BIOL230W Weeks 6 & 7 Material Fall 2013 1
Week 6 Material: Cell Signaling
• Cell to cell communication can take various forms. Juxtacrine signaling
occurs between cells that are in close or direct contact with each other while
paracrine signaling occurs between cells over short distances. Cells involved in
endocrine signaling communicate over long-distances.
• A receptor ligand is a molecule that binds to a cell receptor, causing the protein
to change its conformation to relay a cell signal. Hormones (ex.
norephinephrine), neurotransmitters (ex. GABA), cytokines, and growth factors
are examples of receptor ligands.
• Not all cellular receptors are found on the membrane. Some cellular
receptors for hydrophobic molecules like estrogen that can easily diffuse through
the phospholipid bilayer are found inside the cell.
Signal Transduction – Key Concepts
• Nonpolar receptors are generally found inside the cell since nonpolar molecules
can easily diffuse through the cell’s phospholipid membrane.
• Major changes occurs to a receptor after a ligand binds. Ligand binding
induces part of the receptor to change shape. This conformational change
usually activates enzyme activity in the receptor or frees up another binding site
so that an intracellular signal can bind and relay the signal through the cell.
• Second messengers are molecules that transmit signals from cell surface
receptors to targets inside of the cell in the cytoplasm or nucleus.
• The role of GTP in G-protein coupled receptors: The G-protein coupled
receptor is initially associated with inactivated GDP-bound G-protein. When a
ligand binds to the G-protein coupled receptor, the G-protein becomes activated
and GTP binds to the Gα subunit in the place of GDP, causing the Gα subunit to
separate from the receptor and signal effector proteins down the line.
• Activated G-protein typically complete release second messengers.
• A kinase is an enzyme that catalyzes the addition of a phosphate group to a
• A phosphatase is an enzyme that can remove the phosphate group.
• Kinases can amplify a signaling event by triggering a cascade of phosphorylation
within the cell. For example, activated receptor tyrosine kinases can trigger small
G proteins that trigger guanine nucleotide exchange factors that in turn trigger
more small G proteins to be activated, thereby amplifying the original signal.
• The Ras, Rho, and Raf family of proteins are collectively referred to as
small G proteins.
• Integrin lacks kinase activity, but it can set off a signaling cascade that
involves an increase in kinase activity. Integrin can interact with intracellular
protein kinases such as integrin-linked kinase and adaptor molecules to pass on
signals from the extracellular matrix to regulate the timing of cell proliferation,
differentiation, and programmed cell death. BIOL230W Weeks 6 & 7 Material Fall 2013 2
• Signal transduction occurs when an extracellular signaling molecule binds to a
receptor protein on the surface of a cell membrane and the receptor triggers
intracellular changes to convey the signal into the cell.
• Second messengers are generally found in the cytoplasm or nucleus of the cell.
• Second messengers can be produced and released by enzymes or by the
opening of ion channels. Their function can be regulated so that the signaling
activity is localized to a specific time and part of the cell.
• A primary effector produces the secondary messenger signal.
o The primary effector in the cAMP system is Adenylyl cyclase and the
primary effector in the phosphoinositol system is Phospholipase C.
o The second messenger in the cAMP system is cAMP. The second
messengers in the phosphoinositol system are IP and D3G.
Week 7 Material: Cellular Replication & Mitosis