BIO230 lecture 11 notes
- After a protein has entered the ER via transmembrane transport, they can move to different
cellular compartments through vesicular transport.
- Glycosylation occurs on most soluble and most transmembrane proteins in ER. There are 2
major types of glycosylations:
Occurs on serines and threonines in the Golgi
There is a big precursor that exists preformed in the ER; this is added onto an asparagine
side chain. The abbreviation for asparagine is N.
This precursor is made up of 14 sugar residues consisting of 3 glucoses, 9 mannoses, and
It is linked to a target protein in the ER.
This precursor is anchored on the cytosolic face of the ER by a fatty acid called dolichol
by pyrophosphate bonds, which are high energy bonds that will provide the energy to
transfer this oligosaccharide from this fatty acid to a protein.
The oligosaccharide precursor is transferred by an enzyme called oligosaccharyl
transferase, which is a transmembrane protein with its catalytic site on the ER luminal
side; the catalytic site is the functional site that carries out the specific activity of that
enzyme. In this case, this enzyme is responsible for transferring the precursor onto the
growing side chain. Remember that this is a co-translational process so as the protein is
being synthesized and delivered into the ER, sugar modifications will be occurring.
- There is an amino acid sequence that is preferentially glycosolyated by N-linked glycosylation: it
is asparagine, followed by any amino acid (except proline), and then serine or threonine. If you
find these amino acid sequences in proteins going into the ER, then there is a high chance that
they will be N-linked glycosylated.
- Since this all occurs on the ER lumen side, proteins are only glycosylated on the ER lumen side.
- You get this 14 residue oligosaccharide added onto proteins in the ER. Once this happens, the
oligosaccharide is processed. After the transfer, the 3 glucoses are removed by enzymes called
glucosidases and 1 mannose is removed by an ER mannosidase. That protein can now be
transported to the Golgi by vesicular transport where there will be more modifications.
- Each cisternae (stacks) of the golgi has different enzymes; these different enzymes modify these
sugars by adding or removing sugars from these proteins coming in from the ER. The overall
result is that there will be different modifications to different proteins.
- Purpose of glycosylation of proteins:
Tag to mark state of protein folding
There is a specific enzyme in the ER lumen. There is the oligosaccharide added on to the
protein and the 3 glucoses are removed, in the state when there is still one glucose on
it, the enzyme calnexin grabs onto this protein with one glucose on it and this protein is
unfolded and it recognizes it because of this one glucose residue and it holds onto the
protein and then let it go. If the protein is properly folded, it gets out of the ER. If it is
not folded properly then an enzyme called glucosyl transferase adds the glucose back so
that calnexin will attach to it and make it stay until it gets properly folded. Protect proteins on cell surface from proteases
Some glycosylated proteins have role in cell adhesion
Allow proteins to form correct 3D structure
- Vesicular transport occurs between the ER and other compartments of the cell. It is also called
the secretory pathway because it results in the release of its contents.
- Vesicular transport involves the movement of cargo between compartments. There is a donor
compartment, where vesicles are budding off and it eventually fuses with a target compartment
and release their cargo inside that target component. You are using these lipid-enclosed bubbles
to carry material around from one place to another.
- The proteins that are in the vesicles are called cargo proteins and they are collected at one site
into vesicles: they consist of transmembrane proteins. There are also soluble proteins which are
bound by transmembrane cargo receptors so that they are brought into the vesicle. The vesicles
bud off once these proteins are inside it and then fuse with the target compartment.
- How do vesicles form?