Thursday January 29, 2009
- This is the next lecture, this is about sorting and packaging of proteins in
the Golgi apparatus.
- Just an overview, this is basically to remind us of how proteins destined for
the secretion pathway actually go through this process and we’ll see a little
movie of this in a bit. Basically all proteins or most of them are transcribed,
the genes are transcribed in the nucleus and then translated in the cytoplasm
and proteins that are destined for secretion are transported into the ER shown
on the slide.
- That is the 1st stage, they get in there by transmembrane transport & then
the 2nd stage is movement from the ER to the Golgi & this is through
vesicular transport so that is what we’re going to cover today, is how proteins
move from the ER to the Golgi through the Golgi & then sorted to different
compartments & we will focus on the lysosome today.
- Recognize or appreciate that both soluble proteins & transmembrane
proteins are going through this secretion process.
- So the 1st destination from the ER is the Golgi and that is what we’re going
to cover today and this movie nicely shows what we learned so far about
secretion and where we’re going to.
- Fluorescently labeled membrane proteins start their journey to the plasma
membrane after synthesis in the ER. They are first dispersed throughout the
extensive membrane network of the ER.
- From there they move to exit sites that form in random locations in the
membrane network. At each of these sites, the membrane proteins are
concentrated and packaged into transport vesicles. Clusters of the transport
vesicles fuse to form transport intermediates.
- At the next stage, transport intermediates move along microtubule tracts to
the Golgi apparatus near the center of the cell. The membrane proteins exit
the Golgi apparatus.
- They move in transport vesicles that are now pulled outward by the
microtubules which deliver them to the plasma membrane. Each time a Golgi
derived vesicle fuses with the PM, its content proteins disperse.
- That video nicely shows well first movement in the cell and how proteins
get through the secretion pathway so we’re going to start with the movement
from the ER to the Golgi.
- First just to give us an overview of what the Golgi looks like, we’re all
familiar with this structure but basically you would have the ER up there
and movement of proteins is going from the ER to the first part of the Golgi
which is called the cis Golgi.
- The Golgi can be separated into different cisternae, so these are different
layers of the Golgi. The first one that is encountered would be the cis Golgi
network or the CGN. That leads into the cis Golgi, this cis Golgi network is
the closest one to the ER and that would be called the cis face of the Golgi
that moves to the trans cisternae and eventually to this complex structure of
membranes and vesicles called the trans Golgi network. From there vesicles
or proteins are sorted to their appropriate compartments in the cell like the
coming from the ER, they shed their COP2 coat and through v-SNARE t-
SNARE interaction, these vesicles fuse together to form the vesicular
tubular cluster and this is actually found before the Golgi apparatus.
- The vesicular tubular cluster will move to the Golgi apparatus along the
microtubule network, these motor proteins are actually carrying the
vesicular tubular cluster to the cis Golgi network. The vesicular tubular
network is this network of tubules that is formed from the fusion of newly
formed vesicles that are budding off the ER. These will eventually move
to the Golgi apparatus.
! Escaped ER resident proteins
! Proteins involved in vesicle budding from ER
- In here, there are a lot of proteins that are destined to remain in the ER,
they need to get back to the ER so there is what is called a retrieval
transport. That’s the formation of vesicles from this vesicular tubular
cluster and these vesicles are COP1 coated vesicles, these ones that are
budding off the ER are called COP2, the ones that are in the retrieval
pathway are called the COP1 coated vesicles
- These vesicles are budding off the vesicular tubular cluster and there are
also some coming off the Golgi and they are moving back to the ER and
they’re bringing with them, escaped ER resident proteins, so proteins that
are destined to stay in the ER like the chaperones that bind to unfolded
proteins, and they’ll also include proteins that are involved in vesicle
budding from the ER so formation of these COP2 coated vesicles so
proteins that are specifically required for the formation of these coats will
also be brought back to the ER
- There needs to be this retrieval transport to make sure ER proteins or
proteins that only function in the ER don’t make it past into the Golgi and
into the secretion pathway so there is a way to bring them back. How is
! Bound by KDEL receptor
! Signal bound by COP1 coats
- ER resident proteins, many of them have what are called ER retrieval
signals on them. This is a protein sequence that will signal for that protein
to be retained in the ER and to go into these COPI coated vesicles of the
- Soluble proteins that are ER resident proteins have a signal called the
KDEL sequence, this is for the amino acid code that it encodes, so a
Lysine, aspartic acid, glutamic acid and a leucine. That is the amino acid
sequence that is found in these proteins. This sequence will be bound by
what is called a KDEL receptor so this is a receptor that actually binds to
this amino acid sequence and then this receptor, along with this soluble
ER resident protein will be packaged into this COP1 coat and then a
vesicle will bud off and go back to the ER and bring your ER resident
protein back to its appropriate place.
- Membrane proteins on the other hand have a very different sequence, it is
a KKXX sequence at the C terminus, these are two lysine residues
followed by two other variable AAs at the extreme C terminus of this
protein. This is an AA sequence found on ER resident membrane proteins.
One of these is actually the KDEL receptor so the KDEL receptor has this
sequence at its C terminus, so it will be packaged into the COP1 coated
vesicles. This KKXX sequence will bind to COP1 coats & that is how it is
recruited to these appropriate vesicles – these proteins will be packaged in
there & the membrane proteins will go back to the ER where they belong.
- So soluble proteins have a KDEL sequence, binds to the KDEL receptor.
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BIO120H1 Full Course Notes