Glucose is a good example.
Glucose is a critical product of digestion.
It is an essential energy source for cellular metabolism.
This energy is produced
when glucose is used as a substrate for glycolysis
and then the Krebs or Citric Acid Cycle.
Following the digestion of food,
higher levels of glucose circulate through the blood stream
where it enters different cell types.
In muscle cells
glucose is readily used to produce energy
and is also stored as glycogen,
a secondary short term energy source.
In fat cells, glucose is used for Triglyceride production,
and acts as an important energy reserve molecule.
Here we will illustrate the signaling pathway that occurs
when glucose is at high levels.
This pathway involves multiple proteins and signaling events.
This is termed cytoplasmic signaling.
Different types of cells
perform similar signaling steps
in response to changes in their environment.
In the Protein Recycling animation
we see a group of storage vesicles enriched with GLUT4 proteins
continuously recycling from the cell membrane
to an inactive location in the cytosol.
GLUT4 is a protein that facilitates the movement of glucose into the cell.
When high levels of glucose are detected by beta cells in the pancreas,
insulin is released by the cells.
The insulin circulates through the blood stream
until it binds to an insulin receptor embedded in the cell membrane
of a muscle, fat, or brain cell.
Once the insulin binds to the receptor,
phosphate groups are added
to the intracellular domain of the receptor.
Since the receptor itself
adds the phosphate groups,
the process is called autophosphorylation.
This phosphorylation event
sets off a cascade of molecular events.
The activated receptor protein
then adds a phosphate group to another closely associated protein.
This effectively passes the signal from the receptor
to the next step in the signal pathway.
Proteins that add phosphate groups to another protein
are called kinases.
Kinases are often components of signal pathways,
and phosphorylation is an important component
in the transmission of a signal from one compartment to another.
In this system,
the signal corresponds to the level of blood glucose
and is transmitted from outside to inside the cell.
Next we see a large pool of molecules that are embedded in the membrane
also being phosphorylated.
Other proteins are then in turn phosphorylated,
further transmitting the first extracellular signal
that was originally sent from outside the cell membrane.
So how does this affect the uptake of glucose?
As we mentioned before,
Glut4 is a glucose transporter,
and Glut4 Storage Vesicles are held in a recycling state near the cell membrane.
The vesicles are held mostly in this region
because the RAB proteins that interact
with the motor proteins necessary to move the vesicles to the membrane
are in an inactive state.
The final step in the signal pathway
involves the phosphorylation of a protein
that prevents the RAB proteins from interacting with the vesicles.
When the RAB proteins are no longer inhibited,
the storage vesicles can freely merge with the membrane.
Once the vesicles have merged
many Glut4 proteins are embedded in the membrane
and large quantities of glucose can move into the cell.
It is the signaling pathway
that insures only the correct molecules will be allowed to enter the target cell.