14
Introduction
Together with other mutagenesis studies and functional experiments (Anders et al.,
2011) two clusters of interacting residues were found for both molecules, respectively
(figure 1.7). For DR, one cluster is present in the DRα1 domain close to the peptide N-
terminus and a second cluster is located in the membrane-proximal DRß2 domain. The
contact residues for DM are located in the membrane-distal α1/ß1 domains and the
membrane-proximal α2/ß2 domains. The same lateral interaction surface was confirmed
by functional experiments in which DM was tethered either to the peptide N- or C-
terminus (Stratikos et al., 2002). Enhanced peptide release was observed with DM
covalently bound to the peptide N-terminus, but not to the C-terminus. Although the
overall binding site has been identified, the molecular mechanism of DM-catalyzed
peptide exchange is still unknown.
1.6
Proposed mechanisms for HLA-DM-catalyzed peptide exchange
As described in the previous section, the general interaction site between DM and
MHC II molecules has been identified. However, crucial residues important for peptide
exchange which could elucidate how DM catalyzes peptide exchange are still
unidentified. An attractive potential target for DM activity is the conserved hydrogen
bond network between peptide and MHC II molecules because it is a prominent
sequence-independent feature, and it has been shown that DM acts promiscuously on
different DR alleles and catalyzes the exchange of various peptides (Weber et al., 1996).
For example, Weber et al. discovered that the rate of enhancement of peptide
dissociation catalyzed by DM is directly proportional to the intrinsic dissociation rate of
a peptide from its DR molecule (Weber et al., 1996). Several studies have been already
carried out investigating whether DM targets some of these conserved hydrogen bonds,
but the results have been somewhat conflicting. Sadegh-Nasseri and colleagues
perturbed the conserved hydrogen bond between histidine 81 of the DRß chain and
peptide backbone with a histidine-to-asparagine mutation, which abolished DM
enhancement of peptide dissociation (Narayan et al., 2007). The conserved residue
Hisß81 is located close to the peptide N-terminus and, therefore, also close to the
interaction surface of DM and DR. A ‘hit-and-run’ mechanism was proposed by
Sadegh-Nasseri and colleagues in which DM transiently but repeatedly interacts with a
DR/peptide complex and thereby induces a conformational change leading to disruption