3
$\begingroup$

I haven't read this being stated explicitly, however whenever I read about proteins and glycoproteins this seems to be implied. For example, in this Wikipedia article,

Lipid and proteins on the cell membrane surface often have short carbohydrate chains protruding out from the cell surface, known as glycolipids and glycoproteins.

My question is, is this the case? And if so, why would they only be present on the cell surface/plasma membrane. Would they not be useful elsewhere in the cell for example for vesicle trafficing as receptors for targetting sequences?

One idea I have is that, to the best of my knowledge, the enzymes for glycosylation are present in the endoplasmic reticulum and Golgi lumens (oligosaccharyl transferase in ER, etc) so glycosylation would only occur on the part of the protein/lipid that is topologically outside of the cell, however this brings up more questions:

  • This does not exclude glycoproteins from sticking into certain organelles (i.e. on the other side of the membrane to the cytosol) which is also topologically equivalent to the cell exterior.
  • Why would there not be glycosylation enzymes on the cytosolic side?
$\endgroup$
2
$\begingroup$

Since you asked three questions, I'll answer them one by one.

  • Are glycoproteins and glycolipids present only on the cell surface membrane?

No, glycoproteins have many functions and are certainly not restricted to cell membranes.

Some examples of glycoproteins in blood include fibrinogen, antibodies, miraculin, etc. See this:

Fibrinogen (factor I) is a glycoprotein in vertebrates that helps in the formation of blood clots.

For a more detailed list of glycoproteins, see this wikipedia article.

On the other hand, glycolipids are found only on cell membranes. See this:

Glycolipids are lipids with a carbohydrate attached by a glycosidic bond. Their role is to maintain stability of the membrane and to facilitate cellular recognition. The carbohydrates are found on the outer surface of all eukaryotic cell membranes.

  • This does not exclude glycoproteins from sticking into certain organelles (i.e. on the other side of the membrane to the cytosol) which is also topologically equivalent to the cell exterior.

Glycoproteins, or any proteins, after they are completely processed , don't just float around in the cell and reach their appropriate place. Its obviously not that proteins 'know' where to go, but they are actually taken to their appropriate places. This is known as protein targeting. See this:

Protein targeting or protein sorting is the biological mechanism by which proteins are transported to the appropriate destinations in the cell or outside of it. Proteins can be targeted to the inner space of an organelle, different intracellular membranes, plasma membrane, or to exterior of the cell via secretion. This delivery process is carried out based on information contained in the protein itself. Correct sorting is crucial for the cell; errors can lead to diseases.

There are differences between cell membrane and organelle membrane, which makes them distinguishable for targeting factors.

  • Why would there not be glycosylation enzymes on the cytosolic side?

Glycosylation is a post-translational modification or proteins.

Post-translational modification (PTM) refers to the covalent and generally enzymatic modification of proteins during or after protein biosynthesis. Proteins are synthesized by ribosomes translating mRNA into polypeptide chains, which may then undergo PTM to form the mature protein product.

Only certain proteins require glycosylation, that is why they are translated by ribosomes connected to golgi apparatus or endoplasmic reticulum. If glycosylation enzymes are present everywhere in the cell, then every protein that encounters them would get modified and, possibly, non-functional. Thus, there are special compartments in the cell for such purposes.

EDIT: To answer the questions you asked in comments:

  • are there any glycoproteins found elsewhere inside the cell? For example in the mitochondria?

We aren't sure about that yet. There have been a couple of studies to find out whether there could be glycoproteins in mitochondria, one of which is this:

Isolated intact mitochondria selectively incorporate monosaccharides from nucleotide diphosphate monosaccharides into protein. Fucose, mannose, glucose, and galactose were incorporated by the mitochondria into glycoprotein; xylose was not. Structural integrity of the mitochondria was not necessary for the incorporation of monosaccharide into glycoprotein; mitochondria broken by homogenization also incorporated monosaccharide. The monosaccharides incorporated into glycoprotein were localized in the inner mitochondrial membranes, the same membranes which contain the protein into which leucine is incorporated by the isolated mitochondria.

If there are glycoproteins in mitochondria, they could be at many other places too inside a cell.

  • does the cell not sometimes need glycosylated proteins inside of its cytoplasm? How would it get these proteins in its cytoplasm?

Glycoproteins are not required inside cell (I, or maybe nobody, knows why). Their main function is as receptors or secreted products, thus they are not found inside a cell. However, glycosyltransferases can behave as dynamic glycoproteins and hence are found inside cells. See this:

Glycosyltransferases (GTFs, Gtfs) are enzymes (EC 2.4) that establish natural glycosidic linkages. They catalyze the transfer of saccharide moieties from an activated nucleotide sugar (also known as the "glycosyl donor") to a nucleophilic glycosyl acceptor molecule, the nucleophile of which can be oxygen- carbon-, nitrogen-, or sulfur-based.

glycosyltransferase mechanisms

$\endgroup$
  • $\begingroup$ Metroplex thank you for your reply. I still have a question though; so glycoproteins can be secreted as well as being destined for the plasma membrane, but are there any glycoproteins found elsewhere inside the cell? For example in the mitochondira? I know there will be glycoproteins present inside vesicles which have just come from the plasma memebrane, and these will be returned. But what about glycorproteins being specifically targetted to regions in the cell which are not the plasma membrane and are not excreted? $\endgroup$ – 21joanna12 Dec 28 '16 at 14:53
  • $\begingroup$ Question 2: Also, I understand now why glycosylation enzymes would not be present inside of the cytoplasm, however does this not seem a bit of a shame for the cell? I mean, does the cell not sometimes need glycosylated proteins inside of its cytoplasm? How would it get these proteins in its cytoplasm? It seems impossible, unless they were to be transported in from the ER lumen or from outside of the cell through chennel or carrier proteins, but I have never heard of a channel, carrier big enough to achieve something like this... $\endgroup$ – 21joanna12 Dec 28 '16 at 14:55
  • $\begingroup$ I hope you find the modified answer helpful. Please add the above questions too in your main question. $\endgroup$ – another 'Homo sapien' Dec 28 '16 at 17:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.