make protein kinases available for reuse, allowing cell to respond again to an
extracellular signal

Second Messengers = small, non-protein, water-soluble molecules or ions that spread
quickly throughout a cell by diffusion
o

Most common ones: cAMP and Ca2+ ions
Cyclic AMP (cAMP)
o
Usually activates a serine/threonine kinase called protein kinase A
o
Protein kinase A usually then phosphorylates other various proteins, depending
on cell type
o

Some G protein systems may inhibit adenlylyl cyclase  inhibitory G protein
Ex. Sutherland found epinephrine increases the amount of cAMP in the cytosol
o
Adenylyl cyclase converts ATP to cAMP in response to the epinephrine signal
o
Attachment of epinephrine triggers activation of adenylyl cyclase, which
produces cAMP
o

Phosphodiesterase converts cAMP to AMP, deactivating it, so it is short-lived
Ca2+ Ions
o
More widely used than cAMP as a second messenger
o
Used in both G protein and tyrosine kinase pathways
o
Concentration is usually low in cytosol, but is much higher in the E.R.

In response to signals, second messengers are released: Inositol Triphosphate
(IP3) and diacylglycerol (DA)

Formed from cleavage of a certain kind of phospholipid in the plasma
membrane

IP3 triggers the release of Ca2+ from the ER
3) RESPONSE = Cell signaling leads to regulation of transcription or cytoplasmic activities

The final activated molecule in a signaling pathway may:
o
Function as a transcription factor  turns a gene “on” or “off”
o
Cause the opening or closing of an ion channel
o
Activate a specific enzyme
o
Regulate other cellular activities, like rearrangement of the cytoskeleton