Concept 11.4 Response: Cell signaling leads to regulation of transcription or cytoplasmic activities Flashcards
We now take a closer look at the cell’s subsequent response to an extracellular signal—what some researchers call the
“output response.”
The response at the end of the pathway may occur in the
nucleus of the cell or in the cytoplasm
Many signaling pathways ultimately regulate protein synthesis, usually by turning specific
genes on or off in the nucleus.
Like an activated steroid receptor (see Figure 11.9), the final activated molecule in a signaling pathway may function as a
transcription factor.
The response to this growth factor signal is transcription, the synthesis of one or more specific mRNAs, which will be translated in the
cytoplasm into specific proteins.
In other cases, the transcription factor might regulate a gene by
turning it off
Sometimes a signaling pathway may regulate the activity of proteins rather than causing their
synthesis by activating gene expression
This directly affects proteins that function outside the
nucleus
The final step in the signaling pathway that begins with epinephrine binding activates the
enzyme that catalyzes the breakdown of glycogen.
participate in a variety of pathways, leading to both nuclear and cytoplasmic responses, including cell division
Signal receptors, relay molecules, and second messengers
Malfunctioning of growth factor pathways like the one in Figure 11.15 can contribute to abnormal cell division and the development of
cancer
Whether the response occurs in the nucleus or in the cytoplasm, it is not simply turned ______________ Rather, the extent and specificity of the response are regulated in multiple ways.
“on” or “off.”
Here we’ll consider four aspects of this regulation. First, as mentioned earlier, signaling pathways generally
amplify the cell’s response to a single signaling event.
The degree of amplification depends on the
function of the specific molecules in the pathway
Second, the many steps in a multistep pathway provide control points at which the cell’s response can be further
regulated, contributing to the specificity of the response and allowing coordination with other signaling pathways.
Third, the overall efficiency of the response is enhanced by the presence of proteins known as
scaffolding proteins.
Finally, a crucial point in regulating the response is the
termination of the signal.
Elaborate enzyme cascades amplify the cell’s response to a
signal.
At each catalytic step in the cascade, the number of activated products can be
much greater than in the preceding step
For example, in the epinephrine-triggered pathway in Figure 11.16, each adenylyl cyclase molecule catalyzes the formation of 100 or so cAMP molecules, each molecule of protein kinase A phosphorylates about
10 molecules of the next kinase in the pathway, and so on.
The amplification effect stems from the fact that these proteins persist in their
active form long enough to process multiple molecules of substrate before they become inactive again.
As a result of the signal’s amplification, a small number of epinephrine molecules binding to receptors on the surface of a liver cell or muscle cell can lead to the
release of hundreds of millions of glucose molecules from glycogen.
Consider two different cells in your body—a liver cell and a heart muscle cell, for example. Both are in contact with your
bloodstream and are therefore constantly exposed to many different hormone molecules, as well as to local regulators secreted by nearby cells.
Yet the liver cell responds to some signals but ignores others, and the same is true for the
heart cell.
