The Ca²⁺ response of a smart forisome protein is dependent on polymerization

Forisomes are giant self-assembling mechanoproteins that undergo reversible structural changes in response to Ca²⁺ and various other stimuli. Artificial forisomes assembled from the monomer MtSEO-F1 can be used as smart biomaterials, but the molecular basis of their functionality is not understood. To determine the role of protein polymerization in forisome activity, we tested the Ca²⁺ association of MtSEO-F1 dimers (the basic polymerization unit) by circular dichroism spectroscopy and microscale thermophoresis. We found that soluble MtSEO-F1 dimers neither associate with Ca²⁺ nor undergo structural changes. However, polarization modulation infrared reflection absorption spectroscopy revealed that aggregated MtSEO-F1 dimers and fully-assembled forisomes associate with Ca²⁺ , allowing the hydration of poorly-hydrated protein areas. A change in the signal profile of complete forisomes indicated that Ca²⁺ interacts with negatively-charged regions in the protein complexes that only become available during aggregation. We conclude that aggregation is required to establish the Ca²⁺ response of forisome polymers.