Crystals of phosphorylcholine-binding Fab-fragments from mouse myeloma proteins: preparation and x-ray analysis.

Fab-fragments of several phosphorylcholine-binding mouse-myeloma proteins have been prepared by pepsin digestion; two of these, MOPC 167 and McPC 603, gave large crystals from ammonium sulfate solutions. The Fab-fragment from MOPC 167 crystallizes in a hexagonal space group, but does not diffract to a resolution greater than about 8 A. In contrast, McPC 603 crystals (space group P6(3)) diffract to about 2.7 A. An isotopic double-labeling technique was developed that demonstrated that the 603 crystals bind 1 mol of hapten per mol of Fab-fragment, but with a binding constant significantly lower than that observed in solution. The findings indicate that a three-dimensional model of this homogeneous antigen-binding immunoglobulin can be constructed. Accordingly, a search for heavy-atom derivatives and determination of the primary structure are in progress.