Fig. 1

Affinity maturation of CBTAU-22.1. a Co-crystal structure of Fab CBTAU-22.1 with tau peptide V1088–5. The Fab’s molecular surface is plotted with heavy chain in grey and light chain in white. Tau peptide is plotted in yellow. b Peptide binding is driven by the Ser⁴²² phosphate hotspot. Its binding pocket (left panel) is formed in the groove between light and heavy chains. The phosphate (plotted here as spheres) is buried deeply in the pocket and fully disolvated (central panel). Multiple hydrogen bonds are formed to bind the phosphate hotspot in the pocket (right panel). c Arg⁵⁰ buried by peptide binding, second example of charge-charge interaction between CBTAU-22.1 and Tau. d Design of the Asn³³ → Phe mutant based on the structure of the wild type CBTAU-22.1. Heavy chain Asn³³ (green) interaction with tau is water mediated. Water cavity surrounding the residue in the co-crystal structure is indicated with black arrow in the upper panel. Phenylalanine (magenta, lower panel) has been identified as a mutation with high shape complementarity with the pocket, forming hydrophobic interactions with Leu⁴²⁵. e Association and dissociation profiles for the parental antibody, variant antibodies with either the Ser⁵² → Arg mutation derived by random mutagenesis or the rationally-designed Asn³³ → Phe, and a variant with both mutations combined (dmCBTAU-22.1) to peptide V1089–24 as determined by Octet biolayer interferometry. f Interactions introduced by the heavy chain mutations confirmed by the co-crystal structure of the double mutant. Wild type residues are plotted in green, and the mutations in magenta. Tau peptide amino-acids are plotted in yellow, and the antibody heavy chain in grey Left panel; Asn³³ (green) mutated to Phenylalanine (magenta) resulted in formation of hydrophobic contacts with tau’s Leu⁴²⁵. Right panel: Ser⁵² (green) was mutated to Arginine (magenta) and resulted in addition of a charge-charge interaction with tau’s Asp⁴¹⁸. g Co-crystal structure of Fab dmCBTAU-22.1 with tau peptide V1088–23. The Fabs’ molecular surface is plotted with heavy chain in grey and light chain in white. Tau peptide is plotted in yellow. Tau peptide sequences are listed in Additional file 1: Table S2