Cell Signaling
• Cells communicate in various ways.
– The type of communication used by each cell is
based on the type of information that needs to be
passed.
– The signal transduction pathway is a series of
steps where the surface recognition of a message
(molecule, gas, hormone, ion,...) is converted into
a specific cellular response.
Examples of cell to cell signaling
• Cell junctions - allow molecules to pass freely between cells
– animal - Gap
– plants - Plasmodesmata
• Paracrine signaling - messenger molecules
– secreted by signaling cell
– short distance
– growth factors
• Synaptic signaling - vesicles
– electrical signal triggers vesicle formation
– vesicle fuses with nearby cell releasing the neurotransmitter
• Endocrine signaling - hormones
– long distance
– dumped into the blood stream for widespread delivery
3 Basic Steps
• Reception
• Transduction
• Response
Reception
– cell signal (ligand) binds to a
surface receptor on the outside
(or inside) of the cell
– large molecules and hydrophilic
molecules must bind to a surface
receptor (3-types)
• G-proteins - associated with GTP
– protein receptor binds ligand &
causes a change in G-protein from
GDP to GTP
– activated complex travels to an
enzyme (still in the membrane) and
activates it causing a cellular
response in the cytosol
– Some signal molecules are not
proteins but are considered
"secondary messangers"
» cAMP is the secondary
messenger of a G-protein
Reception
• Tyrosine Kinases
– transmembrane proteins have binding sites and multiple tyrosine
(Tyr) sites in the cytosol
– protein receptor binds the ligands and causes 2 complexes to
move together
– the phosphorylation of the active Tyr by ATP
» the active sites then activate relay proteins to cause a
cellular response
» 1 tyrosine kinase may activate up to ten different processes
Reception
• Ligand-gated Ion channels
– binding of the ligand causes a conformation change and opening
of a gate to ions of a specific charge
• hydrophobic molecules (steroids) and small molecules (NO)
can pass directly through the plasma membrane to bind with
receptors in the cytosol
Transduction
• after binding the surface
protein changes to a form
that brings about cellular
change
– change may be in the
cytoplasm or in the nucleus
– involves the change in
inactive compounds to their
active conformations
– efficiency may be increased
by the presence of scaffolding
proteins
• protein complex with multiple
relay protein receptor sites that
carry out a cascade of reactions
Response
• cellular change brought about by the transduction of the signal
• involves signal amplification
– activated complex during transduction may activate many molecules
before becoming inactive again
• all different cells are specific in their response
– they all have differing amounts and types of protein complexes and
receptors
• inactivation of the receptor occurs as the product is produced and
the molecule departs from its receptor