Designing nanodevices that can change shape and function in response to molecular signals is a key objective in protein design, paving the way for the development of switchable nanomaterials, triggerable drug-delivery systems, and molecular motors. In this talk, I will present recent progress in designing and characterizing a diverse range of switchable protein assemblies. These include protein rings with tunable shape and diameter, cages that disassemble upon peptide binding, and proteins that conditionally form dimers. Notably, these oligomeric systems exhibit allosteric ligand-binding cooperativity on par with, or exceeding, that of natural proteins. Mass photometry played a critical role in their structural and biophysical analysis, allowing us to quantify complex features like conditional oligomerization and cooperativity in unprecedented quantitative detail.