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.