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Figure 4.
Figure 4. Schematic of the OspA/LA-2 Fab complex crystal
lattice. The view is perpendicular to the crystallographic
a-axis, and a one unit cell thick slice is shown. The b and c
lattice repeats are indicated. The two LA-2 Fab dimers in the
crystal asymmetric unit are indicated in light and dark gray,
respectively. Two OspA/Fab complexes that define a single
crystal asymmetric unit are shown in a thick trace. These two
OspA molecules are shaded according to refined C^a-atom B-value
(blue, B < 35 Å2; red, B > 65 Å2). This view
highlights the alternating dense and sparsely packed molecular
layers along the c-axis, corresponding to LA-2 Fab (gray) and
OspA (green) layers, respectively. The 80 Å long OspA
molecules are strongly anchored at their C-terminal tips to the
Fab antigen combining sites, but they extend into the sparse
layer where they are nearly entirely surrounded by solvent. Only
one of the two OspAs in the asymmetric unit (lower OspA in this
view) makes a weak bridging lattice contact with its N-terminal
domain to the adjoining Fab layer, consisting of two salt
bridges between flexible side-chains. These weak lattice
contacts and corresponding thin-plate crystal morphology
resulted in poor diffraction along the c*-axis.
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