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Sea star tenacity mediated by a protein that fragments, then aggregates

Significance Sea stars are emblematic of the seashore. Despite this, their ability to pry open mussels and attach strongly but temporarily to rocks in their environments are poorly understood. Here we report, to our knowledge, the first sequence of a protein, Sea star footprint protein 1 (Sfp1), a primary constituent of the adhesive footprints secreted by sea star tube feet. Sfp1 is unusually large and complex compared with other marine adhesive proteins such as those of mussels. It is translated from a single mRNA and then fragmented into four subunits, which display specific domains that mediate interactions with other proteins present in the adhesive material and on the tube foot surface. After secretion, Sfp1 forms a structural scaffold and appears to provide footprints with cohesion. Sea stars adhere firmly but temporarily to various substrata as a result of underwater efficient adhesive secretions released by their tube feet. Previous studies showed that this material is mainly made up of proteins, which play a key role in its adhesiveness and cohesiveness. Recently, we solubilized the majority of these proteins and obtained 43 de novo-generated peptide sequences by tandem MS. Here, one of these sequences served to recover the full-length sequence of Sea star footprint protein 1 (Sfp1), by RT-PCR and tube foot transcriptome analysis. Sfp1, a large protein of 3,853 aa, is the second most abundant constituent of the secreted adhesive. By using MS and Western blot analyses, we showed that Sfp1 is translated from a single mRNA and then cleaved into four subunits linked together by disulphide bridges in tube foot adhesive cells. The four subunits display specific protein-, carbohydrate-, and metal-binding domains. Immunohistochemistry and immunocytochemistry located Sfp1 in granules stockpiled by one of the two types of adhesive cells responsible for the secretion of the adhesive material. We also demonstrated that Sfp1 makes up the structural scaffold of the adhesive footprint that remains on the substratum after tube foot detachment. Taken together, the results suggest that Sfp1 is a major structural protein involved in footprint cohesion and possibly in adhesive interactions with the tube foot surface. In recombinant form, it could be used for the design of novel sea star-inspired biomaterials.