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Lecture 18

BIOL 3113 Lecture Notes - Lecture 18: Hydrophile, Cell Membrane, Alpha Helix


Department
BIOL
Course Code
BIOL 3113
Professor
Barbara S
Lecture
18

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18 Cell Signaling
Cells receive (from other cells and from their environment), send and interpret a variety
of signals
Signals/information comes in variety of forms and often have to be converted from one
form to another
Signal transduction - process of signal conversion
Signaling cell produces a particular signal molecule that is detected/recognized by the
target cell
Target cell has to have receptor proteins that recognize and respond to the signal
molecule
Types of signaling:
*Endocrine
*Paracrine
*Neuronal
*Contact-dependent
Types of signaling:
Endocrine
signaling cell secretes a signal that travel throughout the body (via blood stream) and
eventually binds to a receptor on the target cell long-range signaling
Paracrine
signaling cell secretes a signal that travels locally and binds to a receptor on a nearby
target cell local signaling
Neuronal
neuron sends an electrical signal along its membrane which is transduced into a chemical
signal where it contacts (synapse) the target cell very rapid local or long-range
signaling
Contact-dependent
a protein on the surface of the signaling cell binds to a receptor protein on the surface of
the target cell intimate local signaling
Receptors
Whether or not a cell responds to a signal and how it responds depends on the nature of
its signal receptors
The type and number of receptors varies from cell type to cell type and over time
Different cells may respond differently to the same signal molecule

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Two types of signal receptors on target cells:
*Cell surface receptors - usually transmembrane proteins
*Intracellular receptors - soluble cytosolic or nuclear proteins
Intracellular signaling pathways
After binding of signaling molecule to its receptor, the message is passed from one
intracellular signaling molecule to another until signal reaches its final target inside the
cell
Receptors relay signals via pathways of molecular interactions within cells - signaling
cascades
Signaling cascades can:
transfer signals
transform signals
amplify signals
distribute signals
modulate signals
Intracellular Receptors
Intracellular receptors are cytosolic or nuclear molecules that bind to chemical signals
(small hydrophobic molecules) that can cross the plasma membrane
Best-studied example - steroid hormone receptor family
Examples of signals that bind to steroid hormone receptors:
*steroid hormones
cortisol
estradiol
testosterone
*thyroid hormones
*vitamin D
*retinoids
Steroid receptor proteins are DNA-binding proteins that regulate genes
ligand-binding changes the configuration of the receptor, altering its ability to
bind to DNA, thereby activating or inhibiting transcription
Nitric oxide (NO) - example of other
molecules acting through intracellular receptors
Cell Surface Receptors
Majority of signaling molecules (proteins, peptides, water-soluble molecules) are too
large or hydrophilic to cross the plasma membrane
These signaling molecules bind to receptors that are transmembrane proteins
Transmembrane receptors receive a signal on the outside of the cell and relay it in a new
form into the cell interior
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Proteins as Molecular Switches
Intracellular signaling proteins can function as molecular switches:
*signal molecules cycle from active to inactive state
*signal can be passed from one molecule to another giving rise to a cascade of signaling
events
Examples:
*Signaling by phosphorylation
signal protein activated by phosphorylation
mediated by a protein kinase
requires ATP
signal protein deactivated by dephosphorylation
mediated by a protein phosphatase
*G-proteins
G-protein activated by binding of GTP
G-protein inactivated by hydrolysis of GTP
Three classes:
*Ion-channel-linked receptors
*G protein-linked receptors
*Enzyme-linked receptors
For many signaling molecules there is more than one type of extracellular receptor (e.g.
acetylcholine acts on skeletal muscle cells via an ion-channel-linked receptor and in heart
muscle cells it works through a G-protein-linked receptor)
*Ion-channel-linked receptors or transmitter-gated ion channels
Mainly responsible for the rapid transmission of signals across synapses in the nervous
system and in the electrically excitable cells such as muscle
signal binding opens a transmembrane channel
change in intracellular ion concentration or membrane potential changes cell function
*G protein-linked receptors
signal binding stimulates cytosolic part of receptor to bind a G protein activates G
protein
the G protein binds to one or more effector proteins and initiates a signaling cascade
*G protein-linked receptors
Largest family of cell surface receptors; mediate response to a large variety of signaling
molecules (hormones, local mediators, neurotransmitters)
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