- We continue with vesicle transport, we’re moving away from the Golgi, the
trans Golgi network, that is basically the major branching point in vesicle
transport and how do vesicles now make it to the outside or the plasma
membrane and release their contents into the outside of the cell (extracellular
- Just to recap, this slide is what we’ve been looking at in terms of vesicle
transport. We looked at vesicle transport last lecture from the trans Golgi
network to the lysosome and how there’s a signal that is added on in the cis
Golgi onto proteins that are to be directed to the lysosome. These are for
example, lysosomal hydrolases that need to end up in the lysosome and
provide the functions of the lysosome in terms of degrading macromolecules.
- Just to recap, the trans Golgi network is a major branching point for proteins
that are to be sorted to different compartments in vesicles. Proteins that move
through the Golgi will end up in the trans Golgi network and the vesicles that
bud off the trans Golgi network can have different destinations.
- We looked at the lysosome, going through either the late endosome or early
endosome, these will eventually mature or become the lysosome.
- But vesicles can also bud off the TGN and make it to the plasma membrane,
this is called exocytosis. There are two major forms of exocytosis that can
occur. One is regulated and one is constitutive and basically what that means
is that when the vesicle buds off, it either goes to the plasma membrane and
fuses automatically, and that is the constitutive version or the vesicle can sit
around in the cytoplasm and wait for a signal in order to fuse to the plasma
membrane. That is called the regulated secretory pathway.
- We’ll look at those two examples so basically, proteins that are going
through the secretion pathway enter the ER, go through the Golgi, there are
two different ways they can go through the Golgi that we looked at and then
they’re sorted out at the major branching point which is the trans Golgi
network to either the lysosome via the early endosomes or to the plasma
membrane in a process called exocytosis.
- We’ll now look at a small animation that goes through the process till where
- The signal sequence of a membrane protein directs the ribosome to the
rough ER. The growing polypeptide chain enters the lumen of the ER through
a protein import channel in the membrane. There the signal sequence is
cleaved, sugars are added and the protein folds. Some enzymes or structural
proteins remain in the ER. Most proteins that are soluble in the ER are
transported in vesicles to the cis Golgi.
- In the cisternal maturation model, the cis Golgi matures to become the
medial Golgi, a new cis Golgi is formed by fusion of the ER vesicles. The
maturation process includes retrograde vesicular transport of resident Golgi
- Proteins destined for secretion after modification in the Golgi are
transported from the trans Golgi to the plasma membrane. When the
membranes fuse, the proteins are released into the extracellular space.
- Soluble proteins on the outside of a cell can reenter by endocytosis and are
sorted to lysosomes for degradation. Other proteins, including lysosomal
enzymes sort directly to lysosomes from the trans Golgi.