Evolution of temporary adhesive proteins in sea stars

Sea stars are able to quickly and repeatedly attach and detach themselves with their hydraulic tube feet. Long thought to be 'suckers', the tube feet indeed contain a duo-gland adhesive system, where adhesive glands secrete the proteinaceous glue and a different gland type produces a de-adhesive substance. Upon voluntary detachment the adhesive material stays attached to the substrate as a footprint, consisting of a structural meshwork on a thin homogenous film. In recent years, the structural characteristics and adhesive composition have been investigated in the forcipulatid species Asterias rubens. Transcriptomic and proteomic analyses revealed the presence of 34 proteins within the footprints of this species [1]. To date, one large (3853 aa) adhesive protein, the sea star footprint protein 1 (Sfp1), has been characterized and was localized in the meshwork of the footprints [2].
Among sea stars, the adhesive proteins have been solely investigated in the species A. rubens. Yet, polyclonal antibodies raised against footprint material of A. rubens led to a strong immunolabelling within the adhesive epidermis of thirteen other asteroid species, indicating conserved components [3]. A BLAST search with the protein sequence of A. rubens Spf1 in publically available sea star transcriptomes revealed a high conservation of this large protein throughout the Asteroidea phylogeny. These results were further confirmed with Sfp1 antibody staining on tube foot sections of various species.
While the research in A. rubens is ongoing, we further started to investigate temporary adhesion in the distantly related valvatid species Asterina gibbosa. The most recent molecular phylogeny of the Class Asteroidea supports a tree in which two main groups apparently diverged early in the evolution of sea stars [4]. According to this phylogeny, A. gibbosa and A. rubens could be considered as distantly-related species as they each belong to one of these two main sea star clades. In A. gibbosa a de novo tube foot-specific transcriptome was made and is currently used to identify adhesive and de-adhesive proteins. The species A. gibbosa was chosen due to its availability, small size, and uncomplicated and fast development under laboratory conditions. These features may allow functional testing of potential adhesive- and de-adhesive genes using RNA interference.
Extending the research on sea star temporary adhesion from A. rubens to more species, will help to identify conserved motifs and properties of the reversible adhesive and might facilitate the development of biomimetic, reversible glues.

1. E. Hennebert, B. Leroy, R. Wattiez, P. Ladurner, J Proteomics 128, (2015).
2. E. Hennebert, R. Wattiez, M. Demeuldre, P. Ladurner, D. S. Hwang, J. H. Waite, P. Flammang, Proc Natl Acad Sci USA 111, (2014).
3. R. Santos, D. Haesaerts, M. Jangoux, P. Flammang, J Morphol, 263, (2005).
4. G. V. Linchangco, D. W. Foltz, R. Reid, J. Williams, C. Nodzak, A. M. Kerr, A. K. Miller, R. Hunter, N. G. Wilson, W. J. Nielsen, C. L. Mah, G. W. Rouse, G. A. Wray, D. A. Janies, Mol Phylogenet Evol 115, (2017).