Processing of
intracellular antigen for
presentation by class I MHC
Proteasomes
are constitutively involved in the routine turnover and cellular
degradation of proteins. Cytosolic proteins destined for antigen
presentation, including viral proteins, are degraded to peptides via the pathway involving these
structures.
In
addition to proteins that are already present in the cytosol, misfolded
and misassembled proteins are transported from the ER back into the cytosol by a
quality‐control process referred
to as ER‐associated protein degradation (ERAD). Proteins that have undergone retrotranslocation from
the ER into the cytosol can
then also be processed for class I presentation, as can proteins derived from mitochondria. Prior to
processing,
polypeptide
antigens are covalently linked to several molecules of the 7.5 kDa protein ubiquitin in an
ATP‐dependent process.
This polyubiquitination targets the polypeptides to the proteasome
(Figure 5.15).
 |
Figure 5.15 Processing of endogenous antigen and presentation by class I MHC.
Cytosolic proteins (a) targeted for degradation become polyubiquitinated by the
addition of several molecules of ubiquitin. The ubiquitinated protein binds to
the 19S regulator of the proteasome (b) which in a ATP‐dependent reaction
removes the ubiquitin, unfolds the protein, and pushes it into the cylindrical
structure of the 20S core proteasome that is made up of 28 subunits arranged in
four stacked rings. The resulting peptides are transported into the endoplasmic
reticulum (ER) by TAP1 and TAP2 (c). Under the influence of the peptide loading
complex (PLC; which comprises TAP1/2 together with calreticulin, tapasin, and
ERp57) the peptides are loaded into the groove of the membrane‐bound class I
MHC. ERp57 isomerizes disulfide bonds to ensure the correct conformation of the
class I molecule. Tapasin forms a bridge between TAP1/2 and the other PLC
components and is covalently linked to ERp57, which in turn is noncovalently
bound to the calreticulin. Following peptide loading the peptide–MHC complex is
released from the PLC (d), traverses the Golgi system (e), and appears on the
cell surface (f) ready for presentation to the T‐cell receptor. Mutant cells
deficient in TAP1/2 do not deliver peptides to class I and cannot function as
cytotoxic T‐cell targets.
Only a
small minority of the peptides produced by the housekeeping proteasome are the optimal length (8–10 amino acids) to fit
into the MHC class I groove; the remainder are either too short or too long. Longer peptides
can be subjected to
additional processing by, for example, cytosolic aminopeptidases (such as leucine
aminopeptidase). Processing
can also occur following transfer into the ER; in humans using the endoplasmic reticulum resident
aminopeptidases (ERAP‐1 and ERAP‐2), and in mice
the ER aminopeptidase
associated with antigen processing (ERAAP). If the peptides are
only slightly too long they can still bind to the
groove and be recognized by TCRs but in this case, which may apply to up to 10% of class I‐bound
peptides, they bulge out from the groove.
The cytokine IFNγ increases the production of three
specialized catalytic
proteosomal subunits, β1i, β2i, and β5i,
which replace the homologous
catalytic subunits in the housekeeping proteasome to produce the immunoproteasome, a structure with modified cleavage specificity that greatly
increases the proportion of 8–10
amino acid long peptides generated. Both proteasome and
immunoproteasome generated
peptides are translocated
into the ER by the heterodimeric transporter associated with antigen processing (made up of TAP1 and TAP2 subunits) (Figure 5.15). The newly synthesized class I
α chain is retained in
the ER by the lectin‐like chaperone calnexin, which binds to the monoglucosylated N‐linked glycan
of the nascent α
chain. Calnexin assists in protein folding and promotes assembly with β2‐microglobulin. The calnexin is then replaced with calreticulin, which has
similar lectin‐like properties and, together with TAP1/2, tapasin, and ERp57
(57 kDa ER thiol
oxidoreductase), constitutes the peptide loading complex (PLC). The tapasin bridge ensures that the
empty class I molecule sits
adjacent to the TAP pores in the ER, thereby facilitating the loading of peptides. Tapasin also plays a peptide‐editing role, ensuring preferential
incorporation of peptides with
high‐affinity binding to the MHC class I molecules. Upon peptide loading, the class I molecule dissociates from the PLC, and the now stable MHC–peptide traverses
the Golgi stack and
reaches the surface where it is a sitting target for the cytotoxic T‐cell.
