MHC II molecule

[inayat]

I would be grateful if someone could help me here.

Below is a molecular model of MHC II with an antigen bound to it. There are three residues which are covalently bound to the MHC class II molecule coloured in pink, but which do not form part of the polypeptide chains: two are attached to the A-chain and one to the B-chain. What type of residues are they? What is the description of this type of modification of the protein? Where in the cell and how did these residues become linked to the polypeptide chain?

These are residues are called NAG1, NAG2 and NAG3. Im guessing these residues have something to do with glycosylation. Am i right? and what type of residues are they? Please help.

[blcr11]

NAG is a quasi-standard abbreviation for N-acetyl glucosamine. It is a modified carbohydrate that can be added to a protein via the N- or O-linked pathways. If this is one of the Ian Wilson MHC Class II structures, the NAGs are N-linked to the side chains of Asn residues. Glycosylation is a post-translational modification that takes place in the golgi body and probably has something to do with labeling the proteins for their final location within or on the cell. Glycosylation doesn’t usually determine the proper folding of the protein. That has already happened, for the most part, before the carbohydrates get added. The MHC I and MHC II molecules each have peptide binding sites and both are involved in presenting antigens to the immune system, but each of them (Class I and II) have unique roles to play and each class acquires their peptides in distinct ways. You want to find a good cell biology text and read the sections on the golgi, and the sorting and glycosylation pathways. And then maybe find a good immunology text that describes the MHC and its organization.