54
Chapter II
Figure 3.3: Measuring affinity of DM to DR1 carrying an N-terminally truncated HA peptide at
different pH using surface plasmon resonance. (A-E) DM and DM mutant (αR98A, αR194A) were
immobilized in consecutive flow cells of streptavidin chips. DR1 carrying an N-terminally truncated HA
peptide variant was injected for five minutes at the indicated concentrations at 25 ºC in citrate phosphate
buffer with the indicated pH. After protein injection, buffer with the indicated pH was
injected to measure
dissociation. Binding in the DM mutant flow cell was subtracted from measurements in the DM wild type
flow cell. Dissociation constants were determined by curve fitting using 1:1 Langmuir model (black
curves). The table in (F) shows the calculated dissociation constants at different pH.
57
Chapter II
Table 3.1: Data collection and refinement statistics of DR1 carrying an N-terminally truncated
HA peptide. The data set was collected at beamline 23-ID-D at GM/CA-CAT at the Advanced Photon
Source of the Argonne National Laboratory, IL, USA. The crystal was grown in 100 mM sodium acetate
(pH 4.3) and 9 % PEG 10,000 at 20 ºC and its dimensions were 700 µm x 70 µm x 50 µm. Values in
parenthesis refer to the highest resolution shell.
Data collection
Space group
C222
1
Cell dimensions
a, b, c (Å)
95.8, 111.4, 211.4
Resolution (Å)
50.0-2.14 (2.18-2.14)
R
merge
(%)
8.4 (34.4)
I/σI
31.7 (5.4)
Completeness (%)
94.6 (90.6)
Redundancy
6.4 (6.2)
Refinement
Resolution (Å)
38-2.14
No. unique reflections
59393 (2862)
R
work
/R
free
(%)
20.8/23.5
Number of atoms (ASU)
Protein
6159
Peptide
159
Water
362
B-factors (Å
2
)
Protein
35.0
Peptide
40.3
Water
38.1
R.m.s. deviations
Bond lengths (Å)
0.007
Bonds angles (º)
1.055
3.3.5
Structure of HLA-DR1 carrying an N-terminally truncated HA peptide
Using molecular replacement the structure of DR1 carrying an N-terminally
truncated HA peptide was solved to 2.14 Å resolution. The two DR1
molecules found in
the asymmetric unit exhibit similar protein conformations with different crystal contacts
with one crystal contact being of particular interest. As can be seen in figure 3.5, the
flexible C-terminus of the DRα chain of a DR1 molecule binds to the partially empty
peptide-binding groove of an adjacent DR1 molecule where normally the peptide N-
terminus is bound. This intermolecular contact, which occupies only one of the two
peptide-binding grooves in the asymmetric unit, is stabilized by hydrogen bonds
between the DRα C-terminus and conserved MHC II residues. The same MHC II
residues are involved in the conserved hydrogen bond network between peptide N-
terminus and MHC II molecule. For example Hisβ81, which generally interacts with the
peptide backbone of residue P-1, builds a hydrogen bond to the carboxyl group of the
DRα C-terminus. Furthermore, the side chain of residue Serα53 forms a hydrogen bond