Glycoproteins and other post-translational modifications..

[Bio-student]

I'm reading up on protein modification at the moment and I'm kinda stumped on something. I understand that proteins become glycoproteins through the addition of polysaccharides post-translation, but since all proteins don't undergo this process, I'm wondering how a cell 'knows' that some proteins need to be glycosylated (or insert modification here) while other proteins don't?

Go easy on me if this is an obvious question...I'm only a first year undergrad. :shy:

[Ygggdrasil]

There are a couple of factors that regulate glycosylation. First is the physical location of the glycosyltransferases that create glycoproteins. Since most of the glycosyltransferase enzymes reside in the ER and Golgi, most proteins need to be trafficked through the ER and/or Golgi in order to be glycosylated (protein trafficking through the secretory system is regulated by short polypeptide sequences at the N-terminus of proteins, called signal peptides). This is why glycosylation is commonly found on transmembrane proteins and secreted proteins but is less common on cytoplasmic proteins (which do not pass through the secretory system).

Another factor is sequence specificity. Most glycosyltransferases recognize a specific sequence of amino acids on the polypeptide to be glycosylated. For example, oligosaccharyltransferase, the enzyme responsible for producing N-linked glycans, recognizes asparagine residues with a serine or threonine in the +2 position. Some glycosyltransferases recognize specific protein folds or structures, confering a higher degree of specificity.

[Andy Resnick]

Just to add to Ygggdrasil's excellent reply, not all proteins are glycosolated. There are numerous post-translational modifications used by the cell:

http://en.wikipedia.org/wiki/Post-translational_modification

[Bio-student]

Thanks both, great help :)

[Angeline Ong]

Is that means as long as a particular protein have signal peptide it show that it is glycosylated. but can a protein that do not have signal peptide but yet still glycosylated?

[Ygggdrasil]

Signal peptides target proteins for trafficking through the secretory system where most of the glycosylatating enzymes reside. Proteins, that lack signal peptides and are not trafficked through the secretory system can still be glycosylated. The primary carbohydrate modification found on nuclear and cytoplasmic proteins is the attachment of N-acetylglucosamine to serine or threonine residues of proteins. This type of modification has been shown to play a number of different biological roles in a number of different pathways.

[Angeline Ong]

thanks a lot for your help, Ygggdrasil..