The ER is the mother of almost all cellular membranes (organelles such as lysosomes,
perisosomes, endosomes, etc). There are profound differences in the various membranes.
As the ER sends out vesicles and tubules, the lipid/protein ratios change. We are going to
propose that the regulation of the lipid/protein contents of the ER, Golgi, plasma
membrane are regulated by the lipids. They are regulated by lipid/protein interaction.
The origin of the ER
When we try to look at the eukaryotic cell, we have to remember that the ER originated
from the plasma membrane of a prokaryote and in some ways the ER has molecular
memory. Notice that one arm of the ER is the nuclear envelop (very different than the
rest of the ER, highly regulated).
Rough ER contains ribosome sitting on the membranes literally translating proteins into
the lumen of the ER.
Smooth ER synthesizes almost all the lipids, not only that, but it is
packaging them for distribution.
Exam question: The ER is a giant tubular network, which extends throughout the whole
cell and can contact any organelle, plasma membrane, and it can even contact itself.
The question we would ask is when the ER contacts an organelle or plasma membrane,
what is it doing? Can the ER really bind tightly to an organelle or to the plasma
membrane? Or is what we see in the electron microscope an association, but is not really
Here is a cartoon showing that the ER and mitochondria can engage in an intimate
conversion. A conversion that may allow the ER to donate, not only calcium to the
mitochondria, but may interact in such a way that the ER can speak to the mitochondrion
when the mitochondrion is going to kill the cell.
We can say that that the ER contacts organelles and that the interaction is an intimate
one. How do you prove it? Here is an experiment. The ER was stained with green
florescent protein. Chloroplasts always fluoresce red and the question here is when the
ER contacts a chloroplast, what really happens? What was done was to measure using
optical tweezer. What are optical tweezers? You can take two very narrow beams of light
and when they are pushed through a cell, they will literally create a magnetic field. This
will allow you to manipulate organelles within a cell. What you can do, the ER was
brought next to the chloroplast and then they the force that would be required to pull
them apart. If the force was very low then the interaction was trivial, but in this case the
force was rather high (400 pN). This indicates that indeed the interaction was very tight.
Here is the secretory and endocytic pathway. The implication here is that the ER is the
source of vesicle carrying things, which goes and processed in the Golgi, which then
sends it out in either vesicles or tubules to the plasma membrane, to secretory vesicles, to
lysosomes and notice that traffic can go in the reverse direction.