AbstractThe metamorphic protein XCL1 switches between two distinct native structures with different functions in the human immune system. This structural interconversion requires complete rearrangement of all hydrogen bonding networks, yet fold‐switching occurs spontaneously and reversibly in solution. One structure occupies the canonical α‐β chemokine fold and binds XCL1's cognate G‐protein coupled receptor, while the other structure occupies a dimeric, all‐β fold that binds glycosaminoglycans and has antimicrobial activity. Both of these functions are important for the biologic role of XCL1 in the immune system, and each structure is approximately equally populated under near‐physiologic conditions. Recent work has begun to illuminate XCL1's role in combatting infection and cancer. However, without a way to control XCL1's dynamic structural interconversion, it is difficult to study the role of XCL1 fold‐switching in human health and disease. Thus, a molecular tool that can regulate the fractional population of the two XCL1 structures is needed. Here, we find by heparin affinity chromatography and NMR that an engineered XCL1 variant called CC5 can trigger a dose‐dependent shift in XCL1's metamorphic equilibrium such that the receptor binding structure is depleted, and the antimicrobial structure is more heavily populated. This shift likely occurs due to formation of XCL1‐CC5 heterodimers in which both protomers occupy the β‐sheet structure. These findings lay the groundwork for future studies seeking to understand the functional role of XCL1 metamorphosis, as well as studies screening for a drug‐like molecule that can therapeutically target XCL1 by tuning its metamorphic equilibrium. Moreover, the proof of concept presented here suggests that protein metamorphosis is druggable, opening numerous avenues for controlling biological function of metamorphic proteins by altering the population of their multiple native states.