61
Chapter II
Figure 3.7: Comparison of the peptide-binding groove of DR1 carrying a full length HA peptide
versus carrying an N-terminally truncated HA peptide. (A, B) The new DR1 structure carrying an N-
terminally truncated HA peptide (green) is superimposed with the previously published DR1 structure
carrying a full length HA peptide (PDB: 1DLH, orange). (A) No major conformational changes of the
peptide-binding groove were observed. (B) The helices of the DRα and DRß chains adjacent to the
peptide N-terminus are further apart in the DR1 structure missing two N-terminal peptide residues, 7.3 Å
versus 6.4 Å. Distances were measured between C
α
atoms of residues Alaα52 and Valß85. (A, B) DR1
molecules and peptides are shown as cartoon representation. (A, B, C, D) Part of the peptide-binding
groove is shown which binds the N-terminal part of the peptide. (A, C, D) DRα chain can be seen in the
foreground and DRβ chain in the background. The peptide extends horizontally with the N-terminus at
the right side. (B) DRα chain can be seen on the left and DRβ chain on the right. The peptide N-terminus
is pointing towards the observer. (C) The peptide-binding groove of the DR1 structure is shown binding
an N-terminally truncated HA peptide with an additional valine at P1 position and residues P-2 and P-1
missing. A coordinating water molecule between His β81 and the peptide N-terminus is shown as filled
circle in blue. (D) The peptide-binding groove of the previously published DR1 structure (1DLH) is
shown binding a full length HA peptide. (C, D) DR molecules are shown as cartoon representation and
bound peptides as stick model. DR1 residues interacting with the peptide N-terminus are labeled and
shown as stick representation. Hydrogen bonds between DR1 and peptide are shown as dotted lines.
62
Chapter II
Next, the conformations of conserved DR residues around the peptide-binding
groove of DR1/HA(P
1,Val
-P
11
) were compared with the conformations of DR residues of
previously published DR structures to detect minor changes of the partially empty
peptide-binding groove. Most of the conserved residues in the peptide-binding area
either overlapped quite well with the conformation of the residues of the newly solved
DR1/HA(P
1,Val
-P
11
) structure or showed general conformational variability like residue
Pheα51. However residue Valβ85, which showed a conserved conformation for
previously published DR structures (see figure 3.8, A), revealed a different
conformation with the valine rotated outwards from the peptide-binding groove (see
figure 3.8, B). At first, the rotation seems to be a subtle conformational change, but is of
particular relevance as Valβ85 is involved in forming the crucial P1 pocket which is
important for peptide-binding. By rotation of Valβ85 the pocket is opened up on one
side (figure 3.8, B). This can be nicely seen in the surface representation of the peptide-
binding groove of DR1/HA(P
1,Val
-P
11
) (figure 3.8, C) and is further illustrated by
comparison with the fully closed P1 pocket exhibited by the DR1 structure carrying a
full length HA peptide (figure 3.8, D). The rotated conformer of Valβ85 was also
observed for the DR1/HA(P
1,Val
-P
11
) molecule exhibiting a peptide-binding groove
interacting with the C-terminus of an adjacent DR1 molecule as described above. As the
C-terminus binds further away than the peptide N-terminus the peptide-binding site is
still destabilized as the tight peptide/MHC II interactions are not compensated (figure
3.9). The conformational change of Valβ85 might occur as the tight interactions with
peptide residues P-1 and P1 are absent and diminished, respectively. The altered
rotamer of Valß85 was not observed for the DR1/CLIP structure missing residue P-2
(Gunther et al., 2010) and may be dependent on the absence of residue P-1 (figure
3.10). With regard to the entire process of peptide release a partial opening of the P1
pocket in consequence of the release of the two N-terminal peptide residues (P-2, P-1)
likely destabilizes the P1 anchor residue and might further facilitate release of the entire
peptide.
63
Chapter II
Figure 3.8: Minor conformational changes of Val β85 were observed affecting the P1 pocket. (A)
The newly solved structure DR1/HA(P
1,Val
-P
11
) was superimposed with other DR structures and the
conserved residue Val β85 is shown as stick model. It can be seen that the conformation (rotamer) of Val
β85 is conserved for DR molecules carrying a peptide containing residue P-1. However Val β85 of
DR1/HA(P
1,Val
-P
11
) exhibits a different conformer. The distance between C
β
-atoms of Val β85 of
DR1/HA(P
1,Val
-P
11
) and other DR molecules is 0.9-1.7 Å. DR1/HA(P
1,Val
-P
11
) (green), DR1/HA (1DLH,
cyan), DR2/MBP (1BX2, purple), DR3/CLIP (1A6A, yellow), DR4/human collagen II peptide (2SEB,
light pink), DR1/CLIP(106-120)
flipped
(3PGC, grey), DR1/CLIP(102-120) (3PDO, purple blue),
DR1/CLIP(106-120)
canonical
(3PGD, orange), DR1/A2(103-117) (1AQD, red), DR2/MBP (1HQR, pink),
DR1/HA (1HXY, pale yellow), DR4/HA (1J8H, purple), DR1/TPI(23-37) (1KLU, grey). (B) In the
crystal structure of DR1/HA(P
1,Val
-P
11
) which is missing two N-terminal peptide residues (P-2, P-1), Val
β85 is rotated outwards from the binding groove opening up the P1 pocket on one side (C). In the crystal
structure of DR1 with a full length HA peptide (orange/yellow, 1DLH) the P1 pocket is fully closed (D).
(B) In yellow the surface representation of DR1 (1DLH) is shown except Val β85 which is shown as stick
model (orange). The structure DR1/HA(P
1,Val
-P
11
) was superimposed and residue Val β85 is shown as
stick model (green). The full length HA peptide of DR1/HA (1DLH) is shown as stick model (orange).
(C) The DR molecule of DR1/HA(P
1,Val
-P
11
) is shown as surface representation (green) and the N-
terminally truncated HA peptide P
1,Val
-P
11
is shown as stick model (orange). (D) The DR molecule of
DR1/HA (1DLH) is shown as surface representation (yellow) and the full length HA peptide as stick
model (orange).
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