Solution of a Fab (26-10)/digoxin complex by generalized molecular replacement

The structures of a Fab fragment of a monoclonal murine antidigoxin antibody (26-10) complexed with digoxin and of a mutant of the Fab itself have been solved by molecular replacement. The solution strategy employed a generalization of molecular replacement. Prior to translation searches, a large number of the highest rotation-function peaks were subjected to a rigid-body refinement against the linear correlation coefficient between intensities of observed and calculated structure factors in which first the overall orientation of the model and then the orientations and translations of the individual domains (V_H, V_L, C_H1 and C_L) were refined. This procedure clearly identified the correct orientation of the search model. Furthermore, it produced a significant and unambiguous solution for the translation search. After rigid-body refinement, the R factor was in the low forties at 8-2.5 and 8-2.7 Å resolution for the Fab mutant and the Fab/digoxin complex, respectively. One round of simulated annealing refinement of all atomic positions reduced the R factor to the low twenties in both cases.