Topic 4 Flashcards
What are the two antigen presentation pathways?
The endogenous pathway which presents cellular products on MHC-I to CD8+ T cells
The exogenous pathway which presents phagocytosed products via MHC-II to CD4+ T cells
What is the endogenous pathway of antigen presentation?
MHC-I is assembled in the endoplasmic reticulum where it receives its peptide, stabilizing the structure, it then moves through the cis, medial and trans golgi apparatus, it is then moved to a transport vesicle which then fuses with the cell membrane allowing the MHC-I and its peptide to be exposed to the external environment
What is the exogenous pathway of antigen presentation?
MHC-II is assembled in the ER and stabilized by the addition of a conserved molecule, it then moves through the cis, medial and trans golgi apparatus until it goes into storage vesicle which fuses with a late endosome or liposome where it will receive a peptide and is then sent to the cell surface
What are the structural differences between MHC-I and MHC-II?
The MHC-I molecule is made of two peptides, one of which contains 3 domains and contains the membrane anchor, the other lacks a membrane anchor and is non-covalently associated with the first peptide, stabilizing it
The MHC-II molecule is made of two, almost identical peptides each with their own membrane anchor
The largest difference between the two molecules is the peptide binding cleft, with MHC-I having a closed cleft so only peptides of 8-11 amino acids can be stored with 2 terminal anchor residues, MHC-II however has an open binding cleft allowing peptides of 8-30+ amino acids with anchors throughout the peptide
How is the MHC-I molecule assembled?
The heavy chain (a 45 kDa glycoprotein composed of 3 alpha subunits) is placed in the ER lumen and stabilized through calnexin (a membrane-anchored chaperone)
Also in the ER membrane is the TAP peptide transporter which associates with Tapasin, when this is joined by Beta-2-microglobulin, calreticulin and Erp57 this can allow the heavy chain to associate with this complex
The TAP transporter will then let a peptide through which can bind to the MHC-I molecule and stabilize it allowing the chaperones to dissociate
What is Tapasin?
48 kDa glycoprotein which stabilizes TAP1/TAP2 which enhances peptide transport
It bridges MHC-I to TAP acting as a structural component, facilitates peptide loading, stabilizes “empty” peptide-receptive MHC complexes and optimizes the peptide repertoire by acting as a peptide editor
How much of the peptide pool is available for antigen presentation by the immune system?
with 1/3 of intracellular proteolysis in mammalian cells is believed to be directed to nascent proteins like DRiPs and only 1% of the peptide pool is believed to be available to the immune system
How are peptides, which are to be presented by MHC-I processed?
The proteasome will degrade two key types of products in an ATP dependent process, correctly folded proteins which slowly become available as they age and defective ribosomal products which rapidly become available as errors occur in the production of proteins
Following proteasomal degradation many peptides are still too large for degradation and as such need to be further broken down Trim-peptidases
What is the proteasome?
This is a multicatalytic protease of around 700 kDa, it contains 2 rings of beta units which contain the active catalytic sites, these are surrounded by rings of alpha units, this 4 ring core is then surrounded by one or two 19S regulators which big up ubiquitin labelled products and feeds them through the core for degradation
What is the immunoproteasome?
This is a different form of the proteasome, with slightly different subunits like PA28, these subunits do not change where residues can be cleaved but shifts the equilibrium to the point where more of the peptides will be cleaved at positions which facilitate antigen presentation
What is the relationship between the immunproteasome and the standard (constitutively expressed) proteasome?
It does not completely replace the constitutive proteasome and has a shorter half-life
What are trim-peptidases?
These are peptidases which degrade proteasome products and exist in the cytosol (Puromycin-Sensitive Aminopeptidase, Thimet Oligopeptidase, Leucine aminopeptidaase and Tripeptodyl protease II) and in the ER (ERAP 1 and 2)
The main function of these is likely to be in protein degradation rather than immune presentation
Are there carboxypeptidases in the cytosol?
No, this is important as a C-terminal anchor is required in order for the peptide to be able to be presented by MHC-I
How is MHC-II assembled?
This requires the invariant chain, which stabilizes the whole complex as it forms it is then edited until only a small region remains in the peptide binding cleft, stabilizing it
HLA-DM can then facilitate the removal of this region and acts as a peptide controller making sure that only those that bind with a high stability
What is the invariant chain?
This is a protein with 4 domains, a short N-terminal cytosolic domain, a single transmembrane domain, Class II associated invariant chain peptide and C-terminal trimerisation motif It has the following functions, acting as a scaffold for facilitating proper folding and assembly of MHCII, blocking premature class II peptide association, direct trafficking of MHCII invariant chain to endosomal pathway, trimerisation of HLA-DR complex
