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Chapter 9: Cell Communication Guided Notes
Terms in this set (50)
A signaling molecule that binds to a specific receptor protein, initiating signal transduction in cells.
A diverse group of membrane receptors that when activated have kinase enzymatic activity. Specifically, they phosphorylate proteins on tyrosine. Their activation can lead to diverse cellular responses.
receptor tyrosine kinase (RTK)
The events that occur within a cell on receipt of a signal, ligand binding to a receptor protein. Signal transduction pathways produce the cellular response to a signaling molecule.
Difference between signal transduction with hydrophobic ligands and hydrophilic ligands.
The location of the receptor can either be intracellular, for hydrophobic ligands that can cross the membrane, or in the plasma membrane, for hydrophilic ligands that cannot cross the membrane.
Cells can communicate through any of four basic mechanisms, depending primarily on the distance between the signaling and responding cells. These mechanisms are
(1) direct contact, (2) paracrine signaling, (3) endocrine signaling, and (4) synaptic signaling.
A type of chemical signaling between cells in which the effects are local and short-lived.
Two cells in direct contact with each other may send signals across gap junctions.
In paracrine signaling, secretions from one cell have an effect only on cells in the immediate area.
In endocrine signaling, hormones are released into the organism's circulatory system, which carries them to the target cells.
Chemical synapse signaling involves transmission of signal molecules, called neurotransmitters, from a neuron over a small synaptic gap to the target cell.
Chemical reaction resulting in the addition of a phosphate group to an organic molecule. Phosphorylation of ADP yields ATP. Many proteins are also activated or inactivated by phosphorylation.
The removal of a phosphate group, usually by a phosphatase enzyme. Many proteins can be activated or inactivated by dephosphorylation.
An enzyme that adds phosphate groups to proteins, changing their activity.
Any of a number of enzymes that removes a phosphate group from a protein, reversing the action of a kinase.
- No extracellular signal-binding site
- Receives signals from lipid-soluble or noncharged, nonpolar small molecules
- Receptors for NO, steroid hormone, vitamin D, and thyroid hormone
- Multipass transmembrane protein forming a central pore
- Molecular "gates" triggered chemically to open or close
Cell-Surface Receptors; Chemically gated ion channels
- Single-pass transmembrane protein
- Binds signal extracellularly; catalyzes response intracellularly
- Phosphorylation of protein kinases
Cell-Surface Receptors; Enzymatic receptors
- Seven-pass transmembrane protein with cytoplasmic binding site for G protein
- Binding of signal to receptor causes GTP to bind a G protein; G protein, with attached GTP, detaches to deliver the signal inside the cell
- Peptide hormones, rod cells in the eyes
Cell-Surface Receptors; G protein-coupled receptors
Membrane receptors include three subclasses
Channel-linked receptors, Enzymatic receptors, and G protein-coupled receptors
Chemically gated ion channels form a pore in the plasma membrane that can be opened or closed by chemical signals. They are usually selective, allowing the passage of only one type of ion.
Enzymatic receptors bind to ligands on the extracellular surface. A catalytic region on their cytoplasmic portion transmits the signal across the membrane by acting as an enzyme in the cytoplasm.
G protein-coupled receptors (GPCRs) bind to ligands outside the cell and to G proteins inside the cell. The G protein then activates an enzyme or an ion channel, transmitting signals from the cell's surface to its interior.
G protein-coupled receptors
A small molecule or ion that carries the message from a receptor on the target cell surface into the cytoplasm.
Steroid hormone receptors affect gene expression
The nonpolar structure allows these hormones to cross the membrane and bind to intracellular receptors. The location of steroid hormone receptors prior to hormone binding is cytoplasmic, but their primary site of action is in the nucleus. Binding of the hormone to the receptor causes the complex to shift from the cytoplasm to the nucleus. As the ligand-receptor complex makes it all the way to the nucleus of the cell, these receptors are often called nuclear receptors.
Intracellular receptors, primarily for steroid hormones, that are found in both the cytoplasm and the nucleus. The site of action of the hormone-receptor complex is in the nucleus where they modify gene expression.
Figure 9.5 Intracellular receptors regulate gene transcription.
Hydrophobic signaling molecules can cross the plasma membrane and bind to intracellular receptors. This starts a signal transduction pathway that produces changes in gene expression.
The primary function of steroid hormone receptors, as well as receptors for a number of other small, lipid-soluble signal molecules such as vitamin D and thyroid hormone,
is to act as regulators of gene expression.
A steroid hormone receptor has three functional domains:
1. a hormone-binding domain,
2. a DNA-binding domain, and
3. a domain that can interact with coactivators to affect the level of gene transcription.
In a steroid hormone receptor's inactive state, the receptor typically cannot
bind to DNA because an inhibitor protein occupies the DNA-binding site. When the signal molecule binds to the hormone-binding site, the conformation of the receptor changes, releasing the inhibitor and exposing the DNA-binding site, allowing the receptor to attach to specific nucleotide sequences on the DNA. This binding activates (or, in a few instances, suppresses) particular genes, usually located adjacent to the hormone-binding sequences.
A protein that functions to link transcriptional activators to the transcription complex consisting of RNA polymerase II and general transcription factors.
Other intracellular receptors act as enzymes
- receptor for nitric oxide (NO)
- small gas molecule diffuses readily out of the cells where it is produced and passes directly into neighboring cells, where it binds to the enzyme guanylyl cyclase
- catalyze the synthesis of cyclic guanosine monophosphate (cGMP), an intracellular messenger molecule that produces cell-specific responses such as the relaxation of smooth muscle cells
Figure 9.6 Activation of a receptor tyrosine kinase (RTK).
These membrane receptors bind hormones or growth factors that are hydrophilic and cannot cross the membrane. The receptor is a transmembrane protein with an extracellular ligand-binding domain and an intracellular kinase domain. Signal transduction pathways begin with response proteins binding to phosphotyrosine on a receptor, and by receptor phosphorylation of response proteins.
Figure 9.7 The insulin receptor.
The insulin receptor is a receptor tyrosine kinase that initiates a variety of cellular responses related to glucose metabolism. One signal transduction pathway that this receptor mediates leads to the activation of the enzyme glycogen synthase. This enzyme converts glucose to glycogen.
Any of a class of protein kinases that activate transcription factors to alter gene expression. A mitogen is any molecule that stimulates cell division. MAP kinases are activated by kinase cascades.
mitogen-activated protein (MAP) kinase
A series of protein kinases that phosphorylate each other in succession; a kinase cascade can amplify signals during the signal transduction process.
Figure 9.8 MAP kinase cascade leads to signal amplification.
a. Each kinase is named starting with the last, the MAP kinase (MK), which is phosphorylated by a MAP kinase kinase (MKK), which is in turn phosphorylated by a MAP kinase kinase kinase (MKKK). The cascade is linked to the receptor protein by an activator protein.
b. At each step the enzymatic action of the kinase on multiple substrates leads to amplification of the signal.
Figure 9.9 Kinase cascade can be organized by scaffold proteins.
The scaffold protein binds to each kinase in the cascade, organizing them so each substrate is next to its enzyme. This organization also sequesters the kinases from other signaling pathways in the cytoplasm.
A member of an extensive family of small G proteins involved in cell signaling. These act in signal transduction as a switch that links a membrane RTK with a kinase cascade.
Figure 9.10 Small G proteins act as molecular switches.
Small G proteins, such as Ras, link external signals to internal signal transduction pathways. External signals activate guanine nucleotide exchange proteins (GEF), which activate the G protein. The G protein can be inactivated by its weak intrinsic GTPase activity, which can be stimulated by activating proteins (GAP).
A receptor that acts through a heterotrimeric (three component) G protein to activate effector proteins. The effector proteins then function as enzymes to produce second messengers such as cAMP or IP3.
G protein-coupled receptor (GPCR)
All G proteins are active when bound to _____ and inactive when bound to _____.
produces the second messenger cAMP
cleaves the inositol phosphates and results in the release of Ca2+ from the ER
A second messenger that is released, along with inositol-1, 4, 5-triphosphate (IP3), when phospholipase C cleaves PIP2. DAG can have a variety of cellular effects through activation of protein kinases.
Second messenger produced by the cleavage of phosphatidylinositol-4, 5-biphosphate.
inositol-1, 4, 5-triphosphate (IP3)
An enzyme that can be activated by GPCR signaling to convert ATP to cyclic AMP, which then acts as a second messenger.
Figure 9.15 Inositol phospholipid and Ca2+ signaling.
Extracellular signal binds to a GPCR activating a G protein. The G protein activates the effector protein phospholipase C, which converts PIP2 to DAG and IP3. IP3 is then bound to a channel-linked receptor on the endoplasmic reticulum (ER) membrane, causing the ER to release stored Ca2+ into the cytoplasm. The Ca2+ then binds to Ca2+-binding proteins such as calmodulin and PKC to cause a cellular response.
Figure 9.17 Different receptors can activate the same signaling pathway.
The hormones glucagon and epinephrine both act through GPCRs. Each of these receptors acts via a G protein that activates adenylyl cyclase, producing cAMP. The activation of PKA begins a kinase cascade that leads to the breakdown of glycogen.
Receptor subtypes can lead to different effects in different cells
- multiple forms of the same receptor
- receptor for epinephrine actually has nine different subtypes, or isoforms
- encoded by different genes and are actually different receptor molecules
- sequences of proteins are similar, especially in the ligand-binding domain, which allows them to bind epinephrine
- differ mainly in their cytoplasmic domains, which interact with G proteins
- leads to different isoforms activating different G proteins, leading to different signal transduction pathways
G protein-coupled receptors and receptor tyrosine kinases can activate the same pathways
- RTKs and GPCRs can activate the MAP kinase cascade
- RTKs and GPCRs can activate phospholipase C
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