Progress in programming antibody fragments to crystallize

Abstract

Completion of the X-ray analysis of the human B7-15A2 Fab opened a new vista (Immunotechnology 3, no. 4). In the crystal lattice, both the λ-type light chain (CL domain) and γ1-type heavy chain (CH1 domain) participated in formation of antiparallel β-pleated sheets with neighboring molecules related to the reference Fab by 2-fold axes. This observation evoked memories of the first description of this type of packing for human Bence-Jones (λ chain) dimers 20 years ago (Ely K.R. et al. Biochemistry 1978;17:158–167). Reexamination of packing interactions in selected crystal systems revealed that the C domains of λ and γ1 chains were structurally amenable to the formation of such cross-molecule β-structures, but κ chain CL domains were not. In the latter, a single proline residue disrupted the order of β-strand 3–3 in the middle of the surface used in λ and γ1 chains for intermolecular interactions with symmetry-related molecules. For the packing of Fv molecules, the VL domains are structurally well suited for analogous packing interactions through antiparallel 4–1 β-strands in adjacent molecules. Such interactions have been shown to provide the driving force in the crystal packing of a human (Pot) Fv from an IgM-κ cryoglobulin. Together, these observations suggest several avenues through which propensity to crystallize can be programmed into the designs of synthetic human Fabs, Fvs and single-chain antibodies.