Self-assembling proteins compose the chemically resistant shell biomaterial of planktonic tintinnid ciliates.

Biomaterials provide superior properties and sustainable alternatives relevant to medicine, textiles, and high-tech applications. Research has mainly focused on animal-derived proteinaceous biomaterials, which remain challenging to reproduce while retaining their remarkable properties. Here, we show that the shell biomaterial of tintinnid ciliates, a lineage of planktonic unicellular eukaryotes, is composed of self-assembling structural proteins. The shells form in sea- and freshwater, are structurally diverse, and exhibit resistance against high temperatures and the strongest chemicals. Combining single-cell transcriptomics with proteomics of the shells, we identify the amino acid sequences of the shell-forming proteins that represent a new family unique to tintinnid ciliates, which we term Tintinnidorin. The proteins are rich in aromatic residues and possess a coherent architecture with flexible, unfolded segments connecting a folded core structure of beta-sheets. These multivalent capabilities facilitate intracellular storage, extracellular self-assembly, wet adhesion, thermostability, and salt tolerance. Tintinnid ciliates and their Tintinnidorin proteins provide an accessible system to elucidate sequence-structure-material relationships and inspire biomaterial design.