Nature shows excellent control over the mechanics of fibrous hydrogels by assembling protein, fibers into bundleS, of well-defined dimeosions. Yet, obtaining artificial materials displaying controlled bundling remains a challenge. Here, we developed genetically engineered protein-based polymers, functionalized with heparin-binding KRSR domains and show controlled' bundling using heparin as a binder. The protein polymer forms fibers upon inereasing the pH to physiological values and at higher concentrations fibrous gels. We show that addition of heparin to the protein polymer with incorporated KRSR domains, induces bundling, which results in faster gel formation and stiffer gels. The interactions are--expected to" be primarily electrostatic and fiber bundling has an optimum when the positive charges of KRSR are approximately in balance with the negative: charges of the heparin. Our study suggests that, generally, a straightforward method to control the properties of fibrous gels is to prepare a fiber former with specific binding domains and then simply adding an appropriate amount of binder.