Biological composites are typically based on an adhesive matrix that interlocks rigid reinforcing elements in fiber composite or brick-and-mortar assemblies. In nature, the adhesive matrix is often made up of proteins, which are also interesting model systems, as they are unique among polymers in that we know how to engineer their structures with atomic detail and to select protein elements for specific interactions with other components. Here we studied how fusion proteins that consist of cellulose binding proteins linked to proteins that show a natural tendency to form multimer complexes act as an adhesive matrix in combination with nanofibrillated cellulose. We found that the fusion proteins are retained with the cellulose and that the proteins mainly affect the plastic yield behavior of the cellulose material as a function of water content. Interestingly, the proteins increased the moisture absorption of the composite, but the well-known plastifying effect of water was clearly decreased. The work helps to understand the functional basis of nanocellulose composites as materials and aims toward building model systems for molecular biomimetic materials.