MOLECULAR REPLACEMENT SOLUTION OF THE STRUCTURE OF APOLACTOFERRIN, A PROTEIN DISPLAYING LARGE-SCALE CONFORMATIONAL CHANGE

The crystal structure of an orthorhombic form of human apolactoferrin (ApoLf) has been determined from 2.8 angstrom diffractometer data by molecular replacement methods. A variety of search models derived from the diferric lactoferrin structure (Fe2Lf) were used to obtain a consistent solution to the rotation function. An R-factor search gave the correct translational solution and the model was refined by rigid-body least-squares refinement (program CORELS). Only three of the four domains were located correctly by this procedure, however; the fourth was finally placed correctly by rotating it manually onto three strands of electron density which were recognized as part of its central beta-sheet. The final model, refined by restrained least-squares methods to an R factor of 0.214 for data in the resolution range 10.0 to 2.8 angstrom, shows a large domain movement in the N-terminal half of the molecule (a 54-degrees rotation of domain N2) and smaller domain movements elsewhere, when compared with Fe2Lf. A feature of the crystal structure is that although the ApoLf and Fe2Lf unit cells appear very similar, their crystal packing and molecular structures are quite different.