Cell Communication
Chapter 11
An overview of Cell Signaling
Signal-transduction pathway
• Process by which a signal on a
cell’s surface is converted into
a specific cellular response is a
series of steps called signaltransduction pathway
Evolutionary connection
• Scientists think that signaling
mechanisms evolved in ancient
prokaryotes and single celled
eukaryotes
• They were adopted for new uses
by multicellular descendents
• Local regulator – a substance that
influences cells in its vicinity
• Transmitting cells can secrete
molecules of a local regulator to
communicate with nearby cells
Paracrine signaling
• Paracrine signaling is a type of
local signaling in animals in
which numerous cells can
simultaneously receive and
respond to the signal of a
single cell in their vicinity
Examples of local regulators that
uses paracrine signaling
• Growth factors
in animal cells
are
compounds
that stimulate
nearby target
cells to grow
and multiply
Synaptic Signaling
• Specialized type of local signaling
that occurs in animal nervous
systems
• An electrical signal that is
transmitted the length of a nerve
cell to its target cell without
touching the target cell
Example of synaptic signaling
• Nerve cells produce a
chemical signal called
a neurotransmitter
that diffuses to a
single target cell that
is almost touching
the signaling cell
Long distance signaling
• Plants and animals use
chemicals called hormones
for signaling
• In animals – hormonal signaling
is called endocrine signaling
–Hormone molecules are
released into vessels of the
circulatory system and travel
to other parts of the body
• In plants
– sometimes they move through
vessels
–More often they are diffused
through the air as a gas
•Example: plant hormone
ethylene is a gas that helps
promote fruit ripening
Direct Contact
• Both plants and animals have cell
junctions that promote the direct
contact between 2 cells
• Signaling substances dissolve in the
cytosol and can pass freely between
adjacent cells
• Animal cells can communicate directly
between the molecules on their cell
surfaces
Three stages of cell signaling
• Earl W. Sutherland discovered how the
hormone epinephrine acts on cells
• Sutherland suggested that cells
receiving signals went through three
processes:
–Reception
–Transduction
–Response
Reception
• First step when the target cell
detects a signal coming from outside
the cell
• Chemical signals are detected when
they bind to a cellular protein in the
cell membrane of the target cell
Transduction
• In this stage the signal is converted
into a form that can bring about a
specific cellular response
• Transduction can occur in a single
step or multiple steps of the signaltransduction pathway.
• Molecules involved in the pathway
are often called relay molecules
Response
• Transduced signal triggers a
specific cellular response
• Cellular response can be anything
from activation of certain genes
in the nucleus to catalysis by an
enzyme
Signal Reception and the
Initiation of Transduction
Signal molecule binds to a receptor protein
causing the protein to change shape
• Signal molecules are
complementary in shape to a
specific site on the receptor and
attaches there
• Like a lock and key
Signal molecule behaves as a ligand
• A ligand is a small molecule that
specifically binds to a larger one
• Sometimes causes receptor protein
to change shape
• For some, this shape change
activates the receptor
Signal receptors
• Most signal are water soluble
molecules that are too large to
pass freely through the cell
membrane
• They must utilize a signal receptor
protein embedded in the cell
membrane to transmit information
from the extracellular environment
to the inside of the cell
3 major types of membrane receptors
• G-protein-linked receptors
• Tyrosine-kinase receptors
• Ion-channel receptors
G-protein-linked receptors
• Large family of receptor proteins
that all have 7 transmembrane
helices
G-proteins
• Act as an on/off switch
• If GDP (guanosine diphosphate) is
bound – the G-protein is inactive
• If GTP (guanosine triphosphate) is
bound – it is active
• G-protein receptor systems are
diverse in function
• They can be shutdown quickly when
the signal molecule is no longer
present
Tyrosine-Kinase Receptors
• These receptors are commonly used for
growth factors
• Characterized by having enzymatic activity
• Tyrosine kinase is an enzyme that catalyzes
the transfer of phosphate groups from ATP to
the amino acid tyrosine on a substrate protein
Different from G-protein receptors
• Binding of a signal molecule does
not cause the shape of the
receptor to change like in the Gprotein receptors
• Can activate more than 1 cellular
response (unlike G-proteins)
How it works
• The ligand binding causes
phosphate groups from ATP
(adenosine triphosphate) to
aggregate (combine) with the
tyrosines forming a
phosphorylated dimer (a protein
consisting of 2 polypeptides)
• This aggregation activates the
tyrosine-kinase receptor to bind to
specific intracellular relay proteins
• The activation of the relay proteins
initiates a signal-transduction
pathway leading to a variety of
specific cellular responses
Ligand gated ion-channel receptors
• Protein pores that open or close in
response to a chemical signal
• Important in the nervous system
Intracellular Receptors
• Some receptors are proteins in the
cytosol or on the nucleus of a cell
• In order for these to work, the signal
must be able to pass through the cell
membrane
• Many of these signals are hydrophobic
so they can pass through easily
• Examples include steroid and thyroid
hormones
Protein phosphorylation
• Most proteins are activated by
adding one or more phosphate
groups to it – this is called
phosphorylation
• Protein kinase is the enzyme that
commonly transfers phosphate
groups from ATP to a protein
Phosphorylation cascade
• Protein kinases are often the relay
molecules in the signaltransduction pathway
• They often act on each other to
trigger another protein to be
phosphylated
• This can occur numerous times
until finally triggering the target
protein to elicit a cellular response