50
Chapter II
DR/peptide complexes (analyte) were diluted into the running buffer before injection
and injected at the appropriate concentration followed by buffer. Chips were
regenerated by injection of HA peptide (50 µM). Binding of the reference flow cell
(DM mut) was subtracted from binding in the flow cell of DM wt. Dissociation
constants were determined by curve fitting (1:1 Langmuir model) using the
BIAevaluation software. For experiments with DM possessing an N-terminally
truncated DMα chain, the DM variant was immobilized
in flow chamber three; whereas,
the DM mutant was immobilized in flow chamber two and DM wild type in flow
chamber four. For analysis flow chamber two was subtracted from flow chambers three
and four, respectively.
3.2.6
Protein crystallization, collection of diffraction data and structure solving
For co-crystallization experiments purified DM and DR1/HA were mixed using a 1:1
ratio, dialyzed against 10 mM MES buffer (pH 6) and concentrated until a total protein
concentration of ~ 10 mg/mL was reached. Initial screening plates were set up at the
Collaborative Crystallization Center in Melbourne, Australia, using the nano-dispensing
crystallization robots mosquito Crystal (TTP LabTech) and Phoenix (Rigaku).
Customized screens were prepared with a Freedom Evo liquid handling robot (Tecan
Group Ltd.) preparing crystallization conditions with a pH equal or lower than 6.5.
Around 480 crystallization conditions were screened. Final crystals of DR1/HA(P
1,Val
-
P
11
) were grown in 2.5 M ammonium sulfate, 10 % DMSO or 100 mM sodium acetate
(pH 4.3), 9% PEG 10,000 at 20 ºC. Protein crystallization was performed using
hanging-drop vapor diffusion. The crystals were mounted using CryoLoops (Hampton
Research) or MicroLoops E (Mitegen) and transferred into a solution containing the
crystallization condition and ~ 5 M xylitol or 38% ethylene glycol as cryo-protectant.
Crystals were flash-cooled in liquid nitrogen. X-ray diffraction data were collected at
beamline 24-ID-E of NE-CAT or beamline 23-ID-D of GM/CA-CAT at the Advanced
Photon Source of Argonne National Laboratory, IL, USA. Crystals were stepwise
exposed to X-ray radiation by moving the X-ray beam along the crystal to minimize
radiation damage. Crystals diffracted up to 3.1 Å and 2.14 Å, respectively, and several
complete data sets were collected from individual crystals.
Diffraction data were indexed and scaled using the software HKL2000 (Otwinowski
and Minor, 1997). The structure of DR1 with the truncated HA peptide was solved by