Structure-based design of covalent nanobody binders for a thermostable green fluorescence protein

The use of green fluorescence protein (GFP) has advanced numerous areas of life sciences. An ultra-thermostable GFP (TGP), engineered from a coral GFP, offers potential advantages over traditional jellyfish-derived GFP because of its high stability. However, owing to its later discovery, TGP lacks the extensive toolsets available for GFP, such as heavy chain-only antibody binders known as nanobodies. In this study, we report the crystal structure of TGP in complex with Sb92, a synthetic nanobody identified from a previous <i>in vitro</i> screening, revealing Sb92's precise three-dimensional epitope. This structural insight, alongside the previously characterized Sb44-TGP complex, allows us to rationally design disulfide bonds between the antigen and the antibody for tighter interactions. Using biochemical analysis, we identify two bridged complexes (TGP A18C-Sb44 V100C and TGP E118C-Sb92 S57C), with the TGP-Sb92 disulfide pair showing high resistance to reducing agents. Our study expands the toolkit available for TGP and should encourage its wider applications.