cell communication chapter 11. an overview of cell signaling
TRANSCRIPT
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Cell Communication
Chapter 11
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An overview of Cell Signaling
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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 signal-transduction pathway
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Evolutionary connection
• Scientists think that signaling mechanisms evolved in ancient prokaryotes and single celled eukaryotes• They were adopted for new uses
by multicellular descendents
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• Local regulator – a substance that influences cells in its vicinity• Transmitting cells can secrete
molecules of a local regulator to communicate with nearby cells
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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
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Examples of local regulators that uses paracrine signaling
• Growth factors in animal cells are compounds that stimulate nearby target cells to grow and multiply
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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
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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
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Long distance signaling
• Plants and animals use chemicals called hormones for signaling
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• 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
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• 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
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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
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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
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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
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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 signal-transduction pathway.• Molecules involved in the pathway
are often called relay molecules
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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
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Signal Reception and the Initiation of Transduction
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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
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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
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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
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3 major types of membrane receptors
• G-protein-linked receptors• Tyrosine-kinase receptors• Ion-channel receptors
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G-protein-linked receptors
• Large family of receptor proteins that all have 7 transmembrane helices
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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
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• G-protein receptor systems are diverse in function• They can be shutdown quickly when
the signal molecule is no longer present
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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
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Different from G-protein receptors
• Binding of a signal molecule does not cause the shape of the receptor to change like in the G-protein receptors• Can activate more than 1 cellular
response (unlike G-proteins)
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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)
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• 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
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Ligand gated ion-channel receptors• Protein pores that open or close in
response to a chemical signal• Important in the nervous system
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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
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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
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Phosphorylation cascade• Protein kinases are often the relay
molecules in the signal-transduction 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
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