Natural protein-based materials are exploring their new applications from the traditional uses. Structural proteins provide scaffolds, whereas functional proteins carry essential biological activities through millions of biochemical reactions. The idea of implementing functionalities into natural structures could provide manufactured protein products with a better fit for human desire. The development of synthetic biology and molecular assembly methods illustrates possibilities for the production of functional structures in-situ, which provides better connections between the functional partners with the structural scaffolds. Protein fibres are one of the natural structures which possess a unique shape and superior mechanical properties. Apart from the natural protein fibres, many disease-related peptides self-assembled into amyloid filaments and fibrous structures. Natural globular proteins could also form fibres through the directed assembly by changing their storage conditions or through fusion. Elongated polyglutamine peptides cause many neurodegenerative diseases as they assemble. In this thesis, a polyglutamine peptide (Q77) was fused with functional partners to direct the protein assembly in vitro. The role of the polyglutamine was studied during assembly and after the formation of a self-supportive fibrous product. The extensibility of traditionally size-limited fibrous materials formed by disease-related peptides was tested experimentally for the first time. The resultant fibrous product with embedded functionalities mimics the structure of silk, but the mechanical behaviour of collagen. Two structurally distinct proteins were chosen as the functional partners for Q77: a monomeric red fluorescent protein (mcRFP), which is relatively small in size and possesses a b-barrel structure, and firefly luciferase (Luc), which is a larger protein with a fragile structure consisting of two mobile domains. Both proteins have been widely used as reporters for intracellular activities with either fluorescence or bioluminescent signal. In this work, the functionalities of both proteins were investigated after Q77 fusion and after assembly towards respective fibrous products. The structural variation of these recombinant proteins resulted in the changes of their functionalities. Finally, a self-supportive fibrous ATP sensor was achieved for the first time with this dual functional protein product.