Concept 11.3: Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell Flashcards
The binding of a specific signaling molecule to a receptor in the plasma membrane triggers the first step in the signal transduction pathway—
the chain of molecular interactions that leads to a particular response within the cell.
the signal-activated receptor activates another molecule, which activates yet another molecule, and so on, until the protein that produces the
final cellular response is activated.
The molecules that relay a signal from receptor to response, which we call relay molecules in this book, are often
proteins.
Keep in mind that the original signaling molecule is not physically passed along a signaling pathway; in most cases, it never even
enters the cell.
At each step, the signal is transduced into a different form, commonly a
shape change in the next protein.
Very often, the shape change is brought about by
phosphorylation.
. An enzyme that transfers phosphate groups from ATP to a protein is generally known as a
protein kinase
Recall that a receptor tyrosine kinase is a specific kind of protein kinase that phosphorylates tyrosines on the other receptor tyrosine kinase in a
dimer.
Most cytoplasmic protein kinases, however, act on proteins different from
themselves.
Another distinction is that most cytoplasmic protein kinases phosphorylate either of two other
amino acids, serine or threonine, rather than tyrosine.
Serine/threonine kinases are widely involved in signaling pathways in
animals, plants, and fungi.
Many of the relay molecules in signal transduction pathways are
protein kinases, and they often act on other protein kinases in the pathway
The sequence of steps shown in the figure is similar to many known pathways, including those triggered in yeast by mating factors and in animal cells by many growth factors.
phosphorylation cascade
The signal is transmitted by a cascade of protein phosphorylations, each causing a
shape change in the phosphorylated protein.
The shape change results from the interaction of the newly added phosphate groups with
charged or polar amino acids on the protein being phosphorylated
The shape change in turn alters the function of the
protein, most often activating it
In some cases, though, phosphorylation instead
decreases the activity of the protein.
About 2% of our own genes are thought to code for
protein kinases, a significant percentage.
A single cell may have hundreds of different kinds, each specific for a different
substrate protein.
Together, protein kinases probably regulate the activity of a
large proportion of the thousands of proteins in a cell.
Among these are most of the proteins that, in turn,
regulate cell division.
Abnormal activity of such a kinase can cause
abnormal cell division and contribute to the development of cancer.
Equally important in the phosphorylation cascade are the _____________________, enzymes that can rapidly remove phosphate groups from proteins, a process called
protein phosphatases, dephosphorylation
By dephosphorylating and thus inactivating protein kinases, phosphatases provide the mechanism for
turning off the signal transduction pathway when the initial signal is no longer present.
Phosphatases also make the protein kinases available for reuse, enabling the cell to respond again to an
extracellular signal.
The phosphorylation-dephosphorylation system acts as a molecular switch in the cell, turning activities
on or off, or up or down, as required.
At any given moment, the activity of a protein regulated by phosphorylation depends on the
balance in the cell between active kinase molecules and active phosphatase molecules.
Not all components of signal transduction pathways are
proteins.
Many signaling pathways also involve small, nonprotein, water-soluble molecules or ions called
second messengers
