Fig. 1. Construction of tunable functional amyloids with variable domain structures through a modular genetic design using E. coli curli as a model amyloid system. (a) Schematic illustration of E. coli curli fibrils, β-helix model of CsgA and amino-acid sequences for CsgA. Curli, adhesive amyloid fibrils of E. coli biofilms, are constituted by the major subunits, CsgA. CsgA contains N-terminal 22 residues that act as a secretion signal through the outer membrane, and five repeating strand-loop-strand motifs mediated by conserved residues (marked in red color), that form the amyloid core.25–27 The glutamine and asparagine residues in each repeating unit are predicted to form a hydrogen bond network that contributes to the extreme stability of the fibrils. (b) Structural illustration of chitin binding domains (CBD), which are C-terminal domains of Bacillus circulans chitinase that can specifically bind to insoluble chitin (PDB code: ; 1ED7),21 and mussel foot proteins (Mfps), which are interfacial adhesion proteins of mussels.28,29 The CBD is a rigid and compact twisted β-sandwich structure that contains two antiparallel β-sheets,21 and Mfp is an intrinsically disordered protein in solution.30 (c) Two groups of gene constructs (CsgA-Mfp3, CBD-CsgA-Mfp3 and Mfp5-CsgA, CBD-Mfp5-CsgA) were created using one-pot isothermal Gibson assembly and tagged with poly-histidine residues to facilitate purification, respectively. Note: N-terminal or C-terminal fusion of a flanking domain (R) onto CsgA proteins was denoted as R-CsgA or CsgA-R, respectively. In a typical construct (CBD-CsgA-Mfp3), CBD, Mfp3 domain was extended from CsgA's N-terminal and C-terminal, respectively.