Cells communicate with each other in multicellular organisms to coordinate functions and development. Unicellular organisms communicate to each other to signal local population density or to collectively produce a protective biofilm.
Three stages characterize cell signalling: signal reception, signal transduction, and cell response.
Our classes have focused on three different types of receptors in the plasma membrane: G protein-linked receptors, receptor tyrosine kinases, and ligand-gated ion channels. There are also specific cytoplasmic receptors that bind hydrophobic signals that diffuse through the plasma membrane.
A signal transduction pathway is the series of steps involved in the conversion of a signal received by the cell at its surface to the cell’s response. These pathways are multistep (relay) cascades of molecular interactions for a couple of reasons: 1- a few signal molecules can be amplified by the transduction pathway to get a large cellular response, and 2- each step in the transduction pathway provides the opportunity to regulate or coordinate the cellular response. Transduction pathways can be mediated by 1) protein phosphorylation (protein kinases) and shut down by dephosphorylation (protein phosphatases) and, 2) secondary messengers such as cAMP, Ca2+, IP3 (Inositol Triphosphate) and DAG (diacylglycerol).
The cell may respond by activating transcription factors (which would regulate–turn on or off–specific genes). Or, cytoplasmic enzymes could be activated. As well, membrane protein channels could be opened or closed. And still, cell activity overall could be influenced to change to promote growth for example. Since each cell has a particular set of signal receptors, each of which have a particular set of relay proteins and a particular set of response proteins, then different cells can respond to different signals OR respond to the same signal in different ways. Also, signal transduction pathways within a single cell may “branch” causing more than one cellular response, OR two or more pathways may interact (“cross-talk”) to mediate a single response.
Try the following problems:
- Explain why G protein-coupled (also called G protein-linked) receptor pathways shut down rapidly in the absence of a signal molecule.
- Name the parts in the diagram of an activated receptor tyrosine kinase dimer.
- What does a protein kinase do?
- What does a protein phosphatase do?
- What is a “phosphorylation cascade”?
- Name the components in the following diagram depicting the steps in a signal transduction pathway that uses cAMP as a secondary messenger.
- Fill in the blanks to review the G protein-coupled pathway that uses Ca2+ as a second messenger.A ______________ binds to a G protein-coupled receptor. An activated ______________ activates the enzyme phospholipase C, which cleaves a _______________ into DAG and ______________, which binds to and opens a ligand-gated channel, releasing ______________ from the ________________.
- How does each of the following inactivation mechanisms discontinue a cell’s response to a signal and maintain the cell’s ability to respond to fresh signals?
- Reversible binding of signalling molecules
- GTPase activity of G protein
- Phosphodiesterase
- Protein phosphatases
HTK General Biology 2 
1. After the G-protein is activated and its GDP molecule is replaced by GTP, it will activate another enzyme. The GTP is then reconverted to GDP and returns to the original receptor. If the signal is absent, the receptor can not replace GDP by GTP again and G-protein remains inactive, shutting down the pathway.
2. a) signal
b) Receptor tyrosine kinase
c) phosphate
d) ATP
e) tyrosine
f) relay proteins
g) cellular response
3. Activate other protein by phosphorylation
4. Removes phosphate groups from other proteins
5. A signal transduction pathway where one protein phosphorylates another
6. a) signal
b) G-linked protein
c) G protein
d) target enzyme, adenyl cyclase (is it always this one?)
e) ATP
f) cAMP
g) another enzyme?
h) cell response
7. signal, G protein, PIP2, IP3, Ca2+, extracellular matrix (is this necessary to know for the test?)
8. 1) deactivation of receptor
2) no enzyme to release 2nd messengers for transduction
3) deactivation of 2nd messengers
4) deactivation of proteins necessary in transduction pathway
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1. The G protein is also a GTPase that hydrolyzes its bound GTP to GDP and inactivates itself.
2. perfect! well, you could say for d) that ATP–>ADP
3 and 4 are perfect answers
5. yes, and also amplifies the message at each step
6. b) G protein-linked RECEPTOR, d) not necessarily always adenyl cyclase, but yes you are correct. g) yes, a kinase if h) is a phosphorylation cascade. Nice work!
7. the last blank is endoplasmic reticulum (no, I don’t ask any questions in this same detail)
8. 1) receptor reverts to its inactive form, 2. activated G proteins are inactivated when the GTPase portion of the protein converts GTP to GDP, 3. yes you are right, phosphodiesterase converts cAMP to AMP, 4. yes again correct, protein phosphatases remove phosphate groups from activated proteins and deactivates/regulates the activity of the proteins in the transduction pathway.
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