Natural selection has endorsed proteins with amazing structures and functionalities that cannot be matched by synthetic means, explaining the exponential interest in developing protein-based materials. Protein self-assembly allows fabricating complex supramolecular structures from relatively simple building blocks, a bottom-up strategy naturally employed by amyloid fibrils. However, the design of amyloid-inspired materials with biological activity is inherently difficult. Here, we exploit a modular procedure to generate functional amyloid nanostructures with tight control of their mesoscopic properties. The soft amyloid core of a yeast prion was fused to dihydrofolate reductase through flexible linkers of different sizes. This enabled us to produce, for the first time, biocompatible protein-only amyloid-like oligomeric nanoparticles with defined dimensions in which the embedded enzyme remained highly active, as assessed by biophysical and enzymatic assays. The modular design allowed one to obtain multifunctional nanoparticles by incorporating the antibody-binding Z-domain to the protein fusion. We show how these assemblies can be exploited for antibody-directed targeting of specific cell types and the localized delivery of methotrexate, resulting in the intracellular uptake of the drug by cancer cells and their death. Overall, the novel protein particles we describe in this work might find applications in areas as diverse as biocatalysis, bioimaging, or targeted therapies.