Protein engineering shows a wide range of possibilities for designing properties in novel materials. Following inspiration from natural systems we have studied how combinations or duplications of protein modules can be used to engineer their interactions and achieve functional properties. Here we used cellulose binding modules (CBM) coupled to spider silk N-terminal domains that dimerize in a pH-sensitive manner. We showed how the pH-sensitive switching into dimers affected cellulose binding affinity in relation to covalent coupling between CBMs. Finally， we showed how the pH-sensitive coupling could be used to assemble cellulose nanofibers in a dynamic pH-dependent way. The work shows how novel proteins can be designed by linking functional domains from widely different sources and thereby achieve new functions in the self-assembly of nanoscale materials.