Describe the structure and functions of MHC II.
Answers
The main function of MHC molecules is to bind to antigens derived from pathogens and display them on the cell surface for recognition by the appropriate T-cells. ... Each MHC molecule on the cell surface displays a molecular fraction of a protein, called an epitope.
Answer: For diagram and explanation refer to the video
https://www.youtube.com/watch?v=BY-0cISzHkQ
Explanation:
MHC- Tightly linked complex of genes encoding for cell surface molecules that are required for antigen presentation and rapid graft rejection.
General organization and inheritance of MHC
Histocompatibility antigens- Rejection of foreign tissues is a result of an immune response to cell surface molecules. This concept is known as histocompatibility antigens.
MHC ENCODES THREE MAJOR CLASSES OF MOLECULES
The MHC complex is a tight collection of genes.
The MHC is referred to as the HLA complex (located on chromosome 6) in humans and H2 complex (chromosome 17 ) in mice
STRUCTURE OF MHC CLASS I
• Class I MHC molecules contain a 45kDa α chain (transmembrane glycoprotein encoded by ABC region of HLA complex) and a 12kDa β2 microglobulin molecule.
• α chain contains 3 external domains (α1, α2 and α3), each contain approx. 90 amino acids
• α chain contains a transmemebrane domain of approx. 25 hydrophobic amino acids followed by a small stretch of charged (hydrophilic) amino acids.
• It also contains a cytoplasmic anchor segment of 30 amino acids
• α1 and α2 interact to form 8 antiparallel β strands spanned by 2 α helices. This forms a deep groove or cleft of approx. 25Å X 10 Å X 11 Å with α helices as sides and β strands as the bottom. This peptide binding cleft is located on the top surface of the MHC molecule.
• It can bind a peptide of 8-10 amino acids
• α 3 and β2 are organized into 2β pleated sheets formed by antiparallel β strands.
• α 3 is highly conserved and contains a sequence that interacts with CD8 molecules of Tc cells
• β2 interacts with α 3 and also with α1 and α2. Interaction of β2 is important for a fully folded conformation.
Assembly of Class I molecules
• β2 microglobulin interact with α3 chain and helps the folding of α chain.
• This structure is stabilized by binding of appropriate peptide to form naïve Class I structure.
• This assembly is transported to the cell surface.
• α 3 and β2 association is important for expression of Class I molecules on cell surface. If no interaction between them,then no active MHC.
STRUCTURE OF MHC CLASS II
• Class II molecules contain a 33kDa αchain (90 amino acids) and a 28kDa βchain (25 amino acids) which are associated by non covalent interactions.
• They are membrane bound glycoproteins with external domain, transmemebrane domain and a cytoplasmic anchor segment.
• There are two external domains: α1 and α2 domain, β1 and β2 domain.
• Peptide binding cleft structure is similar to that in Class I MHC molecules, however class II molecules forms an open pocket instead of a closed pocket (due to lack of conserved residues that bond the terminal residues of short peptides).
Credit: Biomagica