58
Chapter II
to the peptide backbone of residue Leuα211 of the C-terminal DRα chain. Overall the
DRα C-terminus is not as closely and tightly packed in the DR binding groove as can be
observed for the peptide N-terminus in crystal structures of MHC II/peptide complexes
(figure 3.9). However the extended intermolecular interactions show the ability of this
DR region to bind protruding protein parts and might bind certain DM regions in a
similar way during DM/DR interaction after partial peptide release. One hypothesis was
that the N-terminus of the DMα chain could interact with the partially empty peptide-
binding groove of DR in a similar way and was further investigated by surface plasmon
resonance experiments as described in section 3.3.7.
A
B
Figure 3.5: C-terminus of DRα chain of one DR1 molecule in the asymmetric unit binds in the
partially empty peptide-binding group of the other DR1 molecule. (A) The two DR1 molecules of the
asymmetric unit are shown in cartoon representation. The red circle indicates where the flexible C-
terminus of the α chain of one DR1 molecule (blue) binds in the partially empty peptide-binding groove
of the other DR1 molecule (green), normally filled with the peptide N-terminus. (B) In green as cartoon
representation part of the peptide-binding groove is shown where normally the peptide N-terminus binds.
DR residues Hisβ81, Valβ85 and Serα53 are indicated and shown as stick representation. In blue the C-
terminus of the α chain of an adjacent DR1 molecule is shown as stick model. Dotted lines indicate
hydrogen bonds whereas black numbers display the lengths of the hydrogen bonds in Angstrom. The
filled circle in blue indicates a coordinated water molecule.
During refinement of the DR1 structure the peptide and a region of the DRα chain
next to the peptide N-terminus were omitted to reduce model bias as these parts could
possibly reveal conformational changes. After building missing protein and peptide
residues into the observable electron density, extra electron density was visible at the
site where normally the P1 peptide residue is bound (figure 3.6). As the HA peptide
59
Chapter II
which was covalently linked to the DRα chain should be N-terminally truncated with
residues P1, P-1 and P-2 missing (VKQNCLKLATK) this site should be empty unless
other small molecules bound in the P1 pocket. Therefore, small molecules (e.g. TFA,
acetate), which could fit into the extra electron density and were present during
crystallization, were built into the extra electron density. However, none of the small
molecules sufficiently explained the extra electron density as still negative or positive
electron density was observable after model refinement including the small molecules.
Upon closer inspection of the extra electron density it looked like the missing part was
covalently linked to the peptide N-terminus, i.e. an additional residue would be present
at the peptide N-terminus. In fact, when an additional valine was built at the peptide N-
terminus the extra electron density was well explained. The additional valine at the
peptide N-terminus (VVKQNCLKLATK) likely resulted from a peptide contaminant
during peptide synthesis. Such a peptide would preferentially bind to DR1 molecules as
it has higher affinity to DR1 due to the additional valine as P1 anchor compared to the
primarily present peptide lacking a P1 anchor residue (VKQNCLKLATK). To further
prove the presence of an additional valine at P1 position the peptide was eluted from
DR1 molecules used for crystallization experiments and was analyzed by mass
spectrometry (see 3.3.6).
o
-F
c
) of the peptide N-terminus showing extra electron density
(F
o
-F
c
) for an additional peptide residue. The peptide N-terminus and parts of the DR1 molecule are
shown as stick model. The electron density is shown in mesh representation (2F
o
-F
c
: blue, F
o
-F
c
: green).
The peptide extends horizontally with the N-terminus at the right side. In the front parts of residues of the
DRα chain and in the background residue Hisβ81 of the DRβ chain can be seen. The red circle indicates
extra electron density accounting for an additional valine at the peptide N-terminus.
60
Chapter II
Figure 3.7 (A, B) shows an overlay of the peptide-binding groove of the newly
solved DR1 structure carrying an N-terminally truncated HA peptide (green) with the
previously published DR1 structure carrying a full length HA peptide (orange). As can
be seen from the superimposition no major conformational change was observed in the
DR1 region around the partially empty peptide-binding groove and the overall
conformation of the DRα and DRβ chains stayed intact. However, the DRα and DRß
helices normally next to the peptide N-terminus were further apart (0.8-0.9 Å) in the
DR1/HA(P
1,Val
-P
11
) structure compared to the DR1 structure carrying a full length HA
peptide (figure 3.7, B) and also carrying a CLIP(106-120) peptide missing residue P-2
(figure 3.10).
It was surprising that greater conformational changes were not observed since in the
absence of two N-terminal peptide residues (P-2, P-1) several peptide/MHC II
interactions were missing resulting in possible destabilization of the respective DR1
region. For example, two hydrogen bonds normally formed between peptide residue P-2
and DR residues Serα53 and Pheα51 could not be formed (figure 3.7, C, D).
Furthermore, the hydrogen bond between peptide residue P-1 and His β81 could not be
built and instead Hisβ81 formed a bridged hydrogen bond to the amine group of the
peptide N-terminus (P1 residue) that was mediated by a coordinated water molecule.
However, the other three N-terminal hydrogen bonds formed by peptide residues P2 and
P1 with DR1 residues Asnβ82 and Serα53 were unmodified. Concerning the peptide
conformation, peptide residue valine at position P2 exhibited a different rotamer in the
DR1/HA(P
1,Val
-P
11
) structure as had been seen for the same peptide residue in the DR1
structure carrying a full length HA peptide (figure 3.7, C, D).
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