25
Chapter I
with the Prodrg server (Schuttelkopf and van Aalten, 2004). To assess the accuracy of
the crystal structure the
statistical quantities R
free
and R
work
were used (Brunger, 1992).
2.2.2
Preparation of isotope labeled MBP peptide
The 15 amino acid peptide (ENSVVHFFKNIVTSR) was expressed as a C-terminal
in-frame fusion to the trpLE sequence with an N-terminal 9-His tag in the pMM-LR6
vector (gift from S. C. Blacklow, Harvard Medical School, Boston). The trpLE fusion
protein (Staley and Kim, 1994) contains the
E. coli trp leader polypeptide L, the
E. coli
trp polypeptide E with the deletion LE 1413 (Kleid et al., 1981; Miozzari and Yanofsky,
1978) and the T7 promoter-operator sequence allowing high levels of transcription into
mRNA (Studier et al., 1990). Furthermore the trpLE fusion protein directs the
expression of the protein into insoluble inclusion bodies which reduces toxicity.
Transformed
E. coli BL21(DE3) cells were inoculated into 1 L M9 minimal medium
(1 liter: 4 g glucose, 1 mL 2 M MgSO
4
, 1 mL 0.1 M CaCl
2
, 40 mL Centrum
multivitamins solution, 1 g NH
4
Cl, 6 g Na
2
HPO
4
, 3 g KH
2
PO
4
, 0.5 g NaCl) with one or
more stable isotope labels. Cultures were grown at 37 ºC to an absorbance of ~ 0.6 at
600 nm and were induced at 37 ºC with 300 μM isopropyl ß-D-thiogalactopyranoside.
NH
4
Cl and glucose of the M9 minimal media were replaced with
15
NH
4
Cl and
13
C-
glucose (Cambridge Isotope Laboratories), respectively, to label the peptide with the
stable isotopes
15
N and
13
C. Deuteration of the peptide required bacterial growth in
D
2
O. As bacterial growth is slow in D
2
O the level of D
2
O was gradually adjusted by
transferring the bacterial culture from media prepared with 50% D
2
O to media with
80% and finally 100% D
2
O.
Inclusion bodies were extracted with 6 M guanidine HCl, 50 mM Tris (pH 8.0), 200
mM NaCl, 1% Triton X-100, and 5 mM 2-mercaptoethanol. The cleared lysate was
bound to a Ni
2+
-affinity column (Sigma) and washed with five column volumes urea
buffer (8 M urea, 50 mM Tris, pH 8.0, 200 mM NaCl) under reducing conditions (5
mM 2-mercaptoethanol) and five column volumes under non-reducing conditions. After
additional washing with water the protein was eluted with pure formic acid. Water was
added to the eluted protein fractions until a formic acid concentration of 70% was
obtained. As a methionine had been introduced between the peptide sequence and the
fusion protein, both components could be separated by chemical cleavage using CNBr.
0.2 g CNBr for every 1 mL formic acid/protein solution was added and incubated at
26
Chapter I
room temperature for 1 h with exclusion of light and oxygen using a nitrogen stream.
The cleavage rate was monitored using SDS-PAGE.
CNBr was removed by dialysis against water using regenerated cellulose membranes
(MWCO 1000 Da). The dialyzed solution was neutralized. After precipitation of the
cleaved trpLE fusion partner the supernatant which contained the peptide was shock
frozen using liquid nitrogen and subsequently lyophilized. The lyophilized peptide was
dissolved in 50% TFA, loaded onto a C8 column and separated on a gradient of 5%-
95% acetonitrile (0.1% TFA) over 15 column volumes. The purified peptide was
identified by mass spectrometry. Peptide fractions were lyophilized and dissolved in
water to prepare a 1 mM stock solution.
2.2.3
Preparation of HLA-DR2/peptide complexes
For the preparation of the various DR2/peptide complexes soluble DR2/CLIP
complexes were used which had been produced using stably transfected Chinese
hamster ovary (CHO) cell lines in hollow fiber bioreactors as described in (Day et al.,
2003). Soluble DR2/CLIP proteins were purified via affinity chromatography with
monoclonal antibody L243 (American Type Culture Collection). The CLIP peptide
(PVSKMRMATPLLMQA) was covalently attached via a thrombin-cleavable linker at
the N-terminus of the DRß chain. After thrombin cleavage (20 units thrombin/1 mg
protein, 2 h, 30 ºC) the cleaved CLIP peptide was exchanged versus the respective
peptide synthesized by Peptide 2.0. For peptide exchange a 5-fold molar excess of
peptide was added to cleaved DR2/CLIP (0.2 mg/mL) in 50 mM citrate buffer (pH 5.2),
100 mM NaCl and incubated overnight at 30 °C.
During the preparation of DR2 complexes carrying isotope labeled MBP peptides
excess peptide was removed by size exclusion chromatography (Superose 6) after the
peptide exchange reaction. During chromatography the complex was also buffer
exchanged to the final buffer (10 mM Tris, pH 7.0, 100 mM NaCl) and afterwards
concentrated to the final concentration used for NMR experiments.
To prepare unlabeled DR2/MBP complex used for
19
F-NMR experiments, MBP
peptide was synthesized by Peptide 2.0 with a DNP-label at the side chain of a C-
terminal lysine. For peptide loading the protocol was followed as described above with
the modification of adding 50 uM of the small molecule J10-1 to the peptide exchange
reaction. Free peptide was removed by size exclusion chromatography (Superose 12)