Animals like mussels and barnacles produce strong adhesives that don’t dissolve in water. One reason this property has drawn the interest of scientists because of the potential to produce commercial products that mimic these sticky underwater properties.
Yet the starfish, properly called the sea star, requires a different system that allows it to stick–and to unstick–as it moves across a rock or pries open the shell of a mussel. The adhesive piece of this system is characterized for the sea star species Asterias rubens in a paper published recently in PNAS.
The key part is a protein with four subunits and multiple active domains, which the researchers call Sea star footprint protein 1, or Sfp1. Some domains are known to link to metals, others to carbohydrates, and others to proteins.
“Sea stars are very different from mussels or barnacles and this protein is very different from others,” says study lead author Elise Hennebert of the University of Mons, Belgium. “It is a protein that should work differently than other marine adhesives.”
But doing the work was not easy. The first step was to analyze the starfish’s “footprints”, left behind on surfaces. Previous studies by the group had shown that this residue from A. rubens was about 20 percent protein, and they had isolated peptide snippets from this glue. “Collecting the glue was very difficult,” notes Hennebert. In the new work, the team used the most prevalent peptide to pull out and characterize the full protein, “which appeared to be very, very long.” Hennebert said. Further study revealed the subunit structure.
The adhesive system has two components. One is a homogeneous film secreted by one type of adhesive cell in the sea star’s tube feet. This film coats the surface the sea star adheres to, clinging to the ubiquitous film of bacteria that covers undersea surfaces.
On top of this is a fibrous, meshlike layer made largely of protein, secreted by a second type of adhesive cell in the tube feet, and including the newly characterized protein. The researchers hypothesize that the new protein may act as a bridge from the homogeneous film to the sea star’s body, with some parts of the protein sticking to glycoproteins in the epidermis of the starfish while others cling to the homogeneous film or other proteins in the mesh layer.
The new finding helps explain how the sea star sticks, but still mysterious is how it can quickly unstick its feet. “We don’t know anything yet about detachment,” Hennebert says, though they have reason to suspect the key substance is also a protein.
Identifying the detachment protein may be particularly difficult. It does not appear to be within the glue, and it may be water soluble, so it may disappear once it’s secreted. Hennebert tried to isolate it from granules in the tube foot, but that failed. One lead the team has is the transcriptome—the full profile of expressed genes–from the tube feet, which should contain the gene sequence for the detachment protein, but they don’t know what to look for to identify it. So for now, Hennebert says, “It’s completely a mystery.”