70
Chapter II
under the peptide N-terminus (residues P-2, P-1), was mutated to aspartic acid
introducing a charged residue at that site. DM binding of the DR1
mutant was compared
to DM binding of the DR1 wild type using surface plasmon resonance. Both molecules
carried a covalently linked HA peptide missing residues P-2, P-1, P1 and containing a
glycine at position P2 to ensure the respective DR1 region is exposed.
As can be seen in figure 3.15 less DM binding was observed for DR1
mutant than for
DR1 wild type indicating DM binding occurred more slowly for the mutant than for the
wild type. However, the off-rate of the mutant was also slower than the off-rate of the
wild type making it difficult to determine whether the overall affinity of the mutant is
lower. More mutations in the respective DR region have to be tested for DM binding to
further investigate
this hypothesis
Figure 3.14: DR1 residues become accessible once the two N-terminal peptide residues (P-2, P-1)
are absent. The DR1 residues Serα53, Alaα52, Hisß81 and Valß85, which are covered by peptide
residues in the complex of DR1 with full-length HA-peptide, (A) become exposed once the two N-
terminal peptide residues (P-2, P-1) are absent (B). Peptide N-termini with part of the peptide-binding
groove of DR1/HA (1DLH) (A) and DR1/HA(P
1,Val
-P
11
) (B) are shown. (C) Model showing an aspartic
acid residue at position 85 of DRß chain. (A, B, C) DR molecules are shown as cartoon depictions and
peptides are shown as surface representations.
71
Chapter II
Figure 3.15: Comparing DM binding of DR1 wild type and DR1 mutant with residue Valß85
mutated to aspartic acid. DR1 wild type and DR1 mutant (Valß85Asp) carrying HA(P
2,Gly
-P
11
) peptide
were injected at 1 µM. As can be seen DR1 mutant (red) binds to DM with a slower on-rate and a slower
off-rate than DR1 wild type (blue). DR1/peptide complexes were injected for 5 minutes with a flow rate
of 15 µL/min following buffer injection for 5 minutes and injection of full length HA peptide (50 uM).
The experiments were carried out in 50 mM citrate phosphate buffer (pH 5.3), 150 mM NaCl at 25 ºC.
Binding in the flow cell of DM mutant was subtracted from measurements in the DM wild type flow cell.
3.4
Conclusion
To determine the co-crystal structure of DR and DM co-crystallization experiments
were set up with a modified DR1/peptide complex carrying an N-terminally truncated
HA peptide, which showed higher affinity to DM than DR molecules carrying full
length peptides. To determine conditions favoring crystallization of the non-covalently
linked DR/DM complex versus the individual proteins, the affinity of the DR/DM
complex was measured at different pH applying surface plasmon resonance. The
experiments revealed that the pH optimum for DM/DR binding is around pH 5.5 and a
major drop in DM binding was observed between pH 6.5 and pH 7.0 with dissociation
constants of 4.7 μM and 15.4 μM, respectively. The results agree with previous data
measuring increased DM activity at acidic pH present in the late endosome (Sloan et al.,
1995) where DM catalysis happens. Therefore, crystallization experiments were set up
using customized crystallization conditions with pH lower than 6.5.
Unfortunately, the DM/DR complex did not crystallize, however, the DR1 molecule
with an N-terminally truncated HA peptide yielded crystals which revealed a DR1
structure (2.14 Å resolution) carrying a peptide missing two N-terminal peptide residues
(P-2, P-1) and containing an additional valine at position P1.
That the peptide included a
0
100
200
300
400
500
600
0
25
50
75
100
C1091/SP120 (1 uM)
C930/SP120 (1 uM)
time (s)
RU
72
Chapter II
P1 residue was surprising as a synthesized HA peptide variant was used for the
preparation of the DR/peptide complex lacking all three N-terminal peptide residues. As
was confirmed later by mass spectrometry, a side-product of the peptide synthesis likely
bound to DR1 carrying an additional valine at P1 position. The minor species was
probably enriched due to two selection processes, i.e. preferential binding during the
peptide loading step based on higher peptide affinity and favored crystallization based
on higher stability of the DR/peptide complex. SPR experiments revealed that the DR1
complex carrying an HA peptide variant with a valine at P1
position and two N-terminal
peptide residues missing binds to DM, although the extent of DM binding is lower than
that of DR1/peptide complex lacking a P1 anchor. Previously DR1 with an HA peptide
variant containing a tyrosine as P1 anchor (P-2, P-1 missing) displayed only marginal
binding to DM; however, it seems that the presence of valine at P1 position allows DM
binding maybe due to its smaller size. Alternatively the valine may more readily leave
the pocket due to fewer interactions with the pocket in case DM binding requires an
empty P1 pocket to bind DR. That even a full length peptide bound to a DR molecule
exhibits extensive mobility has been observed during the studies described in chapter I.
The inhomogeneity of the DR1/peptide complex carrying at least two different HA
peptide variants may have made it more difficult to crystallize the DR/DM complex, as
DM also exhibits lower affinity to the DR/peptide complex containing a P1 anchor.
The DR1 crystal structure revealed an interesting crystal contact with the flexible C-
terminus of the DRα chain binding to the empty part of the peptide-binding groove of a
neighboring DR1 molecule demonstrating the affinity of this region for extended
peptide strands and protruding protein parts. The resulting hypothesis that DM may
bind to DR in a similar way with the solvent exposed N-terminus of the DMα chain
possibly reaching the DM/DR interface was excluded by SPR experiments in these
studies.
Surprisingly, no major conformational change of DR1 was observed as had been
expected due to loss of DR/peptide interactions and lack of three conserved hydrogen
bonds. However, smaller conformational changes were observed. First, the helices of
the DRα and ß chains normally next to the peptide N-terminus were further apart than in
the DR1 structure carrying a full length HA peptide (Stern et al., 1994) or a CLIP
peptide missing residue P-2 (Gunther et al., 2010). The divergence of the helices is
likely due to the missing interactions with the peptide in between them normally
holding the helices closer together. Although the DR1 structure can not directly be