Nanocellulose shows a high potential as a building block in composite materials because of its high strength, availability, and sustainability. It has been a significant goal to construct composite materials that are able to capture the high mechanical properties of the components. Indeed, there are several reports demonstrating both high strength and stiffness in nanocellulose based materials. However, achieving a high toughness at the same time has been a significantly more difficult challenge. Here we present strategies for both assembly and interface engineering that lead to the enhancement of stiffness, strength, and toughness simultaneously. The approach uses biomimetics and gives insight into overall mechanisms for composite design in biomolecular materials. We use recombinant DNA and protein engineering to create molecules that are adapted for functioning as matrix materials and show how precise structural engineering of these proteins affects materials function.