Transcript Cell Structure and Function

Cell Communication
Chapter 11
Cellular Biology
What you need to know!
• The 3 stages of cell communication.
• How G-protein-coupled receptors receive
cell signals and start transduction
• How receptor tyrosine kinase receive cell
signals and start transduction.
• How a phosphorylation cascade amplifies
a cell signal during transduction.
• How a cell response in the nucleus turns
on genes while in the cytoplasm it
activates enzymes.
• What apoptosis means and why it is
important to normal functioning of
multicellular organisms.
Local Signals
• Adjacent cells have a variety of
junctions: tight, desmosomes, gap
• These adjacent cells communicate via
gap junctions (plasmodesmata in
plants), or cell-cell recognition (antigen
and receptor protein)
• Local regulators work to influence
several localized cells
• Neurotransmitters are released into the
synapse and stimulate the target cell
Long-distance Signaling
• Hormones released by the
endocrine system travel
through the blood and activate
cells elsewhere in the body
• Neurons can send electrical
impulses from the brain to any
part of the body
External Signals  Responses
• Animal cells communicate by contact,
secreting regulators (hormones), or
neurotransmitters
There are three stages of cell signaling:
1. Reception: target cell’s detection of signal
molecule
2. Transduction: converting the signal into a
cellular response
3. Response: The specific cellular response
to the signal
Signaling Overview
Reception
• The ligand or signaling molecule can be
sent throughout the entire body
• Only cells with the appropriate receptor
protein “hear” the signal
• Most receptors are membrane proteins
– Water soluble ligands that cannot cross the
plasma membrane
• Others receptors are intracellular
– Lipid soluble ligands can cross membranes
Examples:
• G Protein-Coupled Receptors
• Receptor Tyrosine Kinases
• Ligand-gated ion channels
G Protein-Coupled Receptors
• Membrane imbedded protein receptor works with
a G protein:
1. Ligand binds to a protein receptor causing a
conformational change which binds the receptor
protein to an inactive G protein
2. A GTP molecule now replaces a GDP molecule in
the G protein which activates the G protein
3. The G protein leaves the receptor protein and
binds to an enzyme at the allosteric site, this
activates the enzyme (which in turn activates
transduction)
4. The G protein hydrolyzes the GTP to GDP, this
inactivates the G protein, which in turn detaches it
from and deactivates the enzyme
G Protein-Coupled Receptors
Receptor Tyrosine Kinases
• Two receptor tyrosine kinases work in
tandem to create multiple responses from
a single ligand:
1. Ligands bind to both tyrosine kinases
2. The two tyrosine kinases combine to form
a dimer this activates the tyrosine region of
the protein
3. The tyrosine regions of the dimer are each
phosphoralized by ATP molecules
4. Each tyrosine substructure activates a
specific relay protein, each relay protein
undergoes a conformational change and
in-turn activates a transduction pathway
Receptor Tyrosine Kinases
Ligand Gated Ion Channels
• Ion channels are simple gates that
open when ligands bind to them
• The ions trigger the cellular response
upon entry
• This are most common in the nervous
system where ligands are
neurotransmitters and the ions
change the polarity of the cell
Intracellular Receptors
• Intracellular receptors require ligands
that can cross the plasma membrane
– Steroids and small gas molecules qualify
• The hormone-receptor complex move
into the nuclease and act as
transcrption factors
• These transcription factors turn on
certain genes by stimulating RNA
polymerase to transcribe the gene(s)
• The gene activation triggers the
response
• The receptor carries out the entire
transduction
Transduction: Phosphorylation
Cascade
• The enzyme activated at the end of
reception is a relay protein that
phosphorylates (activates) protein kinases
• The activated protein kinases in-turn
activate other protein kinases (potentially
amplifying the signal)
• The cascade ends with the
phosphorylation of a protein that triggers
the cell’s response
• Protein phosphatases (PP) remove the
phosphate from the kinases and the final
protein so they can all be reused
Transduction
Transduction: Second
Messengers
• Not all transduction molecules are
proteins
• cAMP is a converted ATP (NA)
that cause a phosphorylation
cascade
• Ca2+ can also act as a second
messenger which, when released
from the ER, activate various
proteins
Response
• Change in cellular function based on
a signal transduction induced by a
ligand
• The final product of many
transduction are transcription factors
that activate genes
• Other pathways simply activate
proteins (i.e. enzymes) already in the
cytoplasm
• Signal transductions can lead to
multiple responses from a single
ligand
Signal Transduction Review
Apoptosis
• Advanced cell signaling which ends in
scheduled cell suicide
• Systematic dismantling and digestion of
the cell
• This prevents the digestive and metabolic
enzymes from spilling out and damaging
adjacent cells
• Crucial in: brain development, the immune
system, and morphogenesis of the fingers
and toes