Generalized Molecular Replacement

The following sections show how the 26-10 Fab fragment complexed with digoxin was solved by generalized molecular replacement (Brünger, 1991c). The space group of the 26-10 Fab/digoxin crystals is $P2_1$ (Strong, 1990), with the b-axis unique, and a non-crystallographic twofold symmetry is present. The following strategy was employed to solve the structure:

1. Self-rotation function.
2. Modification of the elbow angle of a known Fab structure.
3. Cross-rotation function with the modified Fab structure.
4. Filtering the rotation function by $PC$-refinement.
5. Analysis of the $PC$-refinement.
6. Translation function for molecule A, using the $PC$-refined model.
7. Translation function for molecule B.
8. Combined translation function to determine the relative position between A,B.
9. Rigid-body refinement.

In general, one has to try several different starting elbow angles (spaced approximately 10$^{\circ}$ apart) and repeat steps 3-6 until a good solution is found. This strategy is also applicable to other multidomain proteins or PC-refinements of other degrees of freedom. The modification of the Fab example input files should be straightforward. An overview of the strategy is shown in Fig. 19.1.

Figure 19.1: Overview of molecular replacement.