Single-chain Fv multimers of the anti-neuraminidase antibody NC10:: the residue at position 15 in the VL domain of the scFv-0 (VL-VH) molecule is primarily responsible for formation of a tetramer-trimer equilibrium

Single-chain variable fragment of the murine monoclonal antibody NC10 specific to influenza virus N9 neuraminidase, joined directly in the V-L to V-H orientation (scFv-0), forms an equilibrium mixture of tetramer and trimer with the tetramer as the preferred multimeric species. In contrast, the V-H-V-L isomer was previously shown to exist exclusively as a trimer. Computer-generated trimeric and tetrameric scFv models, based on the refined crystal structure for NC10 Fv domain, were constructed and used to evaluate factors influencing the transition between V-L-V-H trimer and tetramer. These model structures indicated that steric restrictions between loops spanning amino acid residues L55-L59 and L13-L17 from the two adjacent V-L domains within the V-L-V-H trimer were responsible for four scFv-0 molecules assembling to form a tetramer. In particular, leucine at position L15 and glutamate at position L57 appeared to interfere significantly with each other. To minimize this steric interference, the site-directed mutagenesis technique was used to construct several NC10 scFv-0 clones with mutations at these positions. Size-exclusion chromatographic analyses revealed that several of these mutations resulted in the production of NC10 scFv-0 proteins with significantly altered tetramer-trimer equilibrium ratios. In particular, introduction of a polar residue, such as asparagine or threonine, at position L15 generated a highly stable NC10 scFv-0 trimer.